Generalized free-energy method used to calculate the high-pressure, high-temperature phase transition in solid CO2.

Abstract

Free-energy techniques are generalized to calculate phase transitions in solids at high pressures, using the constant-pressure Monte Carlo method with periodic, deformable boundary conditions. The procedures are then applied to ${\mathrm{CO}}_{2}$, and a room-temperature phase transition is found at ${\mathit{P}}_{\mathit{t}}$=4.43\ifmmode\pm\else\textpm\fi{}0.01 GPa, into an orthorhombic Cmca structure. The volume and entropy change on transition is found to be \ensuremath{\Delta}V=0.35 ${\mathrm{cm}}^{3}$/mole and \ensuremath{\Delta}S=0.07\ifmmode\pm\else\textpm\fi{}0.14 cal/mol K.