Remarkable anisotropy in rhombohedral Ge2Sb2Te5 compound: a promising thermoelectric material with multiple conduction bands and acoustic-optical branches coupling

First-principles calculations and Boltzmann transport theory are used to evaluate the electronic structure and thermoelectric properties of the high-temperature rhombohedral Ge ₂Sb ₂Te ₅ compound. In this work, we have investigated the phonon spectrum, band structure and density of states with K-H configuration, respectively. In the phonon spectrum, the acoustic and optical branches are coupled, and their combined effect significantly reduces the thermal lattice conductivity. The conduction band contributed by the Ge, Sb, and Te has the characteristics of more multiple valleys and degenerate features. When the electron-doping level is 4.3 × 10 ¹⁹ cm ⁻³ , a large number of conduction bands are activated, thereby enhancing the conductivity and seebeck coefficient. The layered Ge ₂Sb ₂Te ₅ has significant anisotropy, and its thermoelectric performance along the z-axis is excellent. The thermoelectric transport coefficients are calculated using Boltzmann transport theory combined with relaxation time approximation. For p type, the largest ZT value of 1.509 could be obtained with the carrier density of 1.3 × 10 ²⁰ cm ⁻³ at 900 K, while for n type, the considerable ZT value of 3.124 could be obtained with the carrier density of 4.5 × 10 ¹⁹ cm ⁻³ at 700 K.