Crystalline phase transitions and vibrational spectra of silicon up to multiterapascal pressures

Abstract

A composite high-pressure phase diagram for silicon has been predicted for up to 4 TPa. This diagram has been built using a combination of evolutionary algorithm-based structure searches, electronic density functional theory, lattice dynamics of perfect crystals, and anharmonic corrections for the solid state in conjunction with molecular dynamics for evaluating the melt curve. The anharmonic corrections to free energy, arising from both finite-temperature multiphonon interactions and temperature dependence of the lattice axial ratios, play a critical role in properly identifying the solid-solid transition boundaries of the orthorhombic structures. A double hexagonal close-packed structure has been found to be thermodynamically and dynamically stable, sandwiched between the experimentally observed base-centered orthorhombic and hexagonal close-packed structures. In addition, beyond 2.8 TPa, there exists a sequence of face-centered cubic-to-body-centered cubic-to-simple cubic transitions that are accompanied by the localization of electrons in the interstitial spaces between the ions. Supplementing the structural calculations, second- and third-order interatomic force constants were evaluated to compute the phonon vibration modes and linewidths, respectively. This allowed an elaborate analysis of the Raman and infrared spectra for all of the structures of silicon identified so far.