crystals Article LiCrO2 Under Pressure: In-Situ Structural and Vibrational Studies Alka B. Garg 1,2, *, Daniel Errandonea 3 , Julio Pellicer-Porres 3 , Domingo Martinez-Garcia 3 , Swayam Kesari 4 , Rekha Rao 2,4 , Catalin Popescu 5 and Marco Bettinelli 6 1 High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 2 Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India; rekhar@barc.gov.in 3 Departamento de Física Aplicada-ICMUV, MALTA Consolider Team, Universidad de Valencia, Edificio de Investigación, C/Dr. Moliner 50, Burjassot, 46100 Valencia, Spain; Daniel.Errandonea@uv.es (D.E.); Julio.Pellicer@uv.es (J.P.-P.); Domingo.Martinez@uv.es (D.M.-G.) 4 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India; swayam@barc.gov.in 5 CELLS-ALBA Synchrotron Light facility, 08290 Cerdanyola, Barcelona, Spain; cpopescu@cells.es 6 Luminescent Materials Laboratory, Department of Biotechnology, University of Verona and INSTM, UdR Verona, Strada Le Grazie, 15371 34 Verona, Italy; marco.bettinelli@univr.it * Correspondence: alkagarg@barc.gov.in; Tel.: +91-22-2559-5482  Received: 1 December 2018; Accepted: 17 December 2018; Published: 20 December 2018  Abstract: The high-pressure behaviour of LiCrO2 , a compound isostructural to the battery compound LiCoO2 , has been investigated by synchrotron-based angle-dispersive X-ray powder diffraction, Raman spectroscopy, and resistance measurements up to 41, 30, and 10 Gpa, respectively. The stability of the layered structured compound on a triangular lattice with R-3m space group is confirmed in all three measurements up to the highest pressure reached. The dependence of lattice parameters and unit-cell volume with pressure has been determined from the structural refinements of X-ray diffraction patterns that are used to extract the axial compressibilities and bulk modulus by means of Birch–Murnaghan equation-of-state fits. The pressure coefficients for the two Raman-active modes, A1g and Eg , and their mode-Grüneisen parameters are reported. The electrical resistance measurements indicate that pressure has little influence in the resistivity up to 10 GPa. The obtained results for the vibrational and structural properties of LiCrO2 under pressure are in line with the published results of the similar studies on the related compounds. Research work reported in this article contributes significantly to enhance the understanding on the structural and mechanical properties of LiCrO2 and related lithium compounds. Keywords: high-pressure; X-ray diffraction; Raman spectroscopy; equation of state 1. Introduction Layered ABO2 transition-metal oxides, where A (B) is an alkali or noble metal (3d transition metal), have attracted considerable interest due to the wide variety of attributes that are exhibited by them [1–4]. The major crystal structures adopted by these compounds is either delafossite or α−NaFeO2 -type structure, which belong to space group R-3m. In both the structures each element forms a triangular lattice, which follows the stacking arrangement along the c-axis in the sequence B3+ -O2- -A+ -O2- . Difference in the stacking configuration of O2- -A+ -O2- layers distinguishes the two crystal structures. The delafossite structure has a straight stacking, while a zigzag stacking is favoured in α−NaFeO2 [5]. The compounds with A as noble metal (Cu, Ag, Au) adopt the delafossite structure, forming a family of materials showing a rare combination of electrical conductivity and optical transparency widely known as transparent conducting oxides [6,7]. These compounds find a plethora of technological Crystals 2019, 9, 2; doi:10.3390/cryst9010002 www.mdpi.com/journal/crystals Crystals 2019, 9, 2 2 of 18 functions in devices requiring transparent contacts such as solar cells [8], light-emitting diodes [9], and liquid-crystal displays [10]. On the other hand, compounds with A as alkali metal (Li, Na, K) crystallize in the α−NaFeO2 structure. Their study is largely driven by the immense importance of these materials in the electrochemical industry [11–15]. Particularly, the compounds containing lithium have been extensively investigated as cathode materials for lithium-ion batteries, due to their high lithium storage capacity, rate performances, and durability [16]. The first commercialized cathode material, LiCoO2 , which is used in the lithium batteries, still dominates the market of portable electronic appliances due to its high volumetric density and excellent lithium intercalation properties [17]. LiCrO2 , a compound isostructural to LiCoO2 , though, shows a poor reversible lithium intercalation; however, the interest in this compound is due to its multiple electron transfer nature during electrochemical reactions and its large lithium-storage capacity [18]. Furthermore, the addition of a small amount of Cr to many LiBO2 layered materials has shown to induce a drastic enhancement of the electrochemical properties [19]. Reducing the grain size of LiCrO2 also results in the enhancement of the lithium-storage capacity [20,21]. Other than industrially driven research on alkali-metal based compounds, fundamental research on these materials has developed in parallel. The nickelates, chromates, and cobaltates have been the subject of numerous studies, mainly because of their exotic magnetic properties coupled with interesting phase transitions [22–25]. The properties of a material are linked with its crystal structure and they can be tuned by subjecting the material to various thermodynamic variables such as pressure and/or temperature [26]. A few high-pressure studies carried out on delafossite-structured compounds have shown interesting structural behaviour [27–31]. However, the study of the stability of layered α−NaFeO2 –type compounds under pressure is sparse. Based upon first-principles calculations, a structural phase transition to a cubic phase was proposed to exist in LiCoO2 at 3 GPa [32]. However, more recent X-ray diffraction, Raman spectroscopic, and theoretical studies show the stability of this compound up to 26 GPa [33]. On the other hand, recent report of high-pressure investigations on lithium titanate has revealed the improved electrical conductivity of the compound under pressure suggesting its possible uses in the future development of lithium batteries [34]. In the case of LiCrO2 , after a broad literature search we concluded that the structural stability of LiCrO2 against compression has not been explored so far. In this work, we report the influence of pressure at ambient temperature on the structural, vibrational, and electrical properties of LiCrO2 . These properties were investigated by synchrotron X-ray diffraction, Raman spectroscopy, and electrical resistance measurements up to 41, 30, and 10 Gpa, respectively. All three techniques indicate the stability of the ambient crystal structure up to the highest pressure reached in three measurements. The structural details of the ambient rhombohedral phase have been determined at various pressures. The axial compressibility, inter-atomic bond distances, polyhedral compressibility, and isothermal (300 K) P-V equation of state (EOS) of the ambient-pressure phase of LiCrO2 are reported. The pressure dependence of two expected Raman modes, along with their Gruneisen parameter, has been obtained. 