Contributions to anelasticity in langasite and langatate

Maximization of the quality factors Q of langasite (LGS) and langatate (LGT) is necessary for optimal performance of acoustic resonators of these materials in frequency-control and high-temperature sensing applications. In this report, measurements and least-squares analysis of Q ^-1 as a function of ultrasonic frequency and temperature of undoped LGS (100 K to 750 K) and LGT (300 K to 760 K) reveal a superposition of physical effects, including point-defect relaxations and intrinsic phonon-phonon loss. In LGS, these effects are superimposed on a large temperature-dependent background with weak frequency dependence that is interpreted as arising from a relaxation process with a distribution of activation energies. This distributed relaxation is suggested to be a result of anelastic kink migration. No evidence for a significant background of this form is found in the LGT specimen, consistent with the lower measured dislocation etch-pit density of this crystal. The analysis of the dependence of Q ^-1 of LGT on frequency and temperature indicates that, at near-ambient temperatures, the damping in this specimen is close to the intrinsic limit determined by phonon-phonon interactions. Piezoelectriccarrier relaxation, which must occur at sufficiently elevated temperatures, is found not to be a significant contribution to Q ^-1, relative to defect-related contributions, in either LGS or LGT in the measured range of temperatures.