Acoustic loss and electromechanical properties of LiNb_(1-x)Ta_xO_3

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The proposal is part of the research group "Periodic low-dimensional defect structures in polar oxides", which is dedicated to the correlation of defect structure, electron and ion transport and electromechanical properties using the model system lithium niobate-lithium tantalate (LNT).LNT solid solutions offer high thermal stability and high piezoelectric coefficients, which opens up a broad range of emerging applications for piezoelectric structures up to high temperatures. The latter requires the understanding of the interaction of point defects and domain walls as well as its effects on the electromechanical properties. The determination of related phenomena such as acoustic losses is the main focus of this subproject. Beyond conventional piezoelectric single crystals, it must be considered that the loss mechanisms are also governed by the composition of the solid solution and the domain structure. Provided that the loss mechanisms are understood, both parameters offer numerous options for tailoring the crystal properties. Favorable Nb/Ta ratios and domain structures shall be identified for different application fields. For example, for high temperature actuators and sensors or acoustic crystals, it is unknown which Nb/Ta ratios and domain structures result simultaneously in high piezoelectric coefficients, a high Curie temperature, low acoustic losses and high thermal stability.The investigations are planned in a wide temperature range starting at room temperature. Depending on the specific objective and the investigation method, the Curie temperature or the melting temperature represent the upper limit. In particular, acoustic losses, the electrical conductivity of domains and domain walls and the coercive field strength at high temperatures shall be determined and correlated in concert with the results of the other subprojects with the solid solution composition, the type and density of point defects and the domain wall density and charge, so that atomistic mechanisms such as anelastic relaxation can be assigned. Based on these insights, it should be further examined how the acoustic losses can be minimized and whether this can be achieved by doping or a suitable domain structure.The anticipated investigation methods such as resonant ultrasonic spectroscopy, pulse-echo method, laser Doppler vibrometry and electrochemical (micro-)impedance spectroscopy shall be applied up to a temperature of at least 1000 °C at variable oxygen and Li2O partial pressures and supported by modeling of the electromechanical properties.