Pressure-induced changes in the electronic structure and enhancement of the thermoelectric performance of SnS2: a first principles study

The thermoelectric properties of SnS2 have been studied using ab initio calculations with a full potential linearized augmented plane-wave technique and semi classical Boltzmann theory. We studied the thermoelectric properties of SnS2 at 300 K, 500 K and 800 K and hydrostatic pressures of 0 GPa, 10 GPa and 20 GPa. The transport properties were found to be anisotropic owing to the layered structure of SnS2. The electrical conductivity particularly shows strong anisotropy at 0 GPa pressure, presenting a larger value in the a direction than the c direction. Hydrostatic pressure causes the lattice constants to be decreased and induces changes in the electronic structure. There is reduction in the band gap and, as a result, the thermoelectric coefficients are affected. The thermopower becomes nearly isotropic at higher pressures. At 20 GPa we found that electrical conductivity as well as the power factor show a change in anisotropy by presenting a higher value in the c direction than in the a direction, which is obvious at all temperatures. The power factor exhibits an increase at higher pressures and higher temperatures. At 0 GPa and 800 K the power factor is calculated to be 11.89 × 10−4 W K−2 m−1 and 3.15 × 10−4 W K−2 m−1 in the a and c directions, respectively. At 20 GPa and 800 K the maximum value of the power factor is observed in the c direction, which is computed to be 12.10 × 10−4 W K−2 m−1 at a carrier concentration of 4 × 1020 cm−3. By comparing power factor in the c direction at 800 K calculated at 0 GPa and 20 GPa, we observed an enhancement by ∼3.8 times. It is due to this large enhancement that by increasing the pressure to 20 GPa at 800 K the average power factor was also found to increase by 33%. At 20 GPa and 800 K the value of the figure of merit (ZT) was found to be greater than 1 in the c direction. We hope that this study will provide useful information in further enhancing the thermoelectric properties of SnS2.