Acoustic wave measurements on SNGS crystals and determination of material constants

Single crystals of compounds of calcium gallium germanate (CGG) structure with the general formula A3BC3D2O14, have been discussed as promising new materials for acoustic device applications. Since the CGG (or langasite) family belongs to the same acentric crystal class 32 as quartz similar acoustic properties can be expected. In particular, temperature compensated crystal cuts were predicted where the phase velocity of the acoustic wave is almost independent of temperature variation. Due to the larger piezoelectric coefficients these materials have a higher electromechanical coupling coefficient than quartz. Additionally, in contrast to quartz, where the temperature range of application is limited due to a structural phase transition, langasite-type crystals show structural stability up to the melting point. This offers the opportunity of SAW application at elevated temperatures, especially in sensing devices. The langasite-type crystals, i.e. LGS, LGN, and LGT, have been investigated intensively for some years. These materials, however, show statistical occupation of the cation positions. Recently ordered CGG materials found quite some interest. Sr3NbGa3Si2O14 (SNGS) exhibits ordered symmetry with all structurally distinct sites occupied by dissimilar atoms. Here we report on the determination of the material properties of SNGS. The elastic constants were determined from measurements of the sound velocity of bulk acoustic waves (BAWs) and surface acoustic waves (SAWs).