Effects of aluminum substitution on the structure and electric properties of langasite family crystals

The effects of aluminum substitution on the structure and electric properties of langasite (La3Ga5SiO14) family crystals are reported. The maximum Al content x of La3Ga5-xAlxSiO14 (LGAS), La3Nb0.5Ga5.5-xAlxO14 (LNGA), and La3Ta0.5Ga5.5-xAlxO14 (LTGA) crystals grown by the conventional Czochralski technique are x = 0.9, 0.2, and 0.5, respectively. Single-crystal X-ray structural analysis reveals that Al atoms are distributed in all cation sites except for the decahedral one occupied by La, rather than favoring the smallest tetrahedral sites. The electrical properties of the Al-substituted crystals were also compared with those of pure ones. Al substitution was found to affect the piezoelectric constant d14, but not d11. The LGAS and LTGA crystals exhibit higher electric resistivity ρ as a result of Al substitution. Among the Al-substituted and pure crystals, the LTGA crystal featured the lowest temperature dependence of d11 and highest ρ, making it the most suitable of the group for high-temperature piezoelectric applications.