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

Abstract

Eight potassium neodymium silicates have been synthesized as possible fast-ion conductors. Hydrothermal growth experiments in the high-silica region of the SiO_2·H_2O·Nd_2O_3·K_2O system yielded crystals of the following phases (five of which are previously unreported): K_3NdSi_6O_(15), K_3Nd_3Si_(12)O_(32)OH, K_(10)Nd)4Si_(14)O_(39), K_4Nd_2Si_8O_(21), K_3NdSi_8O_(19), K_(12)Nd_2Si_(18)O_(45), K_5Nd_3Si_(20)O_(47), and KNd_9(SiO_4)_6O_2. The compositions and crystallographic data were determined using electron microprobe measurements and precession X-ray photographs, respectively. Single-crystal intensity data for phases K_3NdSi_6O_(15) and K_8Nd_3Si_(12)O_(32)OH 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 K_3NdSi_6O_(15) phase this anisotropy corresponds well to expectations based on structural considerations.

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