Conductivity and crystallography of new alkali rare-earth silicates synthesized as possible fast-ion conductors

Eight potassium neodymium silicates have been synthesized as possible fast-ion conductors. Hydrothermal growth experiments in the high-silica region of the SiO₂·H₂O·Nd₂O₃·K₂O system yielded crystals of the following phases (five of which are previously unreported) :K₃NdSi₆O₁₅, K₈Nd₃Si₁₂O₃₂OH, K₁₀Nd₄Si₁₄O₃₉, K₄Nd₂Si₈O₂₁, K₃NdSi₁₈O_{1 9}, K₁₂Nd₂Si₁₈O₄₅, K₅Nd₃Si₂₀O₄₇, and KNd₉(SiO₄)₆O₂. The compositions and crystallographic data were determined using electron microprobe measurements and precession X-ray photographs, respectively. Single-crystal intensity data for phases K₃NdSi₆O₁₅ and K₈Nd₃Si₁₂O₃₂OH were obtained with a four-circle diffractometer. Of these eight phases, six have been obtained as crystals large enough to permit conductivity measurements along at least one crystallographic axis. Conductivities were measured from 300 to 900 °C in an air atmosphere using either complex impedance techniques or single-frequency ac methods. Our results show that activation energies range from 0.3 to 2.2 eV, that the conduction process does not strictly follow the Arrhenius equation, and that these silicates are significantly anisotropic with respect to conductivity. In the K₃NdSi₆O₁₅ phase this anisotropy corresponds well to expectations based on structural considerations.