ChemComm This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online COMMUNICATION View Journal | View Issue Topochemical nitridation of Sr2FeMoO6† Roberta Ceravola, a Carlos Frontera,a Judith Oró-Solé, a Ashley P. Black, a Open Access Article. Published on 13 February 2019. Downloaded on 7/15/2021 2:48:18 PM. Cite this: Chem. Commun., 2019, 55, 3105 Clemens Ritter,b Ignasi Mata, ac Elies Molins, a Josep Fontcuberta *a and Received 12th December 2018, Amparo Fuertes *a Accepted 13th February 2019 DOI: 10.1039/c8cc09845j rsc.li/chemcomm The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 lower temperatures under NH3 between oxides or carbonates in NH3 at moderate temperatures leads to cubic, Fm3m % double lead to disordered compounds. For instance LaMg1/3Ta2/3O2N12,13 or perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange Sr2FeMoO6xNx14 when prepared from mixtures of binary or ternary interactions determine ferromagnetic order with TC E 100 K. Sub- reactants under NH3 at moderate temperatures shows a high degree stitution of oxide by nitride induces bond asymmetries and local of cation disorder. We have recently demonstrated that ammono- electronically driven structural distortions, which combined with lysis of cation ordered double perovskite oxides at low temperatures Fermi level lowering restricts charge itinerancy to confined regions is a useful synthetic approach of the corresponding oxynitrides and preclude spontaneous long-range magnetic order. Under a as it minimizes the mobility of the cations keeping the order of magnetic field, ferromagnetic correlations expand, favoring charge the precursor oxide.15 delocalization and a negative magnetoresistance is observed. The double perovskite Sr2FeMoO6 (SFMO) is a singular material because it shows metallic conductivity, ferromagnetism (FM) The introduction of nitride in oxidic compounds induces changes above room temperature and a large negative magnetoresistance. in the covalency of bonds, oxidation states of metals and energies These properties arise basically from the existence of a Fe-3d6d– of electronic levels; hence being a useful tool for the design of new Mo-(4d,5s)d mixed electronic configuration that gives rise to a materials.1–4 Using this strategy transition metal perovskite broad, partially occupied conduction band which is fully spin oxynitrides5 have been investigated in the last few years resulting polarized, as illustrated in Fig. 1a and b.16,17 At room temperature in notable applications such as inorganic pigments,6 visible light active photocatalysts,7 and dielectric8 and magnetic materials.9 Double perovskite oxynitrides A2B 0 B00 O6xNx (B = transition metal) are scarcely investigated although they potentially offer the possibilities of finding new properties by combining two different metals in the B sites. Cation order in double perovskites is a determining factor for physical properties and strongly affects the electronic and magnetic interactions between the two transition metals.10 The synthesis of cation ordered double perovskite oxy- nitrides is not straightforward because high temperatures are needed to promote cation ordering, and under these conditions the oxynitrides may decompose into the more stable ternary or quaternary oxides.11 On the other hand, solid state reactions at Fig. 1 (a) Schematic band filling of Sr2FeMoO6. The Fermi level is nearby the a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, bottom of Fe-3dt2gk–Mo-(4d,5s)k bands. (b) In the oxide FeO6 and MoO6 08193 Bellaterra, Spain. E-mail: fontcuberta@icmab.cat, octahedra are regular (top sketch) and the free carriers in the conduction band amparo.fuertes@icmab.es; Fax: +34 935805727; Tel: +34 935801853 (bottom sketch) promote long range ferromagnetic order. (c) Schematic band b Institut Laue-Langevin, 71 Avenue de Martyrs, Grenoble 38000, France filling of Sr2FeMoO4.9N1.1. The oxidation of cations induced by N reduces the c Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, total number of electrons, and the filling of the bands. In the nitrided sample, Spain the Fermi level shifts down to the top of the Fe-3dm band. (d) Localized states † Electronic supplementary information (ESI) available: Experimental details, formed around defect-related potential wells (e.g.: nitride sites, Jahn–Teller and Rietveld refinement information of Sr2FeMoO4.9N1.1 and Sr2FeMoO6. See Fe4+ ions) that trap electrons, hindering carrier mobility, weakening double DOI: 10.1039/c8cc09845j exchange ferromagnetic interactions and cancelling long range ordering. This journal is © The Royal Society of Chemistry 2019 Chem. Commun., 2019, 55, 3105--3108 | 3105 View Article Online Communication ChemComm SFMO crystallizes in the I4/m space group in a superstructure of the perovskite cubic subcell (of parameter ap) with unit cell dimensions O2ap  O2ap  2ap. Its outstanding physical properties are directly correlated with the mixed electronic states of Fe(2+d)+ and This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Mo(6d)+ and the degree of order of the two metals in the 2a(0,0,0) and 2b(0,0,12) sites respectively.18 Antisite occupation of Fe/Mo ions typically leads to a reduction of magnetization and of the strength of the double exchange interaction between the magnetic ions.19,20 On the other hand, it is well known that electron filling of the conducting band in SFMO (mainly of Fe-3d-t2gk–Mo-(4d,5s)k) Open Access Article. Published on 13 February 2019. Downloaded on 7/15/2021 2:48:18 PM. (Fig. 1a and b) can be modified by appropriate aliovalent doping at A-sites, largely modulating its Curie temperature.21,22 In the present work we explore the topotactic introduction of nitride in SFMO. The ultimate goal being the determination of the changes of physical properties in strontium iron molybdenum double perovskite when such substitutions are made at the anionic sublattice, while preserving the chemical ordering of the cationic sublattices. We stress that this approach should, in principle, allow modifying the band filling of Fe–Mo orbitals due to N-promoted cation oxidation, at the cost of perturbing, however, the metal– anion bond network. It will be shown that the obtained oxynitride (Sr2FeMoO4.9N1.1) is ferromagnetic with a relatively high Curie Fig. 2 Rietveld fits to (a) synchrotron X-ray powder diffraction pattern temperature (TC E 100 K) which we associate with the presence (l = 0.41269 Å) and (b) neutron powder diffraction pattern (l = 1.86502 Å) of a ferromagnetic exchange-like magnetic coupling. However, it is of Sr2FeMoO4.9N1.1 at 90 K. In (a) upper and lower ticks mark the reflections found that the presence of N3 and Fe4+ species largely suppresses of Fm3m% Sr2FeMoO4.9N1.1 and I4/m SFMO impurity, respectively. In (b) the long-range carrier mobility and limits the extent of spontaneous two upper ticks mark the nuclear and magnetic reflections of SFMO and the lower tick represents the nuclear reflections of Sr2FeMoO4.9N1.1. The magnetic ordering, which can be however recovered by application insets in (a) show selected regions to illustrate the symmetry change in the of a magnetic field thus giving rise to a negative magnetoresistance. oxynitride with respect to the oxide. The small shoulder at the high 2y side The samples have been characterized by chemical analysis, of the 642 reflection corresponds to SFMO (refined fraction: 10.2(2)%). laboratory and synchrotron powder X-ray diffraction, neutron powder diffraction, electron diffraction, Mössbauer spectroscopy, electrical resistivity and magnetisation measurements. Experi- to cell parameters a = 5.55568(1) and c = 7.90604(2) Å and mental details are provided in the ESI.