2. Experimental Section 2.1. Sample Synthesis and Characterization Polycrystalline sample of LiCrO2 was prepared by solid state reaction route using high purity Li2 CO3 and Cr2 O3 (99.9% purity, Alfa Aesar), taken in 1:1 ratio by weight. The two oxides are meticulously mixed in pestle and mortar, uniaxially compressed into pellets of 13 mm diameter and 5 mm height, and exposed to initial heat treatment at 850 ◦ C for 24 h in programmable resistive furnace, followed by another heating cycle at 1100 ◦ C for 24 h. The resultant product was checked employing X-ray diffraction (angle dispersive mode) using a rotating anode generator with molybdenum target as X-ray source (λ = 0.7107 Å) and Raman spectroscopy (λ = 532 nm laser) for the confirmation of its single phase formation. Crystals 2019, 9, 2 3 of 18 LiCrO2 single crystals were grown by the flux method. The growth was started from a mixture of 0.065 mol of Li2 CO3 , 0.014 mol of PbO, 0.034 mol of B2 O3 , and 0.22 mol of Cr2 O3 , which was heated at 1300 ◦ C and then slowly cooled down to 800 ◦ C and subsequently naturally cooled down to room temperature. After separating the crystals from the flux, their crystal structure and purity were confirmed with powder X-ray diffraction (λ = 1.5406 Å). 2.2. High Pressure Measurements 2.2.1. X-ray Diffraction Powder X-ray diffraction measurements in an Angle-dispersive configuration were carried out at the MSPD-BL04 beamline of the ALBA synchrotron source. A membrane-type diamond-anvil cell (DAC) equipped with 350-µm culet diamonds was used for compressing the sample. Fine particles of the sample with a few Cu grains (to serve as in-situ pressure standard [35]) and Ne, as pressure-transmitting medium, were loaded into a sample chamber made from stainless steel metal gasket with a centred hole of 150 µm in diameter. The gasket was previously pre-indented to a thickness of 50 µm. Special care was taken while loading the sample in the sample chamber to reduce the possibility of the sample bridging the diamonds, which may induce unwanted large pressure gradients in the sample [36,37]. X-ray powder patterns at different pressures were collected using monochromatic X-rays (λ = 0.4246 Å) that were focused to 15 × 15 µm2 . Images of the X-ray diffraction rings were recorded on a Rayonix CCD detector. The sample to detector distance, together with the detector orientation and other calibration parameters, were calculated using the diffraction pattern of LaB6 as standard and the FIT2d software [38]. This software was also used to transform the two-dimensional (2D) diffraction images to one-dimensional (1D) intensity vs. two theta diffraction patterns. The structural analysis was performed with GSAS [39]. 2.2.2. Raman Spectroscopy Two independent data sets of Raman measurements at different laboratories were carried out at several pressures. Both the data sets were collected in back-scattering geometry. In first data set, collected up to 15 GPa on the polycrystalline sample, a Mao-Bell-type DAC, with 16:3:1 methanol-ethanol-water mixture (MEW) as pressure-transmitting medium, was employed. The Raman measurements were performed using a 532 nm laser (5 mW). The spectra were collected using a single monochromator coupled with an edge filter and a thermoelectric-cooled Charge Coupled Devices (CCD). The spectral resolution was 3 cm−1 . In the second data set collected up to 30 GPa on a single crystal, a membrane-type DAC, with Ne as pressure-transmitting medium was used. Raman spectra were excited using a 632.8 nm laser (10 mW). The experimental setup was home-built using a confocal microscope, an edge filter, a single spectrometer, and a thermoelectric-cooled CCD. The spectral resolution of this setup is better than 2 cm−1 . In both sets of measurements, the ruby fluorescence technique [40] was used for in-situ pressure calibration. 2.2.3. Electrical Resistance Electrical resistance measurements up to a maximum pressure of 10 GPa were made using an opposed Bridgman anvil set-up with 12-mm diameter tungsten carbide anvils [41] that is mounted in a hydraulic press. A pair of pyrophyllite gaskets, with a thickness 200 µm each, and with a central hole of 3 mm in diameter, was employed as the pressure chamber. Steatite was used as a pressure-transmitting medium. Pressure was determined by calibrating the high pressure assembly with the known phase transitions of bismuth. A rectangular piece of the sample cut from the well compacted powdered sample, with 2 mm × 1.5 mm × 0.1 mm in size, was used for resistance (four contacts) measurements [42]. A constant current was applied to the sample by means of the outer leads by using a Keithley current source. The voltage drop was measured using a Keithley nano-voltmeter. The measurements were performed at each pressure with two minutes of pressure stabilization time. Crystals 2018, 8, x FOR PEER REVIEW 4 of 19 contacts) measurements [42]. A constant current was applied to the sample by means of the outer leads by using a Keithley current source. The voltage drop was measured using a Keithley Crystals 2019, 9, 2 4 of 18 nano-voltmeter. The measurements were performed at each pressure with two minutes of pressure stabilization time. 3. Results and Discussion 3. Results and Discussion 3.1. Ambient Pressure 3.1. Ambient Pressure As shown in Figure 1, at ambient conditions, the compound adopts the α-NaFeO2 -type layered As shown in Figure 1, at ambient conditions, the compound adopts the α-NaFeO2-type layered rhombohedral structure rhombohedral thatthat structure is described bybyspace is described group R-3m. spacegroup R-3m. FigureFigure 1. (Colour 1. (Colour online) online) Ambient-pressurecrystal Ambient-pressure crystal structure structure of ofthe thelayered layeredstructured LiCrO structured 2 (space LiCrO 2 (space group R-3m). Lithium (green) and Chromium (blue) atoms are octahedrally coordinated with oxygen group R-3m). Lithium (green) and Chromium (blue) atoms are octahedrally coordinated with oxygen atom (red). atom (red). In this structure, the Li atoms are surrounded by edge-sharing oxygen octahedral units, In this structure, the Li atoms are surrounded by edge-sharing oxygen octahedral units, separated separated by triangular planes of Cr atoms, which makes the compound a two-dimensional (2D) by triangular triangularplanes of Cr atoms, antiferromagnet, which makes isostructural thecommon with the compound a two-dimensional battery material LiCoO2 [43]. (2D) Cr triangular atoms antiferromagnet, isostructural with the common battery material LiCoO 2 [43]. are also surrounded by six oxygen atoms. The volume of the lithium octahedron is larger than that Cr atoms are of also surrounded by six oxygen atoms. The volume of the lithium octahedron is the chromium octahedron. An X-ray diffraction pattern measured at ambient conditions and the larger than that of the chromium octahedron. Rietveld