In recent years, the high-temperature stability of piezoelectric single crystals of the langasite (LGS, La3Ga5SiO14) family has been significantly improved. Consequently, such crystals are gaining great practical importance for sensing applications at high temperatures. For example, bulk acoustic waves can be piezoelectrically excited even above 1300 °C . CTGS (Ca3TaGa3Si2O14) is a relatively new synthesized fully-ordered compound of the langasite family with a melting point of 1370 °C. Previous reports indicate significantly lower conductivity and losses at temperatures above 500 °C compared to many other crystals of the langasite family . In order to evaluate the stability of CTGS in more detail, its electromechanical properties, such as electrical conductivity and losses, have to be studied and analyzed as a function of temperature. Further, understanding of the defect chemistry and transport mechanisms at high temperatures is required to evaluate the chemical stability of CTGS. Finally, correlations of electromechanical losses with defects must be established to develop methods for material improvement.
In current work, the electromechanical losses of CTGS are determined as a function of temperature up to 1000 °C and related to loss mechanisms. Y-cut resonators manufactured from Czochralski-grown CTGS crystals by IKZ, Berlin, and Fomos-Materials, Moscow, are used in the study. Obtained results are compared to those of LGS, manufactured by IKZ. These measurements revealed two loss peaks with maxima at temperatures below 450 °C at 5 MHz, which are related to anelastic point defect relaxations. At sufficiently elevated temperatures, i.e. above 700 °C, the shoulder of another peak is observed that is caused by increased electrical conductivity.
The measurements on the samples mentioned above indicate that at temperatures above 500 °C the losses in CTGS are significantly lower than those of LGS. Further, the electrical conductivity begins to influence the total losses in LGS at temperatures above 500 °C, while in CTGS this contribution is visible only above 700 °C. Finally, the analysis of transport kinetics, performed in the temperature range of 1000-1200 °C by application of stable tracer isotope 18O and subsequent secondary ion mass spectrometry, provided oxygen self-diffusion coefficients of CTGS. These coefficients are at least 3 orders of magnitude lower than those of LGS, confirming the outstanding high-temperature stability of CTGS.