Comparison of the local and the average crystal structure of proton conducting lanthanum tungstate and the influence of molybdenum substitution.

We report on the comparison of the local and average structure reported recently for proton conducting lanthanum tungstate, of general formula La28-xW4+xO54+δv2-δ, and the impact of molybdenum-substitution on the crystal structure of the material. Partial replacement of W with 10 and 30 mol% Mo is investigated here, i.e. La27(W1-xMox)5O55.5 for x = 0.1 and 0.3. This study addresses the interpretation and the description of a disordered cation and anion sublattice in this material, which enables the understanding of the fundamental properties related to hydration, transport properties and degradation in lanthanum tungstate. The report shows that Mo-substituted lanthanum tungstate is a promising material as a dense oxide membrane for hydrogen separation at intermediate temperatures.


Introduction
Lanthanum tungstate (LWO) with a general formula corresponding to La 28-x W 4+x O 54+δ v 2-δ is a proton conducting oxide by hydration of intrinsic oxygen vacancies at intermediate temperatures. 1 LWO is a relatively pure proton conductor at low and intermediate temperatures and can therefore be used as electrolyte in proton-conducting solid oxide fuel cells. 2,3,4,5 At higher temperatures, the material exhibits additional p-type and n-type electronic conductivity under oxidizing and reducing conditions, respectively.2 ,3,6 The material can, under reducing conditions, be used as a dense membrane for hydrogen separation due to the ambipolar transport of protons and electrons.2 ,7,8, Hydrogen permeation in LWO is limited by electrons up to ~1000 ºC. More recent materials´ development have shown that partial replacement of W by Mo leads to increased electronic conductivity due to the higher reducibility of molybdenum compared to tungsten, with the advantange of not affecting protonic conductivity significantly.3 ,9,10 Therefore, Mo-LWO exhibits improved ambipolar transport and can be used as for hydrogen separation.
LWO exhibits a fluorite-type crystal structure 11 with the special feature that tungsten dissolves in lanthanum sites to form a stable electrolyte.1 A computational model was obtained from density functional theory (DFT) calculations to describe its real crystal structure for the first time. 1 The model was later verified from a pair distribution function (PDF) study using both time-of-flight neutron and synchrotron data collected at room temperature. 12 Another study has reported an average model based on a combination of neutron and synchrotron data, and a local model based on PDF and EXAFS. 13 The crystal structure of the Mo-substituted material has, to the best of our knowledge, not been reported yet.
The goal of the present study is first to compare the structural model for unsubstituted LWO extracted from DFT calculations reported by Magrasó and co-workers1 ,12 with the average model reported by Scherb et. al. 13 Then, the crystal structure of Mo-substituted LWO La 27 (W 1-x Mo x ) 5 O 55.5 (with x=0.1 and 0.3) will be described based on newly acquired neutron and synchrotron diffraction data. Finally, the electrical properties of the materials will be reported.

Experimental
Nanometric powders of 10% Mo-LWO (Mo 0.1 ; i.e. La 27 W 4.5 Mo 0.5 O 55.5 ) and 30% Mo-LWO (Mo 0.3 , i.e. La 27 W 3.5 Mo 1.5 O 55.5 ) were synthesized by the EDTA combustion method, as described elsewhere.3 The powders were fired at 500-1000 ºC until the organics were removed, and thereafter ground and re-calcined at 1400 ºC for 5 hours in order to obtain single phase with high crystallinity.
Neutron powder diffraction data for Mo 0.1 and Mo 0.3 were collected at D2B beamline of Institut Laue Langevin (Grenoble, France) in high flux mode using λ=1.594 Å, samples were finely ground and enclosed at a cylindrical V sample holder. Synchrotron data were collected at MSPD (BL06, Alba synchrotron, Cerdanyola del Vallès, Spain, 14 ) using multianalyzer detector (MAD). Using the single cut doble Si-111 monochromator, a short wavelength of λ=0.4247 Å was selected in order to reduce the absorbance the tungstate. The fine powder was embedded in a 3 mm in diameter capillary that was rotated during data collection. Data were refined by Rietveld method 15 using FullProf program 16 . For comparison purposes, synchrotron data of the undoped sample (Mo 0 ) were collected at ID31 beamline of European Synchrotron Radiation Facility (Grenoble, France) with λ=0.3999 Å.
Pellets of ~20 mm diameter and ~1 mm thickness were prepared for electrical measurements.
Pt electrodes were painted on each side of the specimens and fired at 1000 ºC for 30 min. The conductivity was measured using an Alpha A impedance spectrometer + POTGAL interface (Novocontrol technologies) in a ProboStat TM measurement cell (NorECs, Norway) by the 2point 4-wire method. Impedance spectra were recorded in the 1 MHz to 0.1 Hz frequency range with an oscillation voltage of 50 mV in wet H 2 . The variation of conductivity with H 2 O was done in oxygen at constant water vapour partial pressure by mixing O2 bubbling through a KBr saturated water solution (pH 2 O~2.5%) or through a P 2 O 5 powder drying stage (pH 2 O~3·10 -5 atm).

