Structural phase transitions in Li2S, Na2S and K2S under compression

First-principles crystal structural searches were carried out for Li2S, Na2S and K2S under compression. Below 50 GPa, they follow same sequence of structural phase transitions, namely, anti-fluorite → anti-cotunnite → Ni2In-type phase. At above 50 GPa, they follow different phase transition sequences and in this pressure region several structural phase transitions are proposed. Calculated electronic density of state functions predicts metallization of Li2S and K2S at 225 and 175 GPa, respectively. Surprisingly, Na2S does not metalize even up to 300 GPa. Calculated structural parameters, equation-of-states and Raman data are in good agreement with the experimental results.