Charge transport and acoustic loss in lithium niobate-lithium tantalate solid solutions at temperatures up to 900 °C

Abstract

Polar oxides, in particular solid solutions of lithium niobate and lithium tantalate (LNT, LiNb1-xTaxO3) are a variable model system to study fundamental materials properties due to miscibility of the edge compounds lithium niobate (LN, LiNbO3) and lithium tantalate (LT, LiTaO3) as well as tunable domain structures. Further new applications in the field of high-temperature actuators are aimed as LNT crystals potentially combine the advantages of the edge compounds with respect to high thermal stability and high piezoelectric coefficients. Beside LN and LT, LNT crystals of two compositions (LiNb0.88Ta0.12O3 and LiNb0.5Ta0.5O3) were grown by the Czochralski technique. Electrical conductivity, lithium diffusion and acoustic loss of LNT at high temperatures were investigated and correlated. Two techniques, namely impedance spectroscopy and resonant piezoelectric spectroscopy (RPS) are applied in this investigation. The electrical conductivity of investigated samples increases linearly in an Arrhenius presentation up to ∼605 °C. After a transition range of 605–620 °C, the conductivity increases linearly again for LT, but the slope is decreased by about 0.15 eV. This indicates that the conductivity below and above the Curie temperature is governed by a slightly different process. In contrast, LN shows a gradually increasing slope in the Arrhenius representation above the transition temperature range mentioned above. Here, a single activation energy cannot be assigned. The increase can be explained by the limited thermal stability of LN at elevated temperatures. Further, the acoustic loss in LNT thickness-shear mode resonators operated at about 2 and 10 MHz is determined by RPS and found to strongly increase above about 450 °C. In addition, the acoustic loss is modelled using independent materials data including electrical conductivity as determined here, and found to be in excellent agreement with the RPS data for the 10 MHz resonator. Based on the modelling, the loss above 450 °C is governed by conductivity-induced piezoelectric/carrier relaxation which can be attributed to lithium migration.