Results and discussion
3.1 Disordered average structure (χ 2 =3.62, R B =3.36% and 3.64% for SPXRD and NPD, respectively), c.f. Table 1, is more accurate than the average description using ‫4ܨ‬ ത 3݉ without the disorder (χ 2 =5.95 and R B =5.89% and 5.97% for SXRPD and NPD, respectively). For the refinements, we have fixed the La/W ratio to be 27/5 (=5.4) by substituting one of the 24 La of the 48h position by one W. Oxygen content has been refined and the result of the stoichiometry is 31.4 oxygen ions at 32f position (O2, around La1) and 24.0 at 96k position (around W1), thus rendering an average six-fold coordination for W1. The total refined oxygen occupancy is, therefore, 55.4, while the theoretical content is 55.5 according to the formula La 28-x W 4+x O 54+δ v 2-δ (δ=3x/2) for x=1. Wyckoff positions while lowering their occupancies accordingly. More precisely, in the average structure one of the La is placed at the 24d (0 ¼ ¼) Wyckoff position but in the disordered structure it is moved away from one of the three mirror planes, along the binary axis and occupies Wyckoff position 48h (0 y y) with half occupancy (and y~¼). This leads to two very near "half" lanthanum ions. In a similar way, one of the oxygen anions is moved from the 32f (xxx) to 96k (xxz) by leaving the ternary axis along the mirror plane, giving rise to three oxygen anions with one third of the occupancy in the average structure.

Disorder description vs. modelled local structure
We have tested this model of the "disordered" LWO description against the data used in ref. [1]. The details of the structure found by the joint refinement of SXRPD and NPD data are printed in Table 1 12 . This theoretical structure has been extensively validated by several methods and has been the key to understand the functional properties of LWO, including hydration thermodynamics, electrical properties, and degradation. 17 It is of major relevance that the DFT model fits very well with the experimental local structure determined by PDF. 12 We have compared the disordered average structure found by diffraction with the local structure found by DFT-PDF by Kalland et al. 12 The objective of this comparison is twofold.
On the one hand, we aim to examine the compatibility between these two descriptions. On the other hand, we intend to understand the instant local environments in the disordered structure. Note that due to the fact that DFT can not model partial occupancies, the comparison is made using a 2x1x1 La 27 W 5 O 55.5 v 0.5 cell, i.e. La 54 W 10 O 111 v 1 . For simplicity, we will refer to the structures as "disordered" and "DFT" structures. The procedure has been the following. First, we have generated all the atoms in the "disordered" cell, and compared their positions with those in the "DFT" structure. We have found, for every atom in the later, the closest atom of the same element in the former (except for the two W placed at La positions that were introduced in the DFT structure). The distance between the two atoms ("DFT" structure vs. the closest in the "disordered" structure), can be understood as a measure of the compatibility between the two. As a result, the mean distance is 0.13 Å for the cations, and 0.19 Å for all the anions. The reason for a larger disagreement of the anion sublattice is due to the oxygens that surround the "special" W that sits on La2 sites, which necessarily leads to shorter cation-oxygen distances for W-O than for La-O. This local arrangement (1 out of 24 positions) is poorly described in the "disordered" average structure. When excluding these oxygen ions, the average difference between the anions of the two structures reduces to 0.16 Å. The list of ions is given as supplementary material. In addition to the substitution of two W atoms in La2 sublattice and one oxygen vacancy in the local structure, one must take into account that DFT calculations are made at zero kelvin temperature, while diffraction data are collected at RT. All these considerations make us to conclude that the disordered structure obtained by diffraction is fully compatible with the local structure found by DFT.
The overall comparison renders that the apparent eight-fold coordination of W1 (with partial anion occupancy) originally found with the ‫ܨ‬ ത 43݉ SG indeed represents disordered octahedra in the local structure, as depicted in Figure 1 (passing through the disordered structure). This local arrangement has been described using ܲܽ ത 3 SG by Scherb et al. 13 . Another interesting result of the comparison between the two structures is the possible role of W2 in the disordered structure. According to DFT calculations, these anions have a 6-fold coordination instead of the 7-fold of La2. This breaks the correlation depicted in Figure 2 between W1O6 octahedra that are connected through a W2 ion.