refinementAn X-ray of the datadiffraction are shown in pattern Figuremeasured atobserved 2. All of the ambientdiffraction conditionspeaks and could the Rietveld be fitted with refinement thedata of the rhombohedral are shown structure in Figure (space 2. group All ofR-3m). This confirms the observed the single diffraction phasecould peaks formation be fitted of the with the compound. The rhombohedral refined structure latticegroup (space parameters R-3m).areThisa confirms = 2.8941(3)the Å single and c phase = 14.391(3) Å. The formation of the corresponding unit-cell volume is V = 104.38(2) Å 3, which is in excellent agreement with the value compound. The refined lattice parameters are a = 2.8941(3) Å and c = 14.391(3) Å. The corresponding reported in the literature [44]. unit-cell volume is V = 104.38(2) Å3 , which is in excellent agreement with the value reported in the literature [44]. Crystals 2018, 8, x FOR PEER REVIEW 5 of 19 Figure 2. Refined ambient conditions XRD pattern of our LiCrO2 sample. It shows the single phase Figure 2. Refined ambient conditions XRD pattern of our LiCrO2 sample. It shows the single phase formation of the compound. Vertical tick marks represent allowed the reflection of the rhombohedral formation of thestructure compound. with R-3mVertical tick marks represent allowed the reflection of the rhombohedral space group. structure with R-3m space group. The ambient pressure volume of LiCrO2 is nearly 8% larger than in the isostructural compound LiCoO2. The Wyckoff positions for Li, Cr and oxygen atoms are 3b (0, 0, 0.5), 3a (0, 0, 0), and 6c (0, 0, z), respectively. The only free parameter is the z parameter of the oxygen atoms. The refined value is 0.7433(5). R-factors of the refinement are Rp = 4.54% and wRp = 6.17% (Table 1). Table 1. Refined structural parameters for LiCrO2 at ambient and high pressures. Ambient Pressure; a = 2.8941 (3)Å; c = 14.391 (3) Å, Rp = 4.5% and wRp = 6.2%. Atom Site x Y Z Crystals 2019, 9, 2 5 of 18 The ambient pressure volume of LiCrO2 is nearly 8% larger than in the isostructural compound LiCoO2. The Wyckoff positions for Li, Cr and oxygen atoms are 3b (0, 0, 0.5), 3a (0, 0, 0), and 6c (0, 0, z), respectively. The only free parameter is the z parameter of the oxygen atoms. The refined value is 0.7433(5). R-factors of the refinement are Rp = 4.54% and wRp = 6.17% (Table 1). Table 1. Refined structural parameters for LiCrO2 at ambient and high pressures. Ambient Pressure; a = 2.8941 (3)Å; c = 14.391 (3) Å, Rp = 4.5% and wRp = 6.2%. Atom Site x Y Z Li 3b 0 0 0.5 Cr 3a 0 0 0 O 6c 0 0 0.7433(4) Li-O Cr-O Cr-Cr/Li-Li Li-Cr Bond length (Å) 2.1134 2.0032 2.9231 2.8941 5.3 GPa; a = 2.8736 (3)Å; c = 14.1367 (2) Å, Rp = 2.65% and wRp = 4.92% O 6c 0 0 0.7496 (4) Li-O Cr-O Cr-Cr/Li-Li Li-Cr Bond length (Å) 2.0353 2.0288 2.8736 2.8816 16.4 GPa: a = 2.8367 (7) Å; c = 13.753 (5) Å, Rp = 3.85% and wRP = 5.82% O 6c 0 0 0.7662 Li-O Cr-O Cr-Cr/Li-Li Li-Cr Bond length (Å) 1.8779 2.1316 2.8367 2.8172 21.8 GPa: a = 2.8199 (8) Å; 13.640 (8) Å, Rp = 3.56 % and wRP = 5.82% O 6c 0 0 0.7736 (4) Li-O Cr-O Cr-Cr/Li-Li Li-Cr Bond length (Å) 1.8185 2.1828 2.8199 2.7961 UsingCrystals 2018, 8, x FOR PEER REVIEW group-theory analysis, the total irreducible representation for the zone centre 6 of 19 optic modes of LiCrO2 is obtained as A1g + 2A Using group-theory 2u + Ethe analysis, g +total 2Euirreducible . Among representation them, two modesfor the (Azone and Eoptic 1g centre g ) are Raman active [45,46]. modesFor the A of LiCrO 2 is 1g mode, obtained the as A1g atomic + 2A 2u + Egshift + 2Eu.of oxygen Among atoms them, two is modes along (A 1g the and Eg)c-axis, are whereas the Raman active [45,46]. For the A 1g mode, the atomic shift of oxygen atoms is along the c-axis, whereas the Eg Eg mode corresponds to vibrations in perpendicular directions. The Raman spectra measured for mode corresponds to vibrations in perpendicular directions. The Raman spectra measured for the the polycrystalline sample of LiCrO2 2isis shown polycrystalline sample of LiCrO in Figure 3. The two Raman active modes E and A1g shown in Figure 3. The two Raman active modes Eg and A1gg appear at 456 and 589 cm −1 , respectively, and matches well with the previously appear at 456 and 589 cm , respectively, and matches well -1 with the previously reportedreported values [46]. values [46]. Figure 3. Raman Figure 3.spectrum of as of Raman spectrum synthesized as synthesizedLiCrO atambient LiCrO22 at ambient pressure pressure and temperature and temperature conditions. conditions. The two Raman The twoactive Ramanmodes are clearly active modes observed. are clearly observed. The Raman spectrum did not show any shoulder peaks of the Eg mode, which are usually associated to the presence of Li vacancies in the material [45]. This observation is consistent with the formation of a stoichiometric compound. The experimental reported value of Eg and A1g modes for the isostructural compound LiCoO2 are 486 and 595 cm−1 (Table 2) [33]. The two modes in this compound are reported to be predominantly Co-O stretching and O-Co-O bending motions, which are supported by Li isotope substitution experiments as well as a comparison to the isostructural Crystals 2019, 9, 2 6 of 18 The Raman spectrum did not show any shoulder peaks of the Eg mode, which are usually associated to the presence of Li vacancies in the material [45]. This observation is consistent with the formation of a stoichiometric compound. The experimental reported value of Eg and A1g modes for the isostructural compound LiCoO2 are 486 and 595 cm−1 (Table 2) [33]. The two modes in this compound are reported to be predominantly Co-O stretching and O-Co-O bending motions, which are supported by Li isotope substitution experiments as well as a comparison to the isostructural compound NaCoO2. In this compound, the Eg and A1g Raman modes are at 486 and 586 cm− 1 , respectively [47]. Notice that by substituting sodium for lithium, the Eg mode does not shift in frequency within experimental resolution. On the other hand, the frequency change of the A1g mode is quite small. These two facts prove that in ABO2 compounds with NaFeO2 -type structure, A1g and Eg frequencies depend predominantly on B cation. In the case of LiCrO2 , the Eg mode has a lower frequency (456 cm−1 ), which corresponds to a weaker restoring force for vibrations of the chromate units than in the cobaltate units due to the larger ionic radii of Cr3+ as compared to the compounds having Co3+ at B site. In the case of LiNiO2 , another isostructural compound with LiCrO2, the Eg and A1g mode frequencies are 465 and 545 cm−1 , respectively [48]. Table 2. Experimental Raman modes along with Gruneisen parameters (γ) for a LiCrO2 and isomorphic compounds. Raman Mode Raman Frequency Gruneisen Compounds B Cationic Radii Assignment (ω) (cm−1 ) Parameter (γ) Eg 486(1) 1.15 [32] LiCoO2 0.545 Å A1g 595(1) 0.79 Eg 456(2) 1.07(2) [this work] LiCrO2 0.615 Å A1g 589(2) 1.18(2) Eg 486 [47] NaCoO2 0.545 Å A1g 586 Eg 465 [48] LiNiO2 0.56 Å A1g 545 3.2. High-Pressure Studies Figure 4 shows the raw 2D