Thermodynamics of the Fast-Conducting Li₃PS₄ Polymorphs from First Principles

We present a first principles study of phase stability among the three Li₃PS₄polymorphs (γ, β, α), among which the high temperature β and α phases have been demonstrated to be promising Li superionic conductors.¹ Despite experimental confirmation of phase transitions among these polymorphs, it is not yet understood why β can be stabilized at room temperature.² To model phase stability in this system, we treat configurational entropy contributions with density functional theory (DFT) calculations and the cluster expansion method, and vibrational contributions with harmonic phonon calculations.<br/>Analysis of DFT relaxations clarifies the nature of Li sublattices and ground state orderings. We predict γ → β and β → α phase transition temperatures that are in good agreement with reported experimental values upon heating.² We find a strong competition between γ and β phases, rationalizing the direct α → γ transition observed upon cooling, as well as the synthetic accessibility of β at room temperature. Configurational and vibrational sources of entropy are also separately examined and quantified. Our theoretical work provides clear thermodynamic understanding which can guide further experimental efforts in stabilizing the superionic conducting α and β Li₃PS₄.<br/><br/><b>References:</b><br/>[1] Homma, K.; Yonemura, M.; Kobayashi, T.; Nagao, M.; Hirayama, M.; Kanno, R. Crystal Structure and Phase Transitions of the Lithium Ionic Conductor Li3PS4. <i>Solid State Ionics</i> <b>2011</b>, <i>182</i> (1), 53–58.<br/><br/>[2] Kaup, K.; Zhou, L.; Huq, A.; Nazar, L. F. Impact of the Li Substructure on the Diffusion Pathways in Alpha and Beta Li3PS4: An in Situ High Temperature Neutron Diffraction Study. <i>J Mater Chem A</i> <b>2020</b>, <i>8</i> (25), 12446–12456.