Materials with low thermal conductivity usually have complex crystal structures. Herein we experimentally find that a simple crystal structure material AgTlI₂(I4/mcm) owns an extremely low thermal conductivity of 0.25 W/mK at room temperature. To understand this anomaly, we perform in-depth theoretical studies based on ab initio molecular dynamics simulations and anharmonic lattice dynamics. We find that the unique atomic arrangement and weak chemical bonding provide a permissive environment for strong oscillations of Ag atoms, leading to a considerable rattling behaviour and giant lattice anharmonicity. This feature is also verified by the experimental probability density function refinement of single-crystal diffraction. The particularly strong anharmonicity breaks down the conventional phonon gas model, giving rise to non-negligible wavelike phonon behaviours in AgTlI₂at 300 K. Intriguingly, unlike many strongly anharmonic materials where a small propagative thermal conductivity is often accompanied by a large diffusive thermal conductivity, we find an unusual coexistence of ultralow propagative and diffusive thermal conductivities in AgTlI₂based on the thermal transport unified theory. This study underscores the potential of simple crystal structures in achieving low thermal conductivity and encourages further experimental research to enrich the family of materials with ultralow thermal conductivity.

Derivatives of the natural mineral colusite with general formula Cu₂₆A₂E₆S₃₂ (A = V, Nb, Ta, Cr, Mo, W; E = Ge, Sn, As, Sb) are an emerging class of excellent thermoelectric materials. Within just a few years, they have been promoted as serious candidates for cost-efficient, environmentally-friendly thermoelectric power generation, reaching performances comparable to tetrahedrites and other advanced thermoelectric sulphides. From ultralow thermal conductivity to the record-high power factor, this review gives an extensive description of the current state of the art regarding the crystal chemistry and electrical and thermal transport properties of colusites.

Bornite Cu₅FeS_4−xSe_x (0 ≤ x ≤ 0.6) compounds have been synthesized, using mechanical alloying, combined with spark plasma sintering (SPS).