image from the CCD detector that was collected at 7.3 GPa. This raw image is representative of the measurement and illustrates the good quality of our data, information which is lost in a standard one dimensional integrated pattern. The evolution of diffraction patterns of LiCrO2 as the pressure is increased is shown in Figure 5a, whereas in Figure 5b a few XRD diffraction patterns collected while pressure unloading is depicted. As marked in the figure, not only the diffraction peaks from LiCrO2 are present. There are also two peaks from the copper used as in-situ pressure calibrant. All of the diffraction peaks assigned to the sample are unequivocally indexed with the ambient-pressure rhombohedral structure. On increasing the pressure, there are no qualitative changes observed in the diffraction patterns. Only shifting of all the peaks to higher angles is found, which is caused by the lattice compression. This evolution of XRD patterns continues until the highest pressure of 41.2 GPa is reached in the present measurements. Similar to CuAlO2 and CuGaO2 delaffosites [30,49], in LiCrO2 also the intensity of the (00k) Bragg peaks and width of the reflections are affected by pressure. However the absence of any extra diffraction peaks indicates the structural stability of the studied material under compression. Refinements of all the diffraction patterns that were collected at various pressures were carried out to obtain the evolution of lattice parameters as a function of pressure. Due to the presence of Cu (111) reflection close to the (104) reflection from the sample, the refinements were carried out by masking these two peaks. Note that Cu being cubic (space group F-3m), the refinement of a single peak, Cu (200) can be used to determine the pressure. For the diffraction patterns up to 21.8 GPa, a Rietveld refinement was carried out, however, due to texture in the diffraction peaks beyond this pressure, only a profile refinement was carried out above 21.8 GPa. This is sufficient to obtain the correct lattice parameters and cell volume. which is lost in a standard one dimensional integrated pattern. The evolution of diffraction patterns of LiCrO2 as the pressure is increased is shown in Figure 5a, whereas in Figure 5b a few XRD diffraction patterns collected while pressure unloading is depicted. As marked in the figure, not only the diffraction peaks from LiCrO2 are present. There are also two peaks from the copper used as in-situ 2019, Crystals pressure 9, 2 calibrant. All of the diffraction peaks assigned to the sample are unequivocally 7 of 18 indexed with the ambient-pressure rhombohedral structure. Crystals 2018, 8, x FOR PEER REVIEW 8 of 19 F-3m), the refinement of a single peak, Cu (200) can be used to determine the pressure. For the diffraction patterns up to 21.8 GPa, a Rietveld refinement was carried out, however, due to texture in the diffraction peaks beyond this pressure, only a profile refinement was carried out above 21.8 GPa. ThisFigure is sufficient to obtain 4. Image the correct of the powder lattice parameters diffraction and data of liCrO cell volume. 2 collected at 7.3 GPa on CCD detector. Figure 4. Image of the powder diffraction data of liCrO2 collected at 7.3 GPa on CCD detector. On increasing the pressure, there are no qualitative changes observed in the diffraction patterns. Only shifting of all the peaks to higher angles is found, which is caused by the lattice compression. This evolution of XRD patterns continues until the highest pressure of 41.2 GPa is reached in the present measurements. Similar to CuAlO2 and CuGaO2 delaffosites [30,49], in LiCrO2 also the intensity of the (00k) Bragg peaks and width of the reflections are affected by pressure. However the absence of any extra diffraction peaks indicates the structural stability of the studied material under compression. Refinements of all the diffraction patterns that were collected at various pressures were carried out to obtain the evolution of lattice parameters as a function of pressure. Due to the presence of Cu (111) reflection close to the (104) reflection from the sample, the refinements were carried out by masking these two peaks. Note that Cu being cubic (space group Figure Figure 5. (a) 5. (a) EvolutionofofX-ray Evolution X-ray diffraction diffraction patterns patternsforforLiCrO LiCrO 2 while 2 whilecompressing; (b) while compressing; (b) while decompressing. decompressing. Diffraction Diffraction peaksfrom peaks fromin-situ in-situ pressure pressure calibrant calibrant(Cu) areare (Cu) also indicated. also Numbers indicated. on on Numbers the right hand side of y-axis denote the pressure the right hand side of y-axis denote the pressure in GPa.in GPa. Figure 6a–d show the representative fitted XRD patterns at 5.3, 16.4, 21.7, and 41.2 GPa, respectively. Crystals 2019, 9, 2 8 of 18 Figure Crystals 6a–d 2018, 8, show x FOR PEERthe representative fitted XRD patterns at 5.3, 16.4, 21.7, and 41.2 GPa, respectively. REVIEW 9 of 19 Figure 6. Observed Figure 6. Observed (red), (red), calculated calculated (green), (green), and and difference difference plot plot (blue) (blue) of of X-ray X-ray powder powder patterns patterns for for LiCrO LiCrO2 (a) at 5.3 GPa, (b) 16.4 GPa, (c) 21.7 GPa, (d) 41.2 GPa. Top and bottom vertical marks indicate 2 (a) at 5.3 GPa, (b) 16.4 GPa, (c) 21.7 GPa, (d) 41.2 GPa. Top and bottom vertical marks indicate Bragg Bragg reflections reflections from from the the sample sample and andpressure pressurecalibrant calibrant(Cu). (Cu). Similar quality of fittings was obtained for all of the pressure points. Structural information Similar quality of fittings was obtained for all of the pressure points. Structural information at at selected pressures and the R-factors of the structural refinements are given in Table 1. Various selected pressures and the R-factors of the structural refinements are given in Table 1. Various bond bond distances are also included in the Table 1. In Figure 7, we present the lattice parameters a and distances are also included in the table 1. In Figure 7, we present the lattice parameters a and c, and c, and the c/a ratio versus pressure. Error bars in both the axes and the axial ratio have also been the c/a ratio versus pressure. Error bars in both the axes and the axial ratio have also been plotted plotted along with the determined results. From these data, the isothermal compressibility along the c- along with the determined results. From these data, the isothermal compressibility along the c- and and a-axes is estimated to be 5.6−3× 10−3 −4(3 × 10−4 ) and −31.24 × 10 −3 (4 × 10−5 ) GPa−1 , respectively. a-axes is estimated to be 5.6 × 10 (3 × 10 ) and 1.24 × 10 (4 × 10−5) GPa−1, respectively. The numbers The numbers in the parenthesis represent the estimated error in the fitting. The values of the axial in the parenthesis represent the estimated error in the fitting. The values of the axial compressibilities indicate a highly anisotropic behaviour of LiCrO2 . The c-axis is more than four times compressibilities indicate a highly anisotropic behaviour of