First And Second Pressure Derivatives Of The Elastic Constants Of Csbr Crystals 222 To 330k Phonon Dispersion Relation

The first and second pressure derivatives of the adiabatic elastic constants of CsBr single crystal were measured at 300, 298, 258.2, and 222°K using pulse interference techniques. The corresponding quantities of the bulk modulus of CsBr are also calculated. The maximum pressures used in the experiments ranged from 6 kbars to 11 kbars. B0/sup T'/ is measured to an accuracy of approximately 5 percent, and B0/sup S''/ to approximately 15 to 30 percent, assuming the systematic error is absent. It is found, however, the value of B0/sup S''/ is strongly affected by the systematic error arising from the choice of model in fitting the data. The widely used quadratic fit is found inadequate for a maximum pressure of 10 kbars. The contribution from the third-order term 1/6 B''0'P3 can not be neglected. The quasi-harmonic approximation is used to derive the theoretical equation of state. Several two- or three-parameter empirical equations of state are also discussed. Using the long-wave method and the present data, the phonon dispersion relation of CsBr is calculated and compared with the experimental results of neutron scattering. 14 tables, 34 figures, 74 references.