† SFMO was prepared by antisite disorder of 11.2(1)% (see Fig. S1 and Table S1, ESI†). treatment in Ar/H2 (99%/1% v : v) at 1100 1C of a 1 : 1 mixture of Neutron powder diffraction data at the same temperature SrFeO3x and SrMoO4. The oxynitride was stabilized by treatment showed reflections indicating ferromagnetic order (see Fig. S2 of 150 mg of SFMO under NH3 in a narrow range of temperatures and Table S2, ESI†). The saturation magnetization of this and gas flow rates. Ammonolysis during 6 hours at 525–550 1C and sample at 2 K was 3.3 mB per f.u. (see Fig. S3, ESI†) which is 1000 cm3 min1 leads to samples containing a high proportion, consistent with the observed low antisite disorder. The syn- between 40 and 50%, of unreacted SFMO together with the chrotron X-ray diffraction pattern of Sr2FeMoO4.9N1.1 (Fig. 2a) oxynitride. The coexistence of both phases was revealed from high showed a symmetry increase induced by nitride to cubic Fm3% m resolution synchrotron powder diffraction data. The analysed in a unit cell with a = 2  ap = 7.87774(2) Å. The reflection nitrogen contents and phase fractions refined by the Rietveld conditions obtained from reconstruction of the reciprocal method in several samples were consistent with a constant lattice by electron diffraction were consistent with this space composition of 1.1 nitrogens per formula in the oxynitride phase. group (see Fig. S4, ESI†). The same space group is shown by the No evidence of the existence of a solid solution Sr2FeMoO6xNx paramagnetic phase of SFMO and reflects the loss of tilting with different N contents was observed. By increasing the tem- around the c axis present in I4/m. perature the oxynitride fraction is enhanced, and the best sample Neutron diffraction patterns have been collected for Sr2Fe- prepared at 575 1C using the same flow rate and reaction time MoO4.9N1.1 from 1.5 K to 450 K and for Sr2FeMoO6 at 90 K. contained ca. 10% of SFMO. Reactions performed at temperatures Ferromagnetic ordering of the oxide secondary phase in the above 575 1C or using longer treatment times (i.e. 10 hours) oxynitride sample is reflected in a change in intensity of some produced cation-disordered samples with the same nitrogen stoi- diffraction peaks (e.g. 101, Fig. S5, ESI†) for patterns collected chiometry. Totally disordered oxynitride perovskites were obtained below 400 K. A detailed examination of the diffraction data at by ammonolysis of the mixture of oxides SrFeO3x and SrMoO4 lower temperatures does not reveal any further enhancement of under similar conditions of temperature and flow rates. any other diffraction peak, or the appearance of purely magnetic Rietveld refinement of synchrotron X-ray powder diffraction reflections. In fact, low temperature (1.5 K) neutron data can be data of the SFMO sample at 90 K in the I4/m space group leads satisfactorily refined assuming that the magnetic reflections 3106 | Chem. Commun., 2019, 55, 3105--3108 This journal is © The Royal Society of Chemistry 2019 View Article Online ChemComm Communication Table 1 Atomic coordinates in the cubic Fm3m % space group, cation and anion occupancies and isotropic temperature factors for Sr2FeMoO4.9N1.1 from the refinement to neutron diffraction data at 90 K using l = 1.86502 Åa Atom Wyckoff site x y z B (Å2) Occ. factor This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Sr 8c 0.25 0.25 0.25 0.78(3) 1 Fe1/Mo1 4a 0 0 0 0.55(3) 0.825/0.175 Fe2/Mo2 4b 0.5 0.5 0.5 0.87(3) 0.175/0.825 O1/N1 24e 0.2550(3) 0 0 1.11(2) 0.817/0.183 Bond lengths (Å) Sr–O,N (12) 2.7853(2), Fe–O,N (6) 2.009(2), Mo–O,N (6) 1.930(2). a Fe/Mo occupation factors were fixed to the refined values from synchrotron diffraction data subject to the ideal stoichio- Open Access Article. Published on 13 February 2019. Downloaded on 7/15/2021 2:48:18 PM. metry (see Table S1, ESI). The N occupation factor was fixed to the analysed stoichiometry. Refined cell parameters were a = 7.87720(9) Å. Agreement factors: w2 = 4.72; Rwp = 3.65%; and RBragg = 2.46%. tobs = hA–Xi/O2hB–Xi, where hA–Xi and hB–Xi are the mean bond distances for A and B sites, respectively, increases from 0.997 in Fig. 3 (a) M–H hysteresis loops of the oxynitride sample; (b) inverse of the magnetic susceptibility of Sr2FeMoO4.9N1.1 (left axis, red symbols), and Sr2FeMoO6 to 1.000 in Sr2FeMoO4.9N1.1 stabilizing the untilted magnetization assigned to ferromagnetic SFMO (right axis, blue symbols), Fm3% m structure. The cell volume increases from 243.025(1) Å3 in obtained after the numerical treatment described in the text and detailed SFMO to 244.441(2) Å3 (normalized to V/2) in Sr2FeMoO4.9N1.1 in the ESI.