ARTICLE pubs.acs.org/crystal
Synthesis and Structure Determination of the High Temperature Form of La2WO6 Mathieu Allix,*,†,‡ Marie-Helene Chambrier,§ Emmanuel Veron,†,‡ Florence Porcher,^ Matthew Suchomel,# and Franc-ois Goutenoire§ †
CNRS, UPR3079 CEMHTI, 1D avenue de la Recherche Scientifique, 45071 Orleans cedex2, France Universite d'Orleans, Avenue du Parc Floral, BP 6749, 45067 Orleans cedex 2, France § Laboratoire des Oxydes et Fluorures, UMR-CNRS 6010, Universite du Maine, 72085 Le Mans Cedex 9, France ^ CEA Saclay, Laboratoire Leon Brillouin, F-91191 Gif Sur Yvette, France # Argonne National Laboratory, Advanced Photon Source, Argonne, Illinois 60439, United States ‡
bS Supporting Information ABSTRACT: This article presents the synthesis, structure determination, and structure analysis of α-La2WO6. This high temperature polymorph was directly observed using laboratory in situ hightemperature X-ray powder diffraction and isolated at room temperature by rapid quenching from 1600 C. Ab initio structure determination has been performed at room temperature by combining electron diffraction results with an analysis of synchrotron and neutron powder diffraction data by charge-flipping algorithm methods. The α-La2WO6 phase is found to crystallize in the Pm21n (No. 31) orthorhombic space group (Z = 6) with cell parameters: a = 16.5513(1) Å, b = 5.52003(3) Å, c = 8.88326(3) Å and a measured density of 6.82(1) g 3 cm3 at room temperature. This previously uncharacterized high temperature α-La2WO6 form (>1450 C) may be described as a regular paving between six [WO6] octahedra alternating with 12 isolated lanthanum atoms. The conductivity properties have been measured and compared to the low temperature (β) polymorph.
’ INTRODUCTION Oxides based on Ln2O3-MO3 (M = Mo and W) chemistries are of significant technological interest for their laser applications,1 ionic conduction,2 catalytic3 and ferroelectric4 properties.5 In particular, Ln2MO6 (M = Mo, W) oxides have been the subject of many crystallographic studies.6 These compounds typically present structural motifs based on the fluorite (CaF2) or Scheelite (CaWO4) structure type. Depending on the lanthanide (Ln) cation radius, three different space groups are commonly observed. Large radius cations (Ln = La to Sm) usually correspond to a I42 m (No. 121) space group crystallization, smaller radius values result in C2/c symmetry, and the cerium-based compound exhibits a Fm3m space group.4 The resulting crystal structure also appears to depend on the exact route of synthetic preparation.7 However, many reported compositions in this family have not been appropriately characterized. This is particularly true for the La2O3WO3 system. For instance, a report describing the existence of a solid solution around the La2WO6 composition has recently been refuted. Although initially reported by Yoshimura8 more three decades ago, the existing literature on La2WO6 primarily addresses only the low temperature β-polymorph.5 To further elucidate this important system, in the present study we report on the synthesis, structure determination, and structure analysis of the previously poorly characterized high temperature α-La2WO6 form.8,9 r 2011 American Chemical Society
