Reversible Antifluorite to Anticotunnite Phase Transition in Li2S at High Pressures

We have investigated the high-pressure behavior of lithium sulfide Li2S by Raman spectroscopy and synchrotron angle-dispersive powder X-ray diffraction in a diamond anvil cell (T=298 K). There occurs a fully reversible phase transition from the antifluorite (Fm3m, Z=4) to an orthorhombic structure at about 12 GPa (increasing pressure). Atomic positions of the orthorhombic phase were refined using the Rietveld method. Near 20 GPa the structure of the high pressure phase is of the anticotunnite type (Pnma, Z=4), which consists of a distorted hexagonal close packed (hcp) array of S atoms, Li(1) atoms occupying all the octahedral sites, and Li(2) atoms situated in one-half of the tetrahedral positions. The Li(1) atoms are shifted toward one of the S atoms so that their actual coordination is square pyramidal (5+1). At lower pressures the orthorhombic phase passes through an intermediate structural arrangement where the Li atoms are disordered, occupying a larger fraction of the tetrahedral sites of the anticotunnite-like hcp sulfur sublattice. Based on the analysis of atomic positions, we discuss the mechanism for the antiflourite to anticotunnite transformation.