The development of materials for microelectromechanical systems, nanoelectronics, sensors and actuators includes piezoelectric materials such as lithium niobate-lithium tantalate solid solutions (LiNb1- xTaxO3, LNT). Indeed, they combine the strengths of both parent compounds, i.e., the high thermal stability of lithium tantalate (LT) and high piezoelectric coefficients and high Curie temperature of lithium niobate (LN). The objective of this study is to assess acoustic losses in bulk LNT and their connection to charge transport processes, particularly at temperatures up to 900°C.

LNT crystals with various Nb/Ta ratios are grown using the Czochralski technique. The study examines how high temperatures and low oxygen partial pressure affect the acoustic loss and electrical conductivity of LNT, comparing the results to those of the edge compounds LN and LT. The investigation employs a variety of experimental approaches, including the utilization of resonant piezoelectric spectroscopy (RPS), resonant ringdown spectroscopy (RRS), and impedance spectroscopy. Through the application of a one-dimensional physical model, the data is examined, enabling the extraction of crucial parameters such as piezoelectric coefficients and elastic modulus as a function of temperature.

Various LNT resonators that were operated in either the thickness-shear mode or the thickness mode are investigated. A notable finding was that the piezoelectric coefficients remain nearly constant up to a temperature of 600°C. Beyond this temperature, a decrease is observed, which has the effect of reducing the electromechanical coupling factor. With increasing temperature, the loss increases not as strong as expected without this effect. To explain the acoustic losses observed in the experiments, both experimental data and modeling techniques are employed. The modeling suggests that the losses observed above 450°C were primarily governed by the piezoelectric/carrier relaxation. The resonators without metal electrodes exhibit significantly lower losses at lower temperatures. Potentially Akhiezer-type losses are visible. Overall, modelling of acoustic losses with the use of the obtained material constants agrees very well with losses determined independently.