LiCrO2. The c-axis is more than four as compressible as the a-axis. This behaviour is opposite to the one observed in delafossites, where the times as compressible as the a-axis. This behaviour is opposite to the one observed in delafossites, c-axis is significantly less compressible than the a-axis. The anisotropic compression that was observed where the c-axis is significantly less compressible than the a-axis. The anisotropic compression that in the axes could be related to the layered framework of the crystal structure. In particular, to the large was observed in the axes could be related to the layered framework of the crystal structure. In particular, to the large compressibility of the Li-O bonds formed between layers of CrO6 octahedral units as will be discussed in the next section. Crystals 2019, 9, 2 9 of 18 compressibility of the Li-O bonds formed between layers of CrO6 octahedral units as will be discussed Crystals 2018,next in the 8, x section. FOR PEER REVIEW 10 of 19 Figure Figure 7. Pressure 7. Pressure dependenceofof(a) dependence (a) lattice lattice parameters parametersand and(b)(b) axial ratio axial (c/a)(c/a) ratio of LiCrO 2 . Symbols of LiCrO 2. Symbols present the experimental data points and the solid lines in (a) are a Birch–Murnaghan present the experimental data points and the solid lines in (a) are a Birch–Murnaghan equation equation of state of (BM–EOS) fit to the data. Error bars have also been plotted in the data. A clear slope change in c/a state (BM–EOS) fit to the data. Error bars have also been plotted in the data. A clear slope change in ratio can be seen. c/a ratio can be seen. As shown in Figure 1, the structure of LiCrO2 along the c-axis can be seen as stacked layers of As6shown LiO in 6Figure -CrO6 -LiO -CrO6 ....1,octahedral the structure units.of AtLiCrO ambient2 along the c-axis conditions (Figurecan be 6seen 1), LiO as stacked and CrO layers of 6 polyhedral LiO6units -CrOare 6-LiO 6-CrO very 6.... distorted slightly octahedralwithunits. two typesAt ofambient conditions bond distances. (Figure The average 1), LiO6 and lithium–oxygen bondCrO6 polyhedral units length (2.113 Å) are in thevery LiO6 slightly octahedrondistorted is 5% largerwith thantwo types Cr-O the average of bond bond distances. Thein average length (2.003Å) the CrO octahedron, with their respective polyhedral volume as 12.51 Å 3 and 10.69 Å3 . A deeper insight lithium–oxygen 6 bond length (2.113 Å) in the LiO6 octahedron is 5% larger than the average Cr-O on the influence bond length (2.003Å) of pressure in the CrOon the crystal structure of LiCrO2 can be obtained 6 octahedron, with their respective from avolume polyhedral quantitative as 12.51 Å3and 10.69 Å3. A deeper insight on the influence of pressure on the crystal structure of comparison of the polyhedral compression [50]. In Figure 8, we have plotted the evolution ofvarious LiCrO2 can bond distances and normalized polyhedral volume of LiO6 and CrO6 , along with the overall cell be obtained from a quantitative comparison of the polyhedral compression [50]. In Figure 8, we have volume with pressure. plotted the evolution of various bond distances and normalized polyhedral volume of LiO6 and CrO6, along with the overall cell volume with pressure. Crystals 2019, 9, 2 10 of 18 Crystals 2018, 8, x FOR PEER REVIEW 11 of 19 Figure 8. (a) Figure 8. (a) Normalized Normalized volume volume ofof LiO LiO66 and and CrO CrO66octahedra along with the overall lattice volume. (b) (b) Cation-cation and anion-cation bond lengths. Note that except Cation-cation and anion-cation bond lengths. except for for the the Cr-O Cr-O bond bond all all other other bonds bonds shrink with pressure. shrink with pressure. The The data data clearly clearly indicates indicates the the compression compression of ofLiO LiO66 polyhedron, polyhedron, whereas whereas the the CrO CrO66 polyhedron polyhedron expands. expands. However, However, as as can can be be seen seen inin the the figure, figure, the the overall overall effect effect of of pressure pressure on on the the crystal crystal structure structure is is the reduction of cell volume. The observed phenomenon could be related with the factfact the reduction of cell volume. The observed phenomenon could be related with the thatthat the the compressibility of Li-O bonds in α-Li2O2 with bulk modulus 75 GPa [51] is much larger when compressibility of Li-O bonds in α-Li O with bulk modulus 75 GPa [51] is much larger when compared compared to to Cr-O Cr-O bonds bonds inin Cr Cr22O O which which is is having having aa bulk bulk modulus modulus of of 205 205 GPa GPa [51]. [51]. In In case case of of LiCrO LiCrO22,, the compressibility of Li-O bonds is additionally favored by the fact that the Li-O bond the compressibility of Li-O bonds is additionally favored by the fact that the Li-O bond length (2.113 length (2.113 Å) is 5% larger than the same bonds in Li O (1.996 Å). Consequently, since the position Å) is 5% larger than the same bonds 2in Li2O (1.996 Å). Consequently, since the position of Li and Cr of Li and Cr atoms are fixed atoms areby the by fixed crystal symmetry, the crystal to compress symmetry, the Li-O to compress the bonds, the oxygen Li-O bonds, atomatom the oxygen moves along moves the along zthe direction towards the Li atoms (change of Oz coordinate), resulting in the reduction z direction towards the Li atoms (change of Oz coordinate), resulting in the reduction of6 LiO6 of LiO unit. Another factorfactor unit. Another contributing towards contributing the large towards shrinking the large of LiOof6 unit shrinking LiO6 could be due unit could beto thetofact due thethat, fact under compression, alkali metals (especially lithium) are highly compressible, that, under compression, alkali metals (especially lithium) are highly compressible, due to the due to the presence of the s electron in its outer shell. As can be seen in the figure, since the oxygen atom presence of the s electron in its outer shell. As can be seen in the figure, since the oxygen atom is is moving towards the lithium, it results in the increase of Cr-O distance. Additionally, Cr being a transition metals, Crystals 2018, Crystals 9, x2 FOR PEER REVIEW 2019, 8, 1211ofof 19 18 moving towards the lithium, it results in the increase of Cr-O distance. Additionally, Cr being a is quite uncompressible, transition metals, is quitedue to the presencedue uncompressible, of dtoelectrons, therefore the presence of d itelectrons, does not therefore contributeitenough does not to the overall compression of contribute enough to the overall CrO 6 units. Since our X-ray diffraction results indicates the compression of CrO6 units. Since our X-ray diffraction results absence of any structural phase transition in the compound, indicates the absence of any structural phase in order to retain transition the ambient in the compound, crystal-structure symmetry, in order to retain the CrO ambient 6 need to expand, while crystal-structure symmetry,LiO 6 compresses. In isostructural compound LiCoO , also, CrO6 need to expand, while LiO6 compresses.2 In