’ EXPERIMENTAL SECTION Laboratory X-ray diffraction (XRD) measurements were performed on a D8 Advance Bruker BraggBrentano diffractometer (Cu Kα radiation) equipped with a Vantec-1 linear detector. In-situ high-temperature diffraction data were collected using a HTK16 Anton Paar chamber. The sample was deposited on a platinum ribbon heating stage. The temperature behavior of this ribbon was previously calibrated using the known phase transitions and thermal expansion of a corundum reference.10 High-intensity and high-resolution synchrotron powder diffraction data were recorded on the 11-BM diffractometer at the Advanced Photon Source (APS), Argonne National Laboratory. Data were collected on a spinning sample (60 Hz) over the 0.565 2θ range with a 0.001 step size at room temperature using a wavelength of λ = 0.412213 Å. To optimize X-ray absorption, a sample with μR ≈ 1.0 was prepared using amorphous silicon grease to coat only the outside of a 0.8 mm diameter polyamide tube with powder. Neutron powder diffraction data were measured on the 3T2 diffractometer (Laboratoire Leon Brillouin, France) for 22 h on approximately 7 g of sample. Data were collected over the 4.5121 (2θ range) with a 0.05 step size at room temperature. Received: August 4, 2011 Revised: September 20, 2011 Published: September 25, 2011 5105
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Figure 1. Differential scanning calorimetry analysis of the β-La2WO6 showing a strong endothermic peak while heating corresponding to the β f α phase transition. During cooling, the corresponding exothermic peak (α f β) is observed with a shift of several degrees. A Setaram MULTI HTC 1600 DSC instrument was used to determine the phase transition temperature. The DSC measurements were carried out on a 50 mg powder sample, using argon as a purging gas and a platinum crucible, with a heating rate of 10 K/min. The density measurement was conducted on an ACCUPIC 1330 (Micromeritics) gas picnometer running under helium. The sample of 3.61 g was measured at a temperature of 30 C ( 1 C. Transmission electron microscopy (TEM) was performed on a Philips CM20 fitted out with an Oxford EDS analyzer. The sample was first crushed in ethanol, and a drop of the solution with the small crystallites in suspension was deposited onto a carbon coated copper grid. The ionic conductivity measurements were performed by complex impedance spectroscopy in the 10 MHz to 1 Hz frequency range with an applied voltage of 100 mV using a Solartron SI 1260 impedance gain phase analyzer with the Smart impedance software11 for data acquisition. The data were acquired on pellets of 5 mm of diameter and 2.5 mm of thickness, with platinum electrodes deposited under a vacuum on both faces. Impedance data were obtained in the 480820 C temperature range while heating and cooling under a nitrogen atmosphere. The sample was allowed to equilibrate for 30 min at each temperature prior to data acquisition in order to ensure thermodynamic equilibrium. The total conductivity was obtained from the resistance value provided from the intersection between the half circle and the Z0 axis on the Nyquist plot.
’ RESULTS Synthesis of α-La2WO6. The first step of α-La2WO6 synthesis was to prepare the well-known low temperature β form using La2O3 and WO3 precursors. The La2O3 powder was dried and decarbonated at 1000 C overnight prior to use. The starting oxides were weighted in stoichiometric proportions and ground together in an agate mortar. The prepared composition was then heated at 1400 C for 12 h in a platinum crucible. The samples were cooled in the furnace from the calcination temperature to ambient in ∼1 h. The X-ray diffraction pattern of this sample was a good match to that reported by Chambrier5 (ICSD reference 246256). A good Rietveld refinement fit of the low temperature β-La2WO6 model5 to this XRD data and the absence of any observed impurity phases confirmed the success of this first synthesis step.
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Figure 2. In-situ X-ray diffractograms recorded versus temperature starting from the β-La2WO6. The α polymorph appears at 1490 C and the β phase remains up to 1540 C.
