Physical properties of piezoelectric oxides

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In the field of functional materials, piezoelectric, ferroelectric and multiferroic systems have repeatedly been investigated in more or less detail in the past because of their potential applications, e.g., as sensors, electronic memories or ultrasonic transducers. This group of crystals includes oxide compounds that crystallize as piezoelectrics or ferroelectrics. Up to now, elastic properties of piezoelectric or ferroelectric oxides have been investigated in some cases as a function of temperature, but in most cases no further investigations have been carried out. This applies in particular to the temperature-dependent acoustic damping phenomena. The changes in mechanical properties can be measured particularly sensitively via the temperature dependence of the elasticity tensor, i.e., the thermoelastic coefficients, and thus with the aid of ultrasound techniques. The piezoelectric properties can also be derived using ultrasound techniques, in particular resonant ultrasound spectroscopy (RUS), as the electromechanical coupling is closely linked to their elastic properties.The aim of the project is to investigate the elastic and piezoelectric properties of selected oxidic crystals (sillenites, gehlenite, fresnoite, gamma-LiAlO2, LiGaO2) using the RUS technique as a function of temperature in order to analyze the basic features of the elastic and thermoelastic behaviour of these materials in a wide temperature range. Furthermore, the understanding of temperature-dependent acoustic damping phenomena and the correlation of crystal structure and thermoelastic properties will be deepened. Since the elastic properties reflect the three-dimensional bonding system of the crystal structure, ultrasound techniques such as RUS provide a highly sensitive probe for the detection of structural instabilities and phase transformations. The results will provide information on (1) the possible occurrence of temperature-dependent acoustic damping in the oxides (e.g. gehlenite, fresnoite) investigated here as was already observed for the sillenites, (2) whether a significant structural change occurs in the sillenites in the high-temperature range associated with the ultrasound damping, and (3) how the temperature-dependent elastic and piezoelectric tensor components of the sillenites change above the temperature limit of the ultrasound damping up to the melting point. Another aspect deals with the temperature-dependent correlation of increasing lithium diffusion of gamma-LiAlO2 and LiGaO2 (ambient temperature to melting point) and the elastic and piezoelectric properties, and a possible occurrence of ultrasound damping. In order to elucidate the possible reasons for a frequently occurring decrease of the piezoelectric effect, high-temperature conductivity and HT-DK measurements together with high-temperature structure determinations and temperature dependent powder-SHG measurements will be performed on a part of these compounds.