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Langasite and gallium phosphate are shown to exhibit piezoelectrically stimulated bulk acoustic waves up to at least 1,400 and 900°C, respectively. Most critical issues are stoichiometry changes due to, e.g. low oxygen partial pressures, and high losses. Therefore, the paper discusses the atomistic transport and defect chemistry of those crystals and correlates them with the electromechanical properties. First, the defect chemistry of langasite is investigated. As long as the atmosphere is nearly hydrogen-free, the transport of charge carriers is governed by oxygen movement. A dominant role of hydrogen is observed in hydrogenous atmospheres. Based on the developed defect model, donors are expected to suppress the oxygen vacancy concentration and, thereby, the loss in langasite. The prediction is proven by niobium doping and found to be valid. A one-dimensional physical model of thickness shear mode resonators is summarized. The analysis of the resonance spectra showed that the loss of the resonators can be described satisfactorily by mechanical and electrical contributions expressed as effective viscosity and bulk conductivity, respectively. The mechanical loss in langasite is significantly impacted by the electrical conductivity due to the piezoelectric coupling. The effect of the piezoelectric coupling on the loss is negligible for gallium phosphate since it shows an extremely low electrical conductivity.

Langasite single crystals show piezoelectrically excited bulk acoustic waves up to 1470 °C and are, therefore, used to prepare high‐temperature functional components such as membranes, cantilevers and field emission tips. The underlying concept includes monolithic structures to avoid thermal stress at elevated temperatures. In order to control the preparation processes, wet chemical etch rates, the transport of dopants and their impact on the materials properties of langasite are determined. Heavily Sr‐doped langasite shows a strong increase in conductivity which is used to realize monolithic electrodes by local doping. Further, the stability of small langasite structures is confirmed up to 1350 °C. Biconvex membranes could be operated in the fundamental and 3rd resonance mode up to 700 °C. The fundamental modes of those 16.3 MHz resonators show quality factors of 500 at the above‐mentioned temperature. Further, field emitter tips of 27 nm in radius are prepared and demonstrated to be operational up to at least 600 °C. The mechanical displacement and the electrical response of vibrating cantilevers is characterized simultaneously and found to be consistent. Finally, the feasibility of sensor film coated membrane arrays to distinguish between CO and H2 at 600 °C is demonstrated.

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