Differential scanning calorimetry (DSC) measurements clearly show the existence of thermal peaks upon heating and cooling (Figure 1). During the heating treatment, an endothermic peak corresponding to the α f β phase transition is observed at Tonset = 1492 C and upon cooling the corresponding exothermic peak from the reverse phase transition appears at Tonset = 1472 C. The presence of the hysteresis indicates a first-order phase transition. This phase transition had been previously observed by different authors: Yoshimura8 (1440 ( 30 C) and Yanovskii12 (around 1400 C), though these authors did not report precise values. On the contrary, no phase transition was mentioned for La2WO6 in the phase diagrams proposed by Casteels,13 Rode,14 and Ivanova.15 Following these thermal measurements, in situ high-temperature X-ray powder diffraction was performed within the 2227 2θ range on heating with 10 C incremental steps up to 1600 C (Figure 2). The first diffraction peaks of the alpha polymorph appeared at 1490 C, in good agreement with the DSC measurements. Both forms then coexist up to 1540 C. At this temperature, the reflections attributed to the β-La2WO6 completely disappear, indicating that the phase transition was complete. In order to enhance the statistics of the measurement and to track the stability of the high-temperature polymorph, 10 scans from 8 to 120 (2θ) were collected, with a total counting time of 2 h. As no degradation of the measurement was observed during this period, the 10 scans were then simply summed. Nevertheless, the background remained noisy over the low 2θ range, which could prevent the detection of small diffraction peaks. The indexation of the summed high temperature X-ray diffraction pattern obtained in situ at 1545 C was undertaken using the TREOR16 and DICVOL17 autoindexing software packages. Platinum peaks were excluded and the first 20 reflections were considered. A solution with high figures of merit could then be obtained by TREOR (M(20) = 124, F(20) = 118(0.005, 35)18 and confirmed by DICVOL (M(20) = 124, F(20) = 116). The symmetry is tetragonal with the following cell parameters: a = 5.5572(2) Å and c = 9.166(1) Å. Yoshimura et al.8 mentioned that a high-temperature La2WO6 phase could be observed, though not as a single phase, by rapid quenching. For this phase, they reported a similar cell (tetragonal symmetry with a = 5.512(2) Å and c = 8.880(2) Å), in good agreement with our results. The difference between the different cell parameters can 5106
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Figure 3. Comparison between the X-ray diffractograms of α-La2WO6: (a) after rapid quenching and (b) in situ at 1545 C. The strong difference observed at 2θ = 20 (002 reflection) is due to preferred orientation along the 00L direction. Inset: zoom of both measurements in the 540 (2θ) range.
be explained by thermal expansion between the quenched sample and the in situ measurement. An ICSD database search using similar cell parameters and symmetry found only one match in the Lu0.8Nd1.2WO6 structure (ICSD 63409 - PDF 01-078-2199).19 This compound crystallizes in the P421m (No. 113) space group. Even though a Le Bail fit showed a relative good agreement between our high temperature data and the Lu0.8Nd1.2WO6 structure, the Rietveld refinement conducted by replacing both Nd and Lu atoms by La led to a poor fit with high reliability factors (Rp = 35%, Rwp = 29%, and χ2 = 9.2). Therefore, in order to obtain additional structural information that could permit a more accurate structure determination of the high temperature form, we have attempted to stabilize α-La2WO6 at room temperature by quenching the sample from high temperature. To this end, a small amount of the β-La2WO6 phase (up to 100 mg) was heated to 1600 C for 15 min in a large platinum crucible permitting only a thin layer of sample covering the bottom of the crucible. The crucible was then rapidly quenched in water. XRD of the obtained sample showed a pure α-La2WO6 phase (Figure 3). The key parameter to ensure the α form stabilization at room temperature, which could not be achieved in previous studies,8 seems to be the combination between the use of a very small amount of sample and a crucible with a large heat exchange area. In order to synthesize a large amount of sample, especially for neutron powder diffraction measurements, the protocol had to be repeated several times. All the obtained samples were then checked by XRD. As no difference could be observed, the samples were then all mixed together. Cell Indexation. Electron diffraction (ED) analysis has been performed on several crystallites resulting from the quenched sample. The sharp spots and the absence of any diffuse streaks (Figure 4) demonstrate the good crystallization of the sample and rule out the existence of any stacking defects. The reconstruction of the reciprocal space obtained by tilting around the crystallographic axes showed a tripling of the cell along a compared to the results obtained by autoindexing programs on the
Figure 4. Electron diffraction patterns of the quenched α-La2WO6 phase recorded along the [001], [010], and [100] directions. White arrows point to forbidden reflections appearing from double diffraction.
