Growth and high-temperature characterization of langasite-family Ca3NbGa3−xAlxSi2O14single crystals

Langasite-family single crystals Ca 3 NbGa 3− x Al x Si 2 O 14 (CNGAS) ( x = 0.1–0.8) were grown by a Czochralski technique along the x -axis. With increasing amount of Al substituent, the lattice constants decreased and the electric resistivity in a high temperature range was improved. The CNGAS ( x = 0.8) specimen had an electric resistivity of 10 10 Ω cm at 350 °C. From the dependence of the resistivity on temperature, an activation energy of about 1.19 eV was obtained. Thermal treatment in oxygen atmosphere was also considered effective for the improvement in electric resistivity. However, the dielectric features associated with loss tangent jump at about 400 °C were not improved by the Al substituent and thermal treatment. Spectroscopic ellipsometry was applied to determine the pseudodielectric function ε = ε 1 + i ε 2 spectrum of the Ca 3 NbGa 3 Si 2 O 14 crystal from 2 to 6 eV at 27 to 450 °C. No crystal structure change was observed from room temperature to 450 °C. The mechanism of the loss tangent change above 400 °C needs further investigation.