isostructural based on Li-O and Co-O compound bond LiCoO distances, the highly incompressible behavior of the CoO6 octahedra 2, also, based on Li-O and Co-O bond distances, the highly has been incompressible reported,of behavior while it is 6the the CoO LiO6 octahedra octahedra has beenthat essentially reported, whilegets it iscompressed and arethat the LiO6 octahedra responsible essentiallyforgets the compressibility of the compound [33]. From our results, we found a change in the compressed and are responsible for the compressibility of the compound [33]. From our results, we axial compressibility beyond found 17 GPa,inwhere a change a clear the axial slope change compressibility in c/a17 beyond ratio GPa,is where seen (Figure a clear 7). slopeThe FWHM change in of c/a various ratio is diffraction peaks along with Cu peaks are shown in Figure 9. seen (Figure 7). The FWHM of various diffraction peaks along with Cu peaks are shown in Figure 9. Figure 9. Full-width Figure Full-width at athalf halfmaxima maxima(FWHM) (FWHM) ofof various diffraction various peaks diffraction from peaks the sample from alongalong the sample with peaks from the pressure calibrant (Cu). with peaks from the pressure calibrant (Cu). One can One can clearly clearly see see an an accelerated accelerated broadening broadening of of the the sample sample lines lines beyond beyond 15 15 GPa GPa while while the the changes in changes in the the width width ofof the the Cu Cu peaks peaksare aremuch muchsmaller. smaller.ItItisisinteresting interestingtotomention mention here herethat in in that case caseof LiCoO also the Bragg peaks were reported to broaden beyond 16 GPa of LiCoO2 also the Bragg peaks were reported to broaden beyond 16 GPa which authors have 2 which authors have attributed to the presence attributed to the of presence non-hydrostaticity due to freezing of non-hydrostaticity dueof to nitrogen freezing pressure medium of nitrogen [33]. However, pressure mediumin the [33]. same report, However, in the theobserved broadening same report, of one of the observed the Ramanofmode broadening one beyond 12 GPamode of the Raman was assigned beyond to 12either GPa was assigned to either anharmonicity or structural effects not detected in diffraction data. Involume anharmonicity or structural effects not detected in diffraction data. In Figure 10, the pressure Figure datathe 10, along with 3rd pressure orderdata volume Birch–Murnaghan along with 3rdequation of state (BM–EOS)equation order Birch–Murnaghan fitted data is shown of state [52,53]. (BM–EOS) fitted data is shown [52,53]. Notice that the pressure media employed in this study solidify belowthe Notice that the pressure media employed in this study solidify below 10 GPa [36,54], therefore 10 described GPa phenomena [36,54], therefore the happening described around 15 GPahappening phenomena cannot be correlated around 15 with GPa the pressure cannot medium be correlated solidification with the pressureand medium should besolidification intrinsic to LiCrO 2 and LiCoO and should 2. be intrinsic to LiCrO2 and LiCoO2. Crystals 2019, 9, 2 12 of 18 Crystals 2018, 8, x FOR PEER REVIEW 13 of 19 Figure 10. (a) Figure (a) Isothermal Isothermalequations equationsofof state data state for for data LiCrO 2 . Solid LiCrO squares 2. Solid are experimental squares data points are experimental data and solid points andline is the solid third line order is the Birch–Murnaghan third equation of order Birch–Murnaghan state fit to equation of the data state fit points. Errorspoints. to the data in the volumeinare Errors thesmaller volumethanare the symbol smaller used. than (b) Top used. the symbol panel (b) showsTopthe same panel data the shows converted to F-f same data plot (see converted the text). to F-f plot (see the text). The fitted The fitted bulk bulk modulus modulus (B (B00)) is is 161(5) 161(5) GPa GPa with with aa pressure pressure derivative derivative of of the the bulk bulk modulus modulus (B (B00')0 ) equal to equal to 5.2(3), 5.2(3),which which is slightly is slightlyhigher than than higher 149(1)149(1) GPa, the experimentally GPa, the experimentallydetermined bulk modulus determined bulk for the isostructural compound LiCoO modulus for the isostructural compound LiCoO2 (Table 3). 2 (Table 3). The proper The proper wayway toto analyze analyze the the order orderof ofBM–EOS BM–EOSneeded neededfor forfitting fittingthetheexperiment experiment is is byby means means of a transformation of a transformation of the ofvolume–pressure the volume–pressure data into data aninto f–F plot, an f–Fi.e.,plot, Eulerian i.e., strain (f) versus Eulerian strainnormalized (f) versus normalized stress (F) plot [55]. Such plot gives a direct indication of the compression behavior.points stress (F) plot [55]. Such plot gives a direct indication of the compression behavior. If the data If the lie onpoints a horizontal 0 data lie on a line of constant horizontal line ofF constant then a second-order BM–EOS (B F then a second-order 0 = 4) is(B BM–EOS needed to fit the data. 0' = 4) is needed to fit If the the dataIf points data. the datalie points on an inclined lie on anstraight inclinedline, the data straight line,will thebe adequately data described by will be adequately a third-order described by a BM–EOS. Positive or negative slopes imply B 0 > 4 and B 0 < 4, respectively. On the other hand, third-order BM–EOS. Positive or negative slopes imply B0' > 4 and B0' < 4, respectively. On the other 0 0 the intercept hand, on the on the intercept F axis thecorresponds F axis correspondsto the value to theof value ambient of pressure ambient bulk modulus, pressure B0 . From our bulk modulus, B0. experiments, a positive slope is obtained, as shown in Figure 10; From our experiments, a positive slope is obtained, as shown in Figure 10; i.e. the pressure i.e., the pressure derivative of the bulk modulus derivative of theis bulk larger than 4, which modulus is larger is than indeed consistent 4, which with consistent is indeed the resultswith that were reported the results that in the were previous paragraph. reported in the previous Figureparagraph. 