High-temperature electrical resistivity and loss tangent of langasite-family Ca^sub 3^Nb(Ga,Al)^sub 3^Si^sub 2^O^sub 14^ single crystals

Langasite-family Ca3Nb(Ga1− x Al x )3Si2O14 (CNGAS) (x = 0–0.6) single crystals were grown by a Czochralski technique along the x-axis. When the Al substituent was 0.5, electrical resistivity was greatly improved to 1010 Ω cm at 500 °C, which was more than two orders of magnitude higher than that of Ca3NbGa3Si2O14 (CNGS). This value is the highest resistivity among members of the langasite family at present. However, the dielectric features associated with loss tangent jump at about 400 °C were not improved by the Al substituent. The bandgap E g was measured by spectroscopic ellipsometry, and E g was changed by both temperature and amount of Al substituent. Real and imaginary dielectric constants at a frequency from 0.1 to 20,000 Hz were measured from 150 to 600 °C. There was no marked difference between Cole–Cole plots of specimens with x = 0.1 and 0.5. Oxygen vacancies associated with Nb ions were discussed for DC conductivity and AC loss tangent.