LATTICE DYNAMICAL THEORY OF THERMAL-EXPANSION AND MODE GRUNEISEN PARAMETERS IN CUBIC BP

Lattice dynamical theory of thermal expansion and mode Gruneisen param eters in cubic boron monophosphide is reported in the quasiharmonic ap proximation within the framework of a second-neighbor rigid-ion model. In this scheme, we optimized the involved force constants by using no nlinear least-squares procedures with constrained parameters and weigh ting of the available data on critical-point phonons and elastic and l attice constants. Theoretical results of the phonon dispersion curves along high-symmetry directions (both at ambient and 89 kbar pressures) , mode Gruneisen parameters, and thermal expansion coefficient (alpha( T)) are compared and discussed with the existing experimental and ab i nitio calculations. Consistent with x-ray data, our calcualtion for th e variation of alpha(T) with temperature in BP is found to be remarkab ly similar to that of beta-SiC, and unlike most other III-V-compounds, it does not attain negative values at lower temperatures.