La10W2O21: An Anion-Deficient Fluorite-Related Superstructure with Oxide Ion Conduction

The crystal structure of La10W2O21, which has to be reformulated (La5.667W0.333)­LaWO14□2, is best described, on average, by a 2 × 2 × 2 anion-deficient fluorite-related superstructure cubic cell, with space group F4̅ 3m, Z = 4, and a = 11.17932(6) Å, similar to Y7ReO14–‑δ. The 32 cations are distributed with lanthanum on the 4a-site, tungsten on the 4b-site, and a partial occupancy of the 24g-site by La (94%) and W. The 56 oxygen atoms occupy four 16e-sites, three of them fully and with an occupancy of 1/2 for the fourth one. Others M10W2O21 (M = Er, Y) adopt a 3 × 2 × 2 fluorite superstructure with W in octahedral sites, whereas W is mainly in tetrahedral sites in La10W2O21. Several powerful techniques such as crystal image furnace synthesis, 139La nuclear magnetic resonance (NMR) and convergent beam electron diffraction (CBED) were used to achieve our results. Transmission electron microscopy (microdiffraction, CBED, and Tanaka patterns) brought us the real symmetry, showing that indeed classical cubic twinning along the 3-fold axis does take place. The surprising La/W mixed site is nicely confirmed by 139La NMR. This compound exhibits interesting fast oxide ion conducting properties, comparable with LAMOX (Lacorre et al. Nature 2000, 404, 856–858) at low temperature. As opposed to many ionic conductors, no temperature structural transition is observed. Its conductivity is about 6.4 × 10–4 S·cm–1 at 700 °C.La₁₀W₂O₂₁的晶体结构可重写为(La₅.₆₆₇W₀.₃₃₃)­LaWO₁₄□₂，其平均结构可通过2×2×2型阴离子缺陷萤石相关立方超结构晶胞最佳描述，空间群为F4̅3m，晶胞内化学式单元数Z=4，晶胞参数a=11.17932(6) Å，与Y₇ReO₁₄₋δ相似。该晶胞内共有32个阳离子位点：镧占据4a位点，钨占据4b位点，24g位点存在La（占有率94%）与W的混合部分占据。56个氧原子占据4个16e位点，其中3个位点完全占据，第4个位点占有率为1/2。 其他通式为M₁₀W₂O₂₁的化合物（M=Er、Y）采用3×2×2型萤石超结构，其中钨位于八面体配位位点；而在La₁₀W₂O₂₁中，钨主要处于四面体配位位点。本研究采用了多种先进合成与表征手段：晶体影像炉合成法、¹³⁹La核磁共振（Nuclear Magnetic Resonance, NMR）、会聚束电子衍射（Convergent Beam Electron Diffraction, CBED）。透射电子显微镜（显微衍射、CBED及田中花样）表征揭示了其真实对称性，证实确实存在沿3次轴的经典立方孪晶现象。令人意外的La/W混合配位位点通过¹³⁹La核磁共振得到了很好的验证。 该化合物展现出优异的氧化物离子快速传导性能，低温下的导电性能可与LAMOX（Lacorre等，《Nature》，2000年，第404卷，第856–858页）相媲美。与多数离子导体不同，该化合物未观察到温度诱导的结构相变。其在700℃时的电导率约为6.4×10⁻⁴ S·cm⁻¹。