Third-Order Elastic Moduli of Barium Fluoride

The complete set of the six third-order elastic moduli for BaF₂ was determined from the change in the sound velocity under hydrostatic and uniaxial pressure. The third-order elastic moduli for BaF₂ were found to be nonisotropic, although the second-order moduli are isotropic. The Grueneisen-mode γ'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.