Crystal Structures and Phase Stability of the Li₂S-P₂S₅ System from First Principles

The Li₂S-P₂S₅ pseudo-binary system has been a valuable source of promising superionic conductors, with α-Li₃PS₄, β-Li₃PS₄, HT-Li₇PS₆, and Li₇P₃S₁₁ having excellent room-temperature Li-ion conductivity >0.1 mS/cm. The metastability of these phases at ambient temperature motivates a study to quantify their thermodynamic accessibility. Through calculating the electronic, configurational, and vibrational sources of free energy from first principles, a phase diagram of the crystalline Li₂S-P₂S₅ space is constructed. New ground-state orderings are proposed for α-Li₃PS₄, HT-Li₇PS₆, LT-Li₇PS₆, and Li₇P₃S₁₁. Well-established phase stability trends from experiments are recovered, such as polymorphic phase transitions in Li₇PS₆ and Li₃PS₄, and the instability of Li₇P₃S₁₁ at high temperature. At ambient temperature, it is predicted that all superionic conductors in this space are indeed metastable but thermodynamically accessible. Vibrational and configurational sources of entropy are shown to be essential toward describing the stability of superionic conductors. New details of the Li sublattices are revealed and are found to be crucial toward accurately predicting configurational entropy. All superionic conductors contain significant configurational entropy, which suggests an inherent correlation between fast Li diffusion and thermodynamic stability arising from the configurational disorder.