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.