Weak Bonding and Anharmonicity in Thermoelectric ZnSb

The Zn─Sb binary system contains two high-performing thermoelectric materials, namely the ordered ZnSb and the disordered ZnSb. Both systems exhibit low thermal conductivity, which is speculated to originate from multicentre bonding within ZnSb-rhombi. Here, the electron density of ZnSb is reported based on multipole modelling of accurate X-ray diffraction data measured at 20 K. Topological analysis reveals that the bond paths in the rhombus are endocyclically strained and that electron density is concentrated within the rhombus rather than along its geometric bonds consistent with a multicentre bond description. However, the electron density is not equally shared between the geometric bonds of the rhombus. Electron density analysis and modelling of low-temperature anharmonicity reveal that one Zn–Sb interaction is weaker than the other. Taken together with the orientation of bonds external to the rhombus structure, an alternative description emerges wherein the multicentre bond is more partially localised along one set of the opposite legs of the rhombus. In this description, the stronger bond can be considered a traditional 2-centre-2-electron bond, while the weaker interaction is coordinative to the covalent bond. The anharmonicity and low thermal conductivity may consequently be understood as Zn rattling along the coordinative interaction.