Dispersion Relations and Elastic-Wave Velocities of External Lattice Vibration Modes Propagating along c-Axes in Scheelites

Abstract

The dispersion relations in CaWO₄, CaMoO₄, SrMoO₄, BaWO₄, BaMoO4, NH₄ReO₄ and ND₄ReO₄ are determined from the two possible sets of force contants given in J. Phys. Soc. Japan 36 (1974) 511 and 916; the words, case I and II, were used to distinguish the two sets of the constants. The sets corresponding to the case II are not suitable to the two perrhenates, as they give imaginary frequencies. The elastic-wave velocities are determined as the gradients of the dispersion relations of the acoustic branches at k=0. The comparison of the theoretical velocities with the observed values suggests that the set corresponding to the case I is reasonable for CaMoO₄.