Screening of Energy-Degenerate Polymorphic Crystals A₂M₂M'Q₄: A New Strategy for Achieving Ultralow Thermal Conductivity

Crystals, characterized by their well-defined lattices, typically exhibit a single most stable configuration for each composition, known as the ground state. However, in this study, we discovered a family of A₂M₂M'Q₄ compounds (A: alkali metals, M & M': transition and main group metals, Q: chalcogenides) that prevalently possess multiple energy-degenerate phases. For a given composition, several structurally distinct polymorphs demonstrate nearly identical formation enthalpies (ΔH < 10 meV/atom). This minimal enthalpy difference suggests that factors such as configurational or vibrational entropy could easily overcome the energy barrier, leading to potential disorder and distortion within the crystal lattice. Our calculation results suggest these materials might exhibit pronounced lattice anharmonicity, significantly impeding the phonon transport, thus lowering thermal conductivity. To rapidly identify such compounds, we proposed a set of energetic descriptors to qualitatively describe the degree of energy degeneracy among polymorphs of a given composition. Using these descriptors, we conducted a high-throughput DFT screening of 1,700 A₂M₂M'Q₄ compounds. Candidates predicted to exhibit disorder or distortion were successfully synthesized, characterized by electron microscopy, followed by thermal conductivity measurements that confirmed their low thermal conductivity. The proposed descriptors and screening methodology offer a novel approach for discovering and designing materials with ultralow thermal conductivity.