in situ high temperature XRD data. The system of reflections suggests an orthorhombic cell with a ≈ 17 Å (5.6 3), b ≈ 5.6 Å, and c ≈ 9 Å. The electron diffraction patterns of the three characteristic planes ([100], [010], and [001)) are presented in Figure 4. No condition limiting the general hkl reflections was observed during the reciprocal lattice reconstruction, leading to the assignment of a primitive lattice. The only extra condition hk0: h + k = 2n due to the presence of an n glide along c restricts the choice to one of the three following space groups: Pmmn, P21mn, and Pm21n. Guided by the symmetry information deduced from electron diffraction, autoindexing analysis using the Dicvol and Treor routines was performed again on the room temperature quenched α-La2WO6 laboratory XRD data. Both the subcell and the tripled cell could be obtained by careful choice of the reflections. However, as the evidence for the weak superstructure peaks is difficult to resolve on the laboratory XRD data, we have subsequently 5107
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Figure 5. Refined diffraction patterns from synchrotron radiation (bottom) and neutron (top) data from the mixed refinement. Red dots represent the observed data; black lines the calculated ones; Bragg ticks the peak positions and the blue curve shows the difference between the observed and calculated patterns. The inset on the refined synchrotron radiation pattern corresponds to an enlargment of the 3040 (2θ) region.
collected synchrotron data in order to enhance the weak features characterizing the superstructure (Figure 5). This unit cell was confirmed by good reliability factors for the synchrotron powder pattern fitted using the Le Bail method.20 The three orthorhombic space groups found by electron diffraction (Pmmn, P21mn, and Pm21n) have been tested. Le Bail fits provided no evidence for a space group preference; all fit the data equally well. Structure Determination. Structure determination of α-La2WO6 was performed using the Superflip21 program included in the Jana software package.22 Starting with the cell parameters determined above, the charge flipping algorithm quickly found a solution and approximate cationic models for all three possible space groups. Six cationic positions were identified, three of
them on a 2a Wyckoff site and the other three on 4b sites. However, no distinction between La and W species was possible at this stage; additional analysis was required to make this assignment. These three possible cationic models were further examined by Rietveld analysis. The most symmetric Pmmn space group was unable to accommodate cation distortions, resulting in very large thermal factors (especially along b) when using anisotropic factors. A better fit was obtained using the Pm21n model, which permitted distortions with correct reliability factors and acceptable thermal parameters. Once the cations were positioned within the cell, some of the most obvious oxygen positions were then localized by inspection of Fourier difference maps. At this point, the assignation of La and W species to their respective cationic positions became feasible due to the 5108
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differentiation between the LaO and WO bond length values, typically within the ranges 2.42.6 Å and 1.72.0 Å, respectively.23 The agreement factors converged rapidly to more acceptable values, and difference Fourier maps inspections permitted the identification of additional oxygen positions. Only two oxygen positions remained not localized at this stage. In order to complete this model with the missing oxygen positions and to obtain more reliable thermal parameters, we then recorded neutron powder diffraction data. The incomplete structural model determined previously by powder synchrotron data refinement was used as a starting model. The two remaining oxygen positions appeared then clearly using Fourier difference maps applied to the refined neutron powder diffraction data. The structural information obtained from the Rietveld refinement on neutron data is presented in Table 1. Interatomic dis-