11a,b showFigure a few representatives 11a,b show a few Raman spectra of LiCrO representatives Raman2 collected spectra with of MEW and Ne as pressure transmitting medium, respectively. The LiCrO2 collected with MEW and Ne as pressure transmitting medium, respectively. The two strong two strong Raman active modes that were Raman observed active modesatthatambient were pressure observedcould be followed at ambient pressureup could to the be highest pressure followed up tointhe both cases highest Crystals 2018,9,8,2x FOR PEER REVIEW Crystals2019, 13 of 14 of 18 19 pressure in both cases without any discontinuous changes being observed either in the frequency or without any discontinuous changes being observed either in the frequency or relative intensity in relative intensity in the entire pressure region indicating the stability of the compound in the studied the entire pressure region indicating the stability of the compound in the studied pressure range. pressure range. Both the modes were observed to shift monotonically towards higher frequency Both the modes were observed to shift monotonically towards higher frequency indicating the usual indicating the usual pressure hardening of phonons due to bond distance reduction under pressure. pressure hardening of phonons due to bond distance reduction under pressure. It is important to It is important to note that we have not observed any change in the Raman mode frequencies note that we have not observed any change in the Raman mode frequencies corresponding to Cr-O corresponding to Cr-O stretching and bending, which could suggest a change in the coordination of stretching and bending, which could suggest a change in the coordination of Cr from octahedral site to Cr from octahedral site to tetrahedral site. tetrahedral site. Table3.3.Compressibility Table Compressibilitydata dataof ofABO ABO2 type type family family of of compounds compounds with with ordered ordered rock rock salt salt structure. structure. 2 LDA and GGA correspond to calculations performed within these approximations. Exp. corresponds LDA and GGA correspond to calculations performed within these approximations. Exp. corresponds toexperiments. to experiments. Compounds Compounds a axis a axis c/a c/a V0 V0 B (GPa) B0 (GPa) B0 0 κa3 (10 B0κ' (10− -3 GPa−1) GPa−1 ) κc (10-3 GPa−1) κc (10−3 GPa−1 ) 0 a Exp Exp 2.8155 4.992 2.8155 4.992 96.4896.48 149(2)149(2)4.1(3) 4.1(3) [32] [32]LiCoO LiCoO22 LDA LDA 2.7921 4.849 2.7921 4.849 91.413 91.413 168.5 168.5 4.67 4.67 GGA 2.8559 GGA 2.8559 4.8994.899 98.838 98.838 142.9 142.9 4.51 4.51 [this work] LiCrO2 Exp 2.8941(3) [this work] LiCrO2 Exp 2.8941(3) 4.9725 4.9725 104.38(2) 104.38(2)161(5)161(5)5.2(3) 5.2(3) 1.34 1.34 4.78 4.78 Figure 11.11.Stacked Figure spectraof LiCrO Stacked Raman spectra of LiCrO at representative 2 at a2 few a few representative pressures (a) pressures (a) with with methanol- methanol-ethanol-water mixture (MEW), (b) with Ne. Numbers at the right hand side y-axis are pressure in ethanol-water mixture (MEW), (b) with Ne. Numbers at the right hand side y-axis are pressure in GPa. GPa.Letter Letterrrin infigures figures(a) (a)and and(b) (b)indicates indicatespressure pressurereleased releaseddata. data. The The two two independent independent experiments experiments givegive consistent consistent results, results, as as can can be be seen seen in in Figure Figure 12, 12, which which shows the pressure dependence of two Raman mode frequencies, which can be shows the pressure dependence of two Raman mode frequencies, which can be expressed asexpressed as Eg: ω(P) = 455.9 + 3.5 P − 0.005P2 Eg : ω(P) = 455.9 + 3.5 P − 0.005P2 A1g: ω(P) = 588.7 + 4.1P − 0.07P2 A1g : ω(P) = 588.7 + 4.1P − 0.07P2 Where ω is the mode frequency in cm−1 and P is the pressure in GPa. The modes have been fitted by Lorentzian where linemode ω is the shapefrequency functionsintocm −1 and obtain thePpeak is theposition. pressure in GPa. The modes have been fitted by Lorentzian line shape functions to obtain the peak position. Crystals 2019, 9, 2 14 of 18 Crystals 2018, 8, x FOR PEER REVIEW 15 of 19 Figure 12. Variation of Raman frequencies shift of LiCrO2 with pressure. Error bars are smaller than the Figure 12. Variation of Raman frequencies shift of LiCrO2 with pressure. Error bars are smaller than symbol size. Circles denote the data with MEW pressure medium; Solid triangles denote the data with the symbol size. Circles denote the data with MEW pressure medium; Solid triangles denote the data Ne pressure medium. Empty triangles denote the data while unloading with Ne pressure medium. with Ne pressure medium. Empty triangles denote the data while unloading with Ne pressure medium. Note that the quadratic term in the above equations is very small. For the given experimental conditions, the integrated intensity remains nearly same from ambient pressure to 15 GPa. However, Note that the quadratic term in the above equations is very small. For the given experimental we do observe the narrowing down of Ag mode with pressures beyond 15 GPa in the hydrostatic data conditions, the integrated intensity remains nearly same from ambient pressure to 15 GPa. However, depicted in Figure 10b. Using this data along with value of zero pressure bulk modulus obtained we do observe the narrowing down of Ag mode with pressures beyond 15 GPa in the hydrostatic from the present X-ray diffraction experiments, zero pressure mode Gruneisen parameter which is data depicted in 10(b). Using this data along with value of zero pressure bulk modulus obtained defined as from the present X-ray diffraction experiments,B0 zero dω pressure mode Gruneisen parameter which is   γ= defined as ω0 dP p=0 B0  dω  is calculated and the values for the Eγg = and A1g mode  ω0  dP  p=0are 1.07(2) and 1.18(2), respectively. Consequently, the influence of pressure on both modes is very similar. The corresponding values for isostructural is calculated compound and2 are LiCoO the 1.15 valuesand for 0.79.the TheEsmaller g and A 1g mode are Gruneissen 1.07(2) of parameter and the 1.18(2), Eg modes respectively. in LiCoO2 as compared with the other compounds remains an incognita. It is worth mentioning values Consequently, the influence of pressure on both modes is very similar. The corresponding here thatfor isostructural compound LiCoO 2 are 1.15 and 0.79. The smaller Gruneissen LiCoO2 was studied using nitrogen as pressure medium, which remains quasi-hydrostatic only up parameter of the Eg modes to 2.4 GPa in [56]. LiCoO 2 as This compared suggests that with the other compounds non-hydrostaticity [41] couldremains an incognita. have affected the results It that is worth were mentioning here that LiCoO 2 was studied using nitrogen as pressure medium, reported for LiCoO2 . Future quasi-hydrostatic experiments would be beneficial to clarify this issue. which remains quasi-hydrostatic On only upcompletely, releasing the pressure to 2.4 GPathe [56]. This suggests original spectrumthat non-hydrostaticity is recovered, corroborating [41]the could have findings affected from XRDthe results results. that The were results present reported for LiCoO highlight . Future quasi-hydrostatic the 2structural experiments stability of the LiCrO would be 2 compound under beneficial to clarify this issue. On releasing the pressure completely, pressure. In earlier measurements on the similar compound LiCoO2 , a significant increase in the the original spectrum is recovered, corroborating the findings from XRD results. The present results highlight integrated intensity of the two modes was reported, which was reversible, ruling out the possibility of the structural stability of pressure the LiCrO induced 2 compoundof reorientations under pressure. the sample. In earlier Those resultsmeasurements suggested theon the similar possible compound changes in the LiCoO 2 , a significant increase in the integrated intensity of the two modes electronic structure of the sample. However, in our present measurements on LiCrO2 , no significantwas reported, which was reversible, changes