Structural stability, dynamical stability, thermoelectric properties, and elastic properties of GeTe at high pressure

摘要

The stability of GeTe in rhombohedral (R3m), face centred cubic (Fm3m), and simple cubic (Pm3m) phases has been studied using density functional perturbation theory. The rhombohedral phase of GeTe is dynamically stable at 0 GPa, while Fm3m and Pm3m phases are stable at 3.1 and 33 GPa, respectively. The pressure-dependent phonon modes are observed in Fm3m and Pm3m phases at F and M points, respectively. The electronic and the thermoelectric properties have been investigated for the stable phases of GeTe. The electronic band gap for rhombohedral and Fm3m phases of GeTe has been observed as 0.66 and 0.17 eV, respectively, while the Pm3m phase shows metallic behavior. We have used the Boltzmann transport equation under a rigid band approximation and constant relaxation time approximation as implemented in BOLTZTRAP code for the calculation of thermoelectric properties of GeTe. The metallic behavior of Pm3m phase gives a very low value of Seebeck coefficient compared to the other two phases as a function of temperature and the chemical potential mu. It is observed that the rhombohedral phase of GeTe exhibits higher thermoelectric performance. Due to the metallic nature of Pm3m phase, negligible thermoelectric performance is observed compared to R3m and Fm3m-GeTe. The calculated lattice thermal conductivities are low for Fm3m-GeTe and high for R3m-GeTe. At the relatively higher temperature of 1350 K, the figure of merit ZT is found to be 0.7 for rhombohedral GeTe. The elastic constants satisfy the Born stability criteria for all three phases. The rhombohedral and Fm3m phases exhibits brittleness and the Pm3m phase shows ductile nature.