Towards an understanding of the anomalous electromechanical behaviour of langasite and related compounds at high temperatures

The elastic properties of La₃Ga₅SiO₁₄ (LGS), La₃Ga_5.5Ta_0.5O₁₄ (LGT), Ca₃Ga₂Ge₄O₁₄ (CGG) and Sr₃Ga₂Ge₄O₁₄ (SGG) have been studied between room temperature and 1300 K using resonant ultrasound spectroscopy. In addition we investigated the dielectric behaviour of LGS and LGT in the range from 300 K to 870 K on samples with different thermal history. All compounds show an exponentially increasing ultrasound attenuation at elevated temperatures. In the case of LGS and LGT the onset is at about 530 K, whereas in CGG and SGG the dissipation increases above 780 K. The gallium germanates exhibit a more pronounced anisotropy of the attenuation effects compared to the lanthanum compounds. In LGS and LGT the ultrasound attenuation is accompanied by a steep increase of the dielectric loss factors. After annealing under high-vacuum at 1000 K for 100 h ε₃₃ drops by about 20% and the temperature dependence of the dielectric loss factors gets considerably smaller. This might indicate a strong influence of oxygen defects on the mobility of certain cations such as Ga³⁺ which are probably responsible for the ultrasound attenuation in LGS and LGT at high temperatures. The anelastic losses in CGG and SGG are likely caused by increasing electrical conductivity.