Defect chemistry based design of monolithic langasite structures for high temperature sensors

High temperature stable piezoelectric crystals open a wide range of new sensor applications due to the ability to monitor environmentally dependent mechanical properties of sensor films deposited onto piezoelectric resonators. An understanding of the defect chemistry of piezoelectric single crystals like langasite (La3Ga5SiO14) aids in establishing their application limits and enables the creation of monolithic structures by appropriate doping. Key defect chemical properties of langasite are outlined with respect to dopants which increase the electrical conductivity or lower the electromechanical losses. We focus here on electronic and ionic transport in heavily Sr-doped langasite since spatially localized doping can be used to create monolithic electrodes. The total electrical conductivity (impedance measurements), the transport of oxygen and dopants (diffusion runs using stable tracers and subsequent secondary ion mass spectrometry) as well as the electromechanical properties (network analysis) are correlated. Strontium doped areas exhibit an increased conductivity by about 4 orders of magnitude, largely governed by oxygen ions. For demonstration of the concept, langasite bulk acoustic wave resonators with Sr-doped monolithic electrodes are operated up to 800 °C. The resonator quality factors are found to be sufficiently high to use such resonators as high sensitivity sensors.