Third-Order Elastic Moduli of Barium Fluoride

The complete set of the six third-order elastic moduli for Ba${\mathrm{F}}_2$ was determined from the change in the sound velocity under hydrostatic and uniaxial pressure. The third-order elastic moduli for Ba${\mathrm{F}}_2$ were found to be nonisotropic, although the second-order moduli are isotropic. The Grueneisen-mode $\gamma \text{'}\mathrm{s}$ and their low- and high-temperature limit were calculated from the third-order elastic moduli, as well as the isotherm ($P-V$ curve) over an extended pressure range. The experimental results for the third-order elastic moduli were compared with theoretically calculated values, and good agreement was found.