Crystal Structure and Oxide-Ion Conductivity along c‑Axis of Si-Deficient Apatite-Type Lanthanum Silicate

We have prepared the highly c-axis-oriented polycrystalline material of Si-deficient apatite-type lanthanum silicate by isothermal heating of the sandwich-type La2SiO5/La2Si2O7/La2SiO5 diffusion couple at 1873 K for 100 h. The resulting polycrystal of La9.50(Si5.87□0.13)­O26, where □ denotes a vacancy in Si site, was characterized using optical microscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the textured thin-plate electrolyte was obtained. The ionic conductivity (σ) along the c-axis steadily increased from 1.6 × 10–2 S/cm to 1.26 × 10–1 S/cm with increasing temperature from 623 to 1073 K. The Arrhenius plot of σ showed the marked slope change at ca. 800 K; the activation energies of conduction were, above and below 800 K, 0.53 and 0.17 eV, respectively. The crystal structure of La9.50(Si5.87□0.13)­O26 at ambient temperature (space group P63/m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The Si-deficient apatite was formed by the extraction of the SiO2 component from the La2O3-excess apatite according to La9.33+2xSi6O26+3x – 1.5xSiO2 → La9.33+2x(Si6–1.5x□1.5x)­O26 (x ∼ 0.087).我们通过对三明治型La₂SiO₅/La₂Si₂O₇/La₂SiO₅扩散偶在1873 K下等温加热100 h，制备出了高度c轴取向的缺硅磷灰石型硅酸镧（apatite-type lanthanum silicate）多晶材料。所得的La₉.₅₀(Si₅.₈₇□₀.₁₃)O₂₆多晶（其中□代表硅位空位）通过光学显微镜、X射线衍射法以及阻抗谱进行了表征。对退火后的扩散偶进行机械加工，得到了织构化薄板电解质。随着温度从623 K升高至1073 K，沿c轴方向的离子电导率（σ）从1.6×10⁻² S/cm稳步提升至1.26×10⁻¹ S/cm。σ的阿伦尼乌斯曲线在约800 K处出现显著的斜率变化；800 K上下的导电活化能分别为0.53 eV和0.17 eV。室温下La₉.₅₀(Si₅.₈₇□₀.₁₃)O₂₆的晶体结构（空间群P6₃/m）显示，与硅原子成键的氧原子（12i位）存在明显的位置无序，同时镧原子（4f和6h位）存在非简谐位移。根据反应式La₉.₃₃+2ₓSi₆O₂₆+3ₓ – 1.5ₓSiO₂ → La₉.₃₃+2ₓ(Si₆–1.5ₓ□1.5ₓ)O₂₆（x≈0.087），缺硅磷灰石相由富氧化镧磷灰石脱除SiO₂组分生成。