tances and bond valence calculation obtained from the neutron refinement are presented in Tables 2 and 3. A combined Rietveld refinement of both synchrotron and neutron diffraction data sets has also been performed. Without a correction for sample absorption, refined thermal displacement parameters were negative due to the high absorption of the La and W elements, especially for the synchrotron measurements. Table 3. Calculated Bond Valences of the α-La2WO6 Structural Model Proposed from Neutron Diffraction Refinements
Table 1. Crystallographic Parameters of α-La2WO6 Obtained from the Rietveld Refinement of Neutron Powder Diffraction Dataa atom
x
position
y
z
atom
coordination
bvc (σ)
atom
coordination
BVC (σ)
W1
6
5.99(9)
O1
4
2.26(4)
W2
6
5.91(8)
O2
4
2.10(4)
La1
8
3.09(4)
O3
4
1.90(4)
La2
8
3.35(4)
O4
3
1.96(5)
La3 La4
9 7
3.23(4) 3.15(4)
O5 O6
4 3
2.18(3) 1.95(4)
O7
2
1.91(5)
O8
6
1.69(2)
O9
3
2.16(4)
O10
4
2.20(3)
Uiso (100)
W1 W2
2a 4b
0.0 0.3321(3)
0.42 0.390(2)
0.2487(7) 0.2309(4)
0.5(1) 0.2(1)
La1
2a
0.0
0.319(2)
0.7885(5)
0.8(1)
La2
2a
0.0
0.911(2)
0.4598(4)
0.7(1)
La3
4b
0.1605(2)
0.415(2)
0.4946(4)
0.2(1)
La4
4b
0.1808(2)
0.943(2)
0.1590(3)
0.3(1)
O1
4b
0.0795(3)
0.180(2)
0.2870(5)
0.5(1)
O2
4b
0.0825(3)
0.663(2)
0.2853(5)
0.7(1)
O3 O4
4b 4b
0.2598(3) 0.0994(3)
0.642(2) 0.634(2)
0.3003(4) 0.7331(5)
0.6(1) 1.2(1)
O5
4b
0.2539(3)
0.178(2)
0.3596(4)
0.3(1)
O6
4b
0.3892(3)
0.551(2)
0.0842(5)
1.0(1)
O7
2a
0.0
0.451(2)
0.0519(6)
1.0(1)
O8
2a
0.0
0.438(2)
0.4952(6)
1.0(1)
O9
4b
0.2705(3)
0.266(2)
0.0709(5)
0.8(1)
O10
4b
0.4041(3)
0.577(2)
0.3754(5)
1.4(1)
a
Cell parameters: a = 16.552(1) Å, b = 5.5198(6) Å, c = 8.8833(6) Å. Space group: Pm21n. Rwp = 8.9%, RBragg = 3.8%, Rexp = 3.1% for 1379 reflections. Measured density = 6.816(2) g 3 cm3, calculated density = 6845 g 3 cm3.
Figure 6. Projection of the O8 antipolyhedron (6-fold coordination). This octahedron is deformed due to the short WO distance compared to the 5 LaO distances.
Table 2. Selected Bond Distances of α-La2WO6 Obtained from the Neutron Refinement distance WO (Å)
distance LaO (Å)
(W1)(O1): 1.90(2) ( 2)
(La1)(O4): 2.43(1) ( 2)
(La3)(O1): 2.62(1)
(W1)(O2): 1.92(2) ( 2)
(La1)(O6): 2.62(1) ( 2)
(La3)(O2): 2.65(1)
(W1)(O7): 1.78(1) (W1)(O8): 2.18(1)
(La1)(O7): 2.45(1) (La1)(O8): 2.68(1)
(La3)(O3): 2.69(1) ( 2) (La3)(O4): 2.64(1)
(W2)(O3): 1.94(2)
(La1)(O10): 2.53(1) ( 2)
(La3)(O5): 2.35(1) ( 2)
(W2)(O4): 1.85(1)
(La2)(O1): 2.51(1) ( 2)
(La3)(O8): 2.662(3)
(W2)(O5): 2.09(1)
(La2)(O2): 2.47(1) ( 2)
(La3)(O10): 2.46(1)
(W2)(O6): 1.83(1)
(La2)(O8): 2.64(2) ( 2)
(La4)(O1): 2.42(1)
(W2)(O9): 1.87(1)
(La2)(O10): 2.35(1) ( 2)
(W2)(O10): 2.03(1)
(La4)(O2): 2.53(1) (La4)(O3): 2.45(1) (La4)(O5): 2.52(1) (La4)(O6): 2.53(1) (La4)(O9): 2.44(1) (La4)(O9): 2.40(1)
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Figure 7. Crystal structures of the β and α-La2WO6 phases. They are compared to the Lu0.8Nd1.2WO6 and CaWO4 structures.
showing up a slightly negative thermal agitation parameter (0.05(1) Å2), which may be explained by the nonstandard synthetic quenching route. The results of this mixed (synchrotron-neutron) Rietveld refinement are presented in Supporting Information and the final corresponding fits are presented in Figure 5.
Figure 8. Temperature dependence of the electrical conductivity for α-La2WO6 (full circle). From 640 C, the plot reports to the α-La2WO6 phase (full star).
