Crystal Structure and Thermoelectric Properties of Novel Quaternary Cu₂MHf₃S₈ (MMn, Fe, Co, and Ni) Thiospinels with Low Thermal Conductivity

Uncovering of the origin of intrinsically low thermal conductivity in novel crystalline solids is among the main streams in modern thermoelectricity. Because of their earth-abundant nature and environmentally friendly content, Cu-based thiospinels are attractive functional semiconductors, including thermoelectric (TE) materials. Herein, we report the crystal structure, as well as electronic and TE properties of four new Cu2MHf3S8 (MMn, Fe, Co, and Ni) thiospinels. The performed density functional theory calculations predicted the decrease of the band gap and transition from p- to n-type conductivity in the Mn–Fe–Co–Ni series, which was confirmed experimentally. The best TE performance in this work was observed for the Cu2NiHf3S8 thiospinel due to its highest power factor and low thermal conductivity. Moreover, all the discovered compounds possess very low lattice thermal conductivity κlat over the investigated temperature range. The κlat for Cu2CoHf3S8 has been found to be as low as 0.8 W m–1 K–1 at 298 K and 0.5 W m–1 K–1 at 673 K, which are significantly lower values compared to the other Cu-based thiospinels reported up to date. The strongly disturbed phonon transport of the investigated alloys mainly comes from the peculiar crystal structure where the large cubic unit cells contain many vacant octahedral voids. As it was evaluated from the Callaway approach and confirmed by the speed of sound measurements, such a crystal structure promotes the increase in lattice anharmonicity, which is the main reason for the low κlat. This work provides a guideline for the engineering of thermal transport in thiospinels and offers the discovered Cu2MHf3S8 (MMn, Fe, Co, and Ni) compounds, as new promising functional materials with low lattice thermal conductivity.揭示新型结晶固体本征低热导率的起源，是现代热电学领域的核心研究方向之一。鉴于铜基硫尖晶石（Cu-based thiospinels）在地壳中储量丰富且环境友好，其作为包括热电（TE）材料在内的功能性半导体极具应用潜力。本研究报道了四种新型Cu₂MHf₃S₈（M分别为Mn、Fe、Co和Ni）硫尖晶石的晶体结构、电子性能及热电性能。本研究开展的密度泛函理论（density functional theory）计算预测，Mn-Fe-Co-Ni系列材料的带隙逐渐收窄，且导电类型由p型转变为n型，该结论已通过实验验证。本研究中，Cu₂NiHf₃S₈硫尖晶石展现出最优的热电性能，这得益于其最高的功率因数与极低的热导率。此外，在所测试的温度区间内，所有合成的化合物均表现出极低的晶格热导率（κlat）。其中Cu₂CoHf₃S₈的晶格热导率在298K时低至0.8 W·m⁻¹·K⁻¹，673K时更是降至0.5 W·m⁻¹·K⁻¹，远低于迄今已报道的其他铜基硫尖晶石。所研究合金的声子传输受到强烈扰动，其主要成因在于其特殊的晶体结构：大尺寸立方晶胞中存在大量空置的八面体空位。通过Callaway方法评估并经声速测试验证，该晶体结构可增强晶格非简谐性，而这正是其低热导率的核心成因。本研究为硫尖晶石的热传输工程化设计提供了指导思路，并将所发现的Cu₂MHf₃S₈（M分别为Mn、Fe、Co和Ni）化合物，作为一类极具应用前景的低热导率功能性新材料推向应用。