High-temperature electrical resistivity and loss tangent of langasite-family Ca₃Nb(Ga,Al)₃Si₂O₁₄ single crystals

Langasite-family Ca₃Nb(Ga_{1- 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.