High-pressure thermal expansion, bulk modulus, and phonon structure of diamond

The thermodynamic properties of diamond at high pressures (up to 1000 Cpa) have been investigated using the ab initio pseudopotential plane wave metho d and the density-functional perturbation theory. The P-V-T equation of sta tes has been calculated from the Helmholtz flee energy of the crystal in th e quasiharmonic approximation. The pressure dependence of the equilibrium l attice constant, bulk modulus, mode Gruneisen parameters, and phonon struct ures has been presented. Some interesting dynamical features of diamond hav e been found at high pressures: (a) The thermal expansion coefficient decre ases with the increase of pressure. At ultrahigh pressure (greater than or equal to 700 GPa), diamond exhibits a negative thermal expansion coefficien t at low temperatures. (b) The phonon frequency at X-4 and L-3' gradually g oes higher than that of X-1 and L'(2), respectively. (c) The unusual overbe nding of the uppermost phonon dispersion curves near Gamma'(25) decreases w ith the increase of pressure. Such overbending results in a maximum in the phonon density of states, which has been invoked in the previous study [Phy s. Rev. B 48, 3164 (1993)] to explain the famous sharp peak in the two-phon on Raman spectrum of diamond. Our present results predict that this sharp p eak near the high-frequency cutoff will decrease with the pressure. [S0163- 1829(99)03237-9].