In order to minimize this effect, an absorption correction term was added to the refinements. This absorption correction value (μr = 2) has been calculated using the web pyFprime utility with appropriate sample input parameters.24 With this added correction, most of the thermal displacement parameters significantly increased and became positive. Only the W2 position was still
’ DISCUSSION Both W atomic positions in the refined α-La2WO6 model represent a usual distorted octahedral environment. The whole structure can thus be described as six [WO6] octahedra units alternating with 12 isolated lanthanum atoms. Seven to 9 oxygen atoms, forming large polyhedra, surround these lanthanum atoms. Bond valence calculations (BVC)25 were performed directly with the Bond Str program26 implemented in the Fullprof Suite. The parameters used were B = 0.37 Å and Ro = 2.172 and 1.921 Å, respectively, for La3+O2 and W6+O2. The values obtained are all within the range of the theoretical values (10%. The largest differences are observed for La2: 3.35(4), O1: 2.26(4), and O8: 1.69(2). The O8 BVC value is the most suspect in regard to its high coordination number (6). The projection of the O8 atom with its first neighbors presented Figure 6 does not show any anomaly, though the coordination number is rather high and unusual for oxygen. Indeed, the 12 independent oxygen positions of the β-La2WO6 low-temperature polymorph exhibited only 3 and 4 coordination number values. The comparison between the structure projections of the 5110
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Crystal Growth & Design two La2WO6 forms is presented Figure 7. The two original features of the β phase have disappeared at high temperature: the noncoordinated oxygen and the [W2O11]10 entity. The fact that the phase transition is of the first order does not allow a direct comparison between the two polymorphic forms. However, it is obvious that α-La2WO6 is more easily classified in the Scheelite crystallographic type than the β-La2WO6 form. The closest related structure, Lu0.8Nd1.2WO6, crystallizes in the P421m quadratic space group with the following cell parameters: a = 5.276(1) Å and c = 8.658(1) Å.19 The projections of the Scheelite CaWO4 and Lu0.8Nd1.2WO6 structures are presented Figure 7. All of these structures are closely related to the fluorite structure (CaF2). The Scheelite structure presents a quadratic cell (aFluorine aFluorine 2aFluorine) due to mixed cationic plans composed of ordered calcium and tungsten species. The A2BX6 structures could be described as an ordered fluorine structure: 3xCaF2 = A2BX6. The cell parameters are related to the ordering of cations and anions. In these three compounds, there are no cationic or anionic vacancies. In opposition, one can find structures related to the fluorite with cationic vacancies (for example La20Mo3O1227) and anionic vacancies (see CaHf4O9028). The structural differences observed between Lu0.8Nd1.2WO6 and α-La2WO6 are due to a stronger ordering leading to a larger cell for the pure lanthanum compound. Ionic conductivity measurements on the α-La2WO6 phase are shown as a Arrhenius plot in Figure 8 and exhibit a remarkable variation of the conductivity versus temperature. Between 480 and 620 C, a linear evolution of the conductivity is observed. At 640 C, the plot shows a drastic decrease which can be linked to the α f β La2WO6 phase transition. This temperature of the metastability limit of the α-La2WO6 phase has been confirmed by thermodiffraction measurements. Above this temperature transition (640 C), the behavior of the plot corresponds to the data already reported for the β phase.5 One can notice that α-La2WO6 is more conducting than β-La2WO6 within 1 order of magnitude (5.5 104 S/cm at 620 C). The Nyquist plot does not show a Warburg behavior at low frequencies as the other members of the La2O3WO3 phase diagram (La18W10O57,29 La10W2O21,30 La6W2O15,31 and La2W2O930), confirming that lanthanum tungstates members are less conductive than lanthanum molybdates. It should be noted that conductivity values obtained in this work are likely underestimated due to the lack of density in our pellet which cannot be sintered at high temperature (due to α f β phase transition).
