Large difference of lattice thermal conductivity between thermoelectric compounds Cu3VSe4 and Tl3VSe4: Elasticity, anharmonicity and chemical bonding from first-principles calculation

The elasticity has a great impact on thermoelectric efficiency and working service life as thermoelectric device. In this work, the large difference of lattice thermal conductivity (KL) between thermoelectric materials Cu3VSe4 and Tl3VSe4 was investigated by calculating their second-order (Cij) and third-order elastic constants (Cijk). The mechanical properties and thermal expansion coefficients were also evaluated. These results show that Cij of Cu3VSe4 and Tl3VSe4 are relatively overall weak. Particularly, in contrast to Cu3VSe4, Tl3VSe4 displays larger negative Cijk, revealing its stronger intrinsic anharmonicity. The relatively smaller elasticity of Tl3VSe4 derives from its weaker Tl-Se and V-Se chemical bonds as compared with that of Cu3VSe4. Finally, the increased anharmonicity results in a non-linearly mechanical behavior and reducing the KL with the rise of temperature. The research provides insight into the relationship of elasticity, anharmonicity and thermal expansion and KL in view of elasticity and will be useful for designing and optimizing of thermoelectric materials with high performance.