First-principles study of materials properties at high pressures
A material under compression tends to change its crystal structures. The current work focuses on understanding of such phase transitions which are exhibited by silicon under high compression. Silicon tends to exhibit various different crystal structures other than from its generally occurring cubic diamond structure. Transition from diamond structure to beta tin (beta-Sn) structure was remarkable as it came to the metallic phase from the usual semi-conductor phase. On further compressed it transforms to a simple hexagonal (sh) phase. Pseudopotential method was used to estimate the variation happening in the total energy of the different exhibited crystal structures. The first principle calculations of these energies by a steady increment or decrement of the initial volumes of the respective crystal structures were fitted using the least-square fitting method and were used to determine the pressure (P) and the enthalpy (H) under which the crystal is at. All these Density Functional Theorem (DFT) calculations were executed with the help of quantum-espresso package which provided higher accuracy than most of the other methods available. For the non-cubic structures like beta-Sn, simple hexagon, the equilibrium c/a ratios have been evaluated for corresponding constant volumes. Based on that the lattice constant (a) was determined respectively. The estimated enthalpy and pressure values of the different exhibited structures are plotted each other which provided the transition pressures from one structure to another. Volume changes at transition pressure were also determined.
Keywords: pseudopotential, transition pressure