Morphology and oxide-ion conductivity of flux grown single crystals of BaO-doped lanthanum silicate oxyapatite

Abstract Well-faceted and chemically homogeneous single crystals of BaO-doped lanthanum silicate oxyapatite were grown by a BaCl2 flux method. They were characterized by optical microscopy, scanning electron microscopy, electron probe microanalysis (EPMA), X-ray diffractometry (XRD), and electrochemical impedance spectroscopy. The developed faces were identified for the two types of single crystals with different habit. The face indices were {100}, {101}, {110}, and {210} for the prismatic crystals, and {100}, {101}, {111}, and {001} for the isometric ones. With prismatic crystals, the general formula was derived from the numbers of cations determined by EPMA and structural data refined by XRD to be (La10−xBax)Σ=10(Si5.5+0.25x□0.5−0.25x)Σ=6O26 (1.78 ≤ x ≤ 1.81), where □ denotes a vacancy in Si site. We determined the oxide-ion conductivity (σ) of the prismatic single crystal of (La8.22Ba1.78)(Si5.94□0.06)O26 (x = 1.78) along the elongation direction, which is parallel to the c-axis. The σ-value steadily increased from 7.69 × 10−3 to 5.30 × 10−2 S cm−1 with increasing temperature from 823 to 1073 K. When compared at the same temperature, the σ-values were >230 times higher than those of the randomly grain-oriented polycrystal with the same chemical composition, the latter of which ranged from 1.74 × 10−5 S cm−1 at 823 K to 2.24 × 10−4 S cm−1 at 1073 K. The remarkable difference in conductivity between the single crystal and randomly grain-oriented polycrystal was found to be principally induced by the marked conduction anisotropy in the crystal structure.