Disorder description of Mo-doped system
We have used the "disordered" structural description to obtain the joint refinement for Mo- Examining the local structure of both compounds, in a similar fashion as for the undoped case (bond distances are reported in the tables), we find that the estimated valence of O1 is -1.90 (for both Mo-10% and Mo-30%). This is below the expected value of -2, and very similar to the value found for the undoped case.

Electrical conductivity at intermediate temperatures
This local description also allows the detailed study of the environment of oxygen anions. O2 are all connected to one La1 and to three La2/W2 positions (bond distances are printed in Table 1). According to bond valence sum (BVS) calculations, their charge would be -2.003 (-2 within the errors). Besides, O1 is bonded to W1 and also to three La2/W2 but BVS calculation render a charge of -1.89, slightly below -2. This result is for the average situation.
In the local disordered structure one must expect that when the true local bond is towards a W ion, the charge of the O would be much lower than this. This could indicate that these oxygen ions are more susceptible to accept H atoms.  Figure 7, which clearly shows that the ambipolar conductivity increases significantly with molybdenum concentration. Mo-substituted LWO is, therefore, proposed as the next generation materials for improved H 2 separation in dense ceramic membranes.
Several research groups are testing hydrogen separation mostly in bulk form 10,22 , and more The variation of the conductivity with water vapour partial pressure in O 2 (at 500ºC) and the variation with temperature in wet H 2 for LWO54 and the Mo-doped compositions are shown in Figures 5 and 6. These materials are mixed ionic-electronic conductors depending on the temperature and atmosphere3 , 9 . Under oxidizing conditions, the conductivity of the materials increases largely with increasing water partial pressure. This is in accordance with the formation of conducting protons following the exothermic nature of the hydration reaction: advanced architectures such as hollow fibers 23 or thin films 24,25 with modified surface for increased surface kinetics are expected to come in the near future.

Conclusions
In summary, we have investigated the structural and transport properties of Mo-substituted  [13] reproduces diffraction data (neutron and synchrotron x-ray powder diffraction) very satisfactorily. In addition, we have examined the compatibility of this disordered model with the local structure found by DFT calculations and found a quite good agreement. From this comparison, we conclude that W is, both dynamically and locally, in an octahedral environment, and that these octahedra are oriented in alternating directions. We also confirm that the structure can be locally described in the ܲܽ3 ത space group and that the BVS calculations in this local environment indicate that O1, surrounding the positions shared by La and W, present an average valence of around -1.9. This value is expected to be significantly lower (in absolute value) for those oxygen anions surrounding W ions. We interpret this as an indication that proton transport may take place through these oxygen ions (those bonded to a W ion).
Rietveld refinements of neutron and synchrotron powder diffraction data of Mo-substituted compounds also confirm the disordered structure is preserved for up to 30 % Mo substitution.
In these cases, the valence of O1 positions does not change with Mo substitution and, accordingly, the proton conductivity does not vary along the studied series.
In accordance with the conducting protons formation thanks to the exothermic hydration reaction, the conductivity of doped and undoped LWO largely increases with water partial pressure under oxidizing conditions. The high ambipolar conductivity and high stability towards acidic gases, makes Mo-substituted LWO a potential material for substituting current unstable state-of-the-art materials for dense ceramic hydrogen permeation membranes.