AB-INITIO STUDY OF THE VOLUME DEPENDENCE OF DYNAMICAL AND THERMODYNAMICAL PROPERTIES OF SILICON

Motivated by the negative thermal expansion observed for silicon betwe en 20 K and 120 K, we present an ab initio study of the volume depende nce of interatomic force constants, phonon frequencies of transverse-a coustic TA(X) and TA(L) modes, and of the associated mode Gruneisen pa rameters. The influence of successive nearest-neighbor shells is analy zed. Analytical formulas, taking into account interactions up to secon d-nearest neighbors, are developed for phonon frequencies of TA(X) and TA(L) modes and the corresponding mode Gruneisen parameters. We also analyze the volume and pressure dependence of various thermodynamic pr operties (specific heat, bulk modulus, and thermal expansion), and poi nt out the effect of the negative mode Gruneisen parameters of the aco ustic branches on these properties. Finally, we present the evolution of the mean-square atomic displacement and of the atomic temperature f actor with the temperature for different volumes, for which the anomal ous effects are even greater.