† indicating that the larger ionic radius of nitride outweighs the contraction caused by the oxidation of the cations (see Table 1 and Table S2, ESI†). The presence of Fe ions in an intermediate arise solely from the SFMO phase (ordered moment of Fe E Fe3.1+ valence state can be assessed by Mössbauer spectroscopy. 4.3(6) mB). This allows us to discard the presence of spontaneous Indeed, the Mössbauer spectrum of the oxynitride can be fitted to long-range magnetic order beyond a coherence length of a few the superposition of two contributions, a main component that nanometers. The presence of the ferromagnetic contribution accounts for 84(2)% of the area and a minor component corres- arising from the residual SFMO is clearly visible in the room- ponding to the remaining area (Fig. 4(a)). The isomer shifts (IS), temperature M(H) curve shown in Fig. 3(a). Indeed, the measured relative to a-Fe, are 0.53(1) and 0.06(2) mm s1, respectively, magnetic moment (E0.3 mB per f.u.) is roughly that expected which are in the range of expected values for Fe3+ and Fe4+ ions. from a 10% fraction of SFMO. Interestingly, the low temperature The corresponding quadrupolar splittings (of about 0.56(1) and M(H) magnetization is significantly larger (E1.5 mB per f.u.) than 0.21(2) mm s1, respectively) show a hierarchy of values typical expected from this SFMO fraction. In order to explore the for Fe3+ and Fe4+ ions. In this refinement the small contribution magnetic properties of Sr2FeMoO4.9N1.1 we have measured M(T) (distributed within a large velocity range due to the Zeeman curves at different applied fields (3, 5, 7, 9 and 11 kOe). We splitting) of the residual SFMO is not included. further assume that the total magnetization at every temperature The electrical resistivity of the oxynitride sample was measured can be expressed as a sum of ferromagnetic and paramagnetic- by using a two contact probe configuration on as-prepared pellets. like contributions: M(T,H) = MOX(T) + wON(T)H. This allows As shown in Fig. 4(b), the resistivity displays a semiconductor-like separation of the two contributions to the magnetization: that behaviour. The concentration of the metallic SFMO residual arise from the FM oxide fraction (MOX), and that from the fraction is well below the 3D percolation limit, this indicates that paramagnetic-like oxynitride (wON(T)H) (further details are given Sr2FeMoO4.9N1.1 is insulating. In fact, the resistivity data cannot be in the ESI†). These two contributions (MOX) and the inverse of described by an activated law but by a variable range hopping the susceptibility (wON) are plotted in Fig. 3(b). The inverse (VRH) model, suggesting electron motion within localized states. susceptibility shows a Curie–Weiss behavior with a positive y Still a large negative magnetoresistance is measured below 100 K. value indicating dominant ferromagnetic interactions within At first sight, this observation is in contrast to the observed the oxynitride. Below 100 K, obviously Sr2FeMoO4.9N1.1 is no insulating character of the oxynitride. However, a simple and longer a paramagnetic material but its magnetic response gets coherent picture can be obtained by noticing that: (a) in the gradually saturated upon cooling. The enhancement of MOX pristine material, the conduction band is populated by minority below this temperature is attributed to a field-induced ferromagnetic spin electrons of Fe-3d and Mo-4d,5s parentages, as depicted in contribution of the oxynitride. To rationalize the nature of