Structure-property relationships in Wittichenite-related phases with ultra-low thermal conductivity

Cu3BiS3 adopts a complex structure in which three distinct trigonal planar sites are partially occupied by copper ions. This, together with the presence of the heavy metal bismuth and its lone pair which introduces anharmonicity results in an exceptionally-low thermal conductivity of 0.38 W-1 m-1 K-1 at room temperature. This decreases on heating, reaching a minimum value at ca. 423 K, before rising at higher temperatures. Partial substitution of copper by silver reduces the thermal conductivity by up to 26% to ultra-low values of 0.2 W m-1 K-1 at room temperature, while retaining the same temperature dependence as that of the parent phase. DSC data show clear evidence of a phase transition in the region of the minimum in thermal conductivity. Moreover, DSC reveals the existence of a second phase transition in the parent phase at sub-ambient temperatures (250 K) that persists at low levels of silver substitution (x <= 0.08). We wish to use powder neutron diffraction to investigate the detailed structure of the silver-substituted phases, including site preferences for substitution, the evolution of structure through the phase transitions identified by DSC and the relationship of the higher temperature transition to the minimum in thermal conductivity.

Cu₃BiS₃具有复杂的晶体结构，其中三个不同的三角平面位点被铜离子部分占据。这一结构特征，加之重金属铋的存在及其引入非简谐性的孤对电子，使得该材料在室温下的热导率极低，仅为0.38 W·m⁻¹·K⁻¹。该热导率随温度升高而降低，在约423 K时达到最小值，随后在更高温度下逐渐回升。采用银部分取代铜可使热导率最高降低26%，在室温下达到0.2 W·m⁻¹·K⁻¹的超低水平，且其热导率随温度的变化规律与母相完全一致。差示扫描量热法（Differential Scanning Calorimetry，DSC）数据显示，在热导率达到最小值的温度区间内存在明确的相变证据。此外，DSC分析还发现，母相在室温以下的250 K处存在第二相变，且该相变在银取代量较低（x ≤ 0.08）的样品中依然存在。本研究拟采用粉末中子衍射技术，对银取代相的精细晶体结构展开研究，具体包括取代反应的位点偏好性、通过DSC观测到的相变过程中的结构演化，以及高温相变与热导率最小值之间的关联。