Acoustic loss in langasite and langanite

Resonant ultrasonic loss Q^-1 in plano-convex unplated Y-cut disks of langasite and langanite was measured as a function of frequency and temperature with the aim of identifying dominant internal mechanisms that will degrade the performance of electronic oscillators. In both materials, the dependence on temperature is similar to that previously reported for langatate. An anelastic peak appears with maxima in the 150-280 K range at frequencies between 2 MHz and 14 MHz. The dependencies of this peak on temperature and frequency is consistent with a point-defect relaxation. The width of the peak is greater than that of a Debye peak, which indicates that the relaxation has a distribution of activation energies. The peak appears at higher temperatures in langasite than in langatate, and this may explain the lower room-temperature Q^-1 reported thus far in langatate. Additional anelastic peaks appear at elevated temperatures. The peaks are superimposed on a component of the loss that increases monotonically with temperature according to an approximate Arrhenius expression.