Computational study of the effects of phosphorus doping on structural, electronic, and thermoelectric properties of Mg2Si0.5Sn0.5 alloy

Abstract

Energy consumption is increasing, necessitating the development of more efficient energy conversion materials. Thermoelectric materials are used to convert waste heat to electricity. Recently, Mg2X (X = Si, Ge, Sn) and their solid solutions have gained increased attention as interesting thermoelectric materials in temperatures ranging from 500 to 800 K due to their nontoxicity, environmental friendliness, and abundance. In this work, the effects on structural, electronic, and thermoelectric properties of P-doped Mg16Si4Sn4 alloy were studied by the first principles pseudopotential plane wave method based on density functional theory (DFT). Geometry optimization is done from Self Consistent Field (SCF) calculations by quantum espresso software. The lattice constants, formation energy, and cohesive energy of Mg16Si4Sn4, Mg16Si3Sn4P, Mg16Si2Sn4