these Fig. 1a; in a fully ordered SFMO, a periodic arrangement of the magnetic interactions electron counting is compulsory. The regular coordination polyhedra exists and the resulting conduction substitution of O2 by N3 in Sr2FeMoO6 leading to Sr2Fe- band is relatively broad and smooth; the ferromagnetic coupling MoO4.9N1.1 induces the oxidation of Mo(6d)+ to Mo6+ and of between localized magnetic moments of 3d5(Fe3+) core-states is Fe(2+d)+ to Fe(3+x)+ (x E 0.1) as a charge compensation mechanism. mediated by the free electrons from nominal Fe(2+d)+/Mo(6d)+ The oxidation of (B,B0 ) cations and larger Sr–X (X = anion) bond (Fig. 1b); (b) in the oxynitride, N-insertion first oxidizes the metal distance induced by nitride favour the transition from tetragonal network, reduces the amount of available electrons and pushes to cubic symmetry. The observed tolerance factor calculated as down the Fermi level. As illustrated in Fig. 1c, according to the This journal is © The Royal Society of Chemistry 2019 Chem. Commun., 2019, 55, 3105--3108 | 3107 View Article Online Communication ChemComm In conclusion, the double perovskite oxynitride Sr2FeMoO4.9N1.1 with high cation order and cubic Fm3% m symmetry is obtained by topotactic nitridation of Sr2FeMoO6 under NH3 using high flow rate and moderate temperature conditions. This process, although This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. preserving the cationic order of the oxide, produces dramatic changes in the magnetic structure, mainly related to the lowering of the Fermi level associated with the oxidation of the transition metals induced by nitride, and to carrier localization prompted by disorder induced by the presence of different anions and electron-driven local distortions. These results illustrate the Open Access Article. Published on 13 February 2019. Downloaded on 7/15/2021 2:48:18 PM. complex interplay of phenomena triggered by nitridation affecting the electronic properties of perovskites. This work was supported by the Spanish Ministerio de Ciencia, Universidades e Investigación, Spain (Projects MAT2017-86616-R, MAT2017-85232-R, MAT2015-71664-R, MAT2015-67593-P and ENE2015-63969) and by Generalitat de Catalunya (2017SGR1377, 2017SGR581). ICMAB acknowledges financial support from MINECO through the Severo Ochoa Program (SEV-2015-0496). We thank the Institut Laue Langevin (ILL) and Alba synchrotron for the provision of beam time. Fig. 4 (a) Mössbauer spectrum of Sr2FeMoO4.9N1.1. The continuous black Conflicts of interest line is the fit of the spectra using two quadrupolar-split resonant lines as indicated. (b) Magnetoresistance (MR = [r(H)  r(0)]/r(0)) of the oxynitride There are no conflicts to declare. sample at different temperatures; inset: logarithm of the resistance (at zero field) as a function of T1/4 (variable range hopping behaviour). Notes and references electron counting and the Mössbauer results, the Fermi level is 1 A. Fuertes, Mater. Horiz., 2015, 2, 453. placed just under the top of the 3d5xm spin-up sub-band, 2 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, anticipating electron itinerancy and establishing a ferromagnetic 293, 269. 3 V. Bachmann, C. Ronda, O. Oeckler, W. Schnick and A. Meijerink, double exchange coupling between Fe(3+x)+ magnetic moments. Chem. Mater., 2009, 21, 316. This is in agreement with y 4 0 observed from magnetic suscepti- 4 G. Hitoki, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and bility data. Next, it breaks the symmetry of the oxygen-bond K. Domen, Chem. Commun., 2002, 1698. 5 R. Marchand, C. R. Acad. 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Rev. B: Condens. Matter Mater. Phys., 2004, 70, 094405. cool magnetization data (not shown) is also compatible with this 22 D. Rubi, C. Frontera, J. Nogués and J. Fontcuberta, J. Phys.: Condens. scenario. Matter, 2004, 16, 3173. 3108 | Chem. Commun., 2019, 55, 3105--3108 This journal is © The Royal Society of Chemistry 2019