’ CONCLUSION The α-La2WO6 compound has been observed in situ at 1545 C and isolated at room temperature by rapid quenching. Its crystal structure has been elucidated ab initio from synchrotron and neutron powder diffraction data. The initial model has been determined using the charge flipping algorithm with cell parameters and space group determined by electron diffraction. The high temperature allotropic α-La2WO6 phase crystallizes within the orthorhombic space group Pm21n (a = 16.5513(1) Å, b = 5.52003(3) Å, c = 8.88326(3) Å). The structure is built from four different lanthanum polyhedra (79 fold coordinated) and two distinct tungsten octahedra. The α-La2WO6 polymorph exhibits conductivity values an order of magnitude higher than the β phase.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Crystallographic parameters obtained from refinement of mixed synchrotron and neutron powder diffraction data (Table SI1). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: (+33) 2 38 25 55 26. Fax: (+33) 2 63 81 03.
’ ACKNOWLEDGMENT The use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. The neutron experiments realized at the Laboratoire Leon Brillouin in Saclay have been supported by Project No. 9719. ’ REFERENCES (1) Kumaran, A. S.; Babu, S. M.; Ganesamoorthy, S.; Bhaumik, I.; Karnal, A. K. Crystal growth and characterization of KY(WO4)(2) and KGd(WO4)(2) for laser applications. J. Cryst. Growth 2006, 292 (2), 368–372. (2) Lacorre, P.; Goutenoire, F.; Bohnke, O.; Retoux, R.; Laligant, Y. Designing fast oxide-ion conductors based on La2Mo2O9. Nature 2000, 404 (6780), 856–858. (3) Alonso, J. A.; Rivillas, F.; Martinez-Lope, M. J.; Pomjakushin, V. Preparation and structural study from neutron diffraction data of R2MoO6 (R = Dy, Ho, Er, Tm, Yb, Y). J. Solid State Chem. 2004, 177 (7), 2470–2476. (4) Brixner, L. H.; Sleight, A. W.; Licis, M. S. Ln2MoO6-type rareearth molybdates - preparation and lattice-parameters. J. Solid State Chem. 1972, 5 (2), 186–190. (5) Chambrier, M. H.; Kodjikian, S.; Ibberson, R. M.; Goutenoire, F. Ab-initio structure determination of beta-La2WO6. J. Solid State Chem. 2009, 182 (2), 209–214. (6) (a) Sillen, L. G.; Lundborg, K. La2MoO6, a lanthanum oxymolybdate with a layer structure. Z. Anorg. Allg. Chem. 1943, 252 (1/2), 2–8. (b) Efremov, V. A.; Tyulin, A. V.; Trunov, V. K. Actual structure of tetragonal Ln2O2MoO4 and factors, determining the distortion of structure-forming coordination polyhedra. Koord. Khim. 1987, 13 (9), 1276–1282. (7) Klevtsov, P. V.; Kharchenko, L. Y.; Klevtsova, R. F. Crystallization and polymorphism of rare-earth oxymolybdates of composition Ln2MoO6. Kristallografiya 1975, 20 (3), 571–578. (8) Yoshimura, M.; Rouanet, A. High-temperature phase relation in system La2O3-WO3. Mater. Res. Bull. 1976, 11 (2), 151–158. (9) Yoshimur., M; Rouanet, A.; Sibieude, F.; Foex, M. High temperature phase of lanthanide tungstates, Ln2O3.WO3 (Ln = La, Ce, Nd, Sm, Dy and Y). C. R. Hebdomadaires Seances L Acad. Sci. Ser. C 1974, 279 (21), 863–865. (10) Taylor, D. Thermal-expansion data: III sesquioxides, M2O3, with the corundum and the A-, B- and C-M2O3 structures. Br. Ceram. Trans. J. 1984, 83 (4), 92–98. (11) Solartron Materials Research and Test Software; Solartron Analytical: Farnborough, Hampshire, United Kingdom, 2004. (12) Yanovskii, V. K.; Voronkova, V. I. Refinement of phaseequilibria in the La2O3-WO3 system close to the composition 1-1. Inorg. Mater. 1983, 19 (3), 375–379. (13) Casteels, F. G.; Brabers, M. J.; Depaus, R. Thermodynamic stability and phase-equilibria in the system La-Th-W-O. Rev. Int. Hautes Temp. Refract. 1979, 16 (4), 424–436. 5111
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