Elastic constants of langasite-type crystals determined by bulk and surface guided acoustic modes

We have determined the elastic constants of langasite-type crystals LGS (langasite), LGN (langanite), and LGT (langataite) from measurements of the sound velocity of bulk and surface guided acoustic modes. Surface acoustic waves (SAW) and leaky waves were measured by thermoelastic laser excitation. The average measurement error was about 1 m/s. The angular dispersion of the propagation velocity was obtained by mounting the sample on a computer controlled rotation stage. Starting with the elastic tensor determined from bulk acoustic waves, we optimized the data set by investigating the influence of the elastic tensor components on the angular dispersion of surface guided waves. This procedure is particularly useful for accurate determination of the non-diagonal elements of the elastic tensor. Acoustic velocity calculations based on the new set of elastic constants show an increased agreement with experimental data compared to the data set derived from bulk waves and previously published material data.