Acoustic loss in LiNb1-xTaxO3 at temperatures up to 900 °C

Lithium niobate-lithium tantalate solid solutions are new piezoelectric crystals that enable to combine the advantages of their edge compounds with respect to the high thermal stability of lithium tantalate and the high Curie temperature of lithium niobate. The specific objective of this study is the determination of the acoustic losses of bulk resonators with varying Nb/Ta ratios and their correlation with charge transport processes at temperatures up to 900°C and at reduced oxygen partial pressures. Several experimental techniques including resonant piezoelectric spectroscopy and contactless resonant ringdown spectroscopy are used to determine the acoustic losses of the resonators operated in the thickness-shear mode and thickness mode. Further, the electrical conductivity is determined by impedance spectroscopy. A one-dimensional physical model for vibrating plates is fitted to the data to extract key parameters such as piezoelectric coefficients and elastic modulus as a function of temperature. Noncontacting determination of loss excludes the impact of metal electrodes and reveals up to 300 °C values in the order of Akhiezer-type losses. Resonators operated at 2 MHz show a rapid loss increase above about 450 °C which is attributed to the piezoelectric/carrier relaxation. The latter follows from atomistic models using the key parameters mentioned and the electrical conductivity. The modelling includes variation of the resonance frequency and suggests higher resonance frequencies to lower the acoustic loss.