areruling out the observed possibility in the integrated of pressure intensityinduced of the two reorientations modes ruling of out the sample. Those results the possibility of any suggested the possible changes major electronic changes in the sample. in the electronic structure of the sample. However, in our present measurements on LiCrO To further confirm 2, no significant changes are observed in the integrated intensity of the two the absence of any electronic changes induced by pressure in the sample, we modes ruling carried out theresistance out electrical possibilitymeasurements of any major electronic on a wellchanges compacted in the sample. sample of LiCrO2 up to 10 GPa. To further confirm the The results are shown in Figure 13. absence of any electronic changes induced by pressure in the sample, we carried out electrical resistance measurements on a well compacted sample of LiCrO2 up to 10 GPa. The results are shown in Figure 13. Crystals 2019, 9, 2 15 of 18 Crystals 2018, 8, x FOR PEER REVIEW 16 of 19 13. Pressure Figure 13. Figure Pressurevariation variationof electrical resistance of electrical of LiCrO resistance . The 2data of 2LiCrO . Theindicates absence of data indicates structural absence of or electronic transition in the compound. structural or electronic transition in the compound. The initial The initial fall fallof ofthe theresistance resistanceupup to to 3 GPa is typical 3 GPa of measurements is typical of measurements in the inBridgman press.press. the Bridgman Such a phenomenon Such a phenomenon is usually due todue is usually theto improvement the improvement of theof ohmic contact the ohmic formation contact between formation the leads between the and the sample in the initial stage of compression. On the other hand, the leads and the sample in the initial stage of compression. On the other hand, the behavior of the behavior of the resistance beyond 3 GPa resistance corresponds beyond to the intrinsic 3 GPa corresponds to thesemiconducting behavior behavior intrinsic semiconducting of the sample, of the which sample,shows whicha monotonic shows decrease in a monotonic the resistance, decrease followed by in the resistance, almost aby followed constant almost value beyond a constant 4 GPa. value This kind beyond 4 GPa.of behavior indicates no major pressure-induced changes in the electronic structure This kind of behavior indicates no major pressure-induced changes in the electronic structure of the of the sample [57]. The behavior sample of the [57]. The resistance behavior of thewith pressurewith resistance therefore confirms pressure that there therefore confirmsare no major that therechanges in the are no major electronic structure of LiCrO 2 up to 10 GPa. changes in the electronic structure of LiCrO2 up to 10 GPa. 4. Summary 4. Summary To summarize, the structural, vibrational, and electronic behavior of LiCrO2 under pressure To summarize, the structural, vibrational, and electronic behavior of LiCrO2 under pressure has has been investigated. All three techniques indicate the structural stability of the compound in the been investigated. All three techniques indicate the structural stability of the compound in the pressure range explored. A detailed structural analysis reveals the compression of LiO octahedra, pressure range explored. A detailed structural analysis reveals the compression of LiO66 octahedra, whereas CrO octahedral units expand. However, the overall effect of pressure on the unit cell is the whereas CrO66 octahedral units expand. However, the overall effect of pressure on the unit cell is the volume reduction. The analysis of the structural data shows an anomaly in the c/a ratio, which is volume reduction. The analysis of the structural data shows an anomaly in the c/a ratio, which is also also reflected in the FWHM of the Bragg peaks. The frequency of the two Raman modes of LiCrO reflected in the FWHM of the Bragg peaks. The frequency of the two Raman modes of LiCrO22 increases with pressure. The corresponding pressure coefficients and Grüneisen parameters have been increases with pressure. The corresponding pressure coefficients and Grüneisen parameters have determined. On pressure release both X-ray diffraction and Raman data show the reversibility of the been determined. On pressure release both X-ray diffraction and Raman data show the reversibility sample behavior. The determination of polyhedral and unit-cell compressibilities and the correlation of of the sample behavior. The determination of polyhedral and unit-cell compressibilities and the this information with the study of pressure effects in lattice vibrations is a step forward in extending the correlation of this information with the study of pressure effects in lattice vibrations is a step understanding of the structural and mechanical properties of LiCrO2 . Finally, Raman and resistance forward in extending the understanding of the structural and mechanical properties of LiCrO2. measurements suggest that little changes occur in the electronic structure of LiCrO under pressure. Finally, Raman and resistance measurements suggest that little changes occur 2in the electronic structure of LiCrO2 under Author Contributions: pressure. Conceptualization, sample synthesis and data analysis, A.B.G. and D.E; Raman Measurements, Author R.R, S.K. and Contributions: J.P.-P.; XRD data collection conceptualization, sampleD.E., J.P.-P., D.M.-G., synthesis and dataandanalysis, C.P.; Single crystaland A.B.G. synthesis, D.E; M.B. The manuscript is written through contributions of all authors. All authors have given approval to the final Raman Measurements, R.R, S.K. and J.P.; XRD data collection D.E., J.P., D.M., and C.P.; Single crystal version of the manuscript. The authors declare no competing financial interest. synthesis, M.B. The manuscript is written through contributions of all authors. All authors have Funding: Three of the authors (D. Errandonea, J. Pellicer-Porres, and D. Martinez-Garcia) are thankful for the given approval to the final version of the manuscript. The authors declare no competing financial financial support to this research from the Spanish Ministerio de Ciencia, Innovación y Universidades, the Spanish interest. Research Agency, and the European Fund for Regional Development under Grant No. MAT2016-75586-C4-1-P and by Generalitat Funding: Valencia Three of the through authors the grant Prometeo/2018/123 (D. Errandonea, EFIMAT. J. Pellicer-Porres, and D. Martinez-Garcia) are thankful for the financial Acknowledgments: XRD experiments were performed at MSPD beamlineInnovación support to this research from the Spanish Ministerio de Ciencia, y Universidades, at ALBA Synchrotron with the collaboration Spanish of ALBA Research staff. and the European Fund for Regional Development under Grant No. Agency, MAT2016-75586-C4-1-P and Conflicts of Interest: The by Generalitat authors declare noValencia conflict through the grant Prometeo/2018/123 EFIMAT. of interest. Crystals 2019, 9, 2 16 of 18 References 1. Mackenzie, A.P. 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