Determination of single-crystal elastic moduli of LiREF4 (RE = Y, Gd, and Tb) by resonant ultrasound spectroscopy

Abstract

The tetragonal fluoro-scheelite [Formula: see text] compounds ([Formula: see text]) have been shown to exhibit a variety of useful optical and magnetic properties. While [Formula: see text] has been widely studied, many of the fundamental thermodynamic properties of other members of this family remain unknown. Here, we report the complete elastic tensors ([Formula: see text]) of single-crystalline [Formula: see text], [Formula: see text], and [Formula: see text] using resonant ultrasound spectroscopy and density functional theory (DFT). We compare the results for [Formula: see text] with prior experimental results using time-of-flight ultrasound methods. This is the first report, however, of the experimental elastic tensors of [Formula: see text] and [Formula: see text]. The present results point to a softening of the elastic moduli of the [Formula: see text] system when Y is replaced by the larger ionic radius of Tb or Gd. Furthermore, we find that just 0.3% doping with Nd on the Y site also leads to a slight softening of the moduli. The variation of the elastic moduli as a function of temperature up to [Formula: see text] was also measured. A nearly linear softening of all seven independent elastic moduli was observed with increasing temperature. Phonon dispersions and phonon density of states obtained by DFT support the experimental finding of a significantly higher sound velocity due to lighter Y atoms in [Formula: see text], as compared to heavier [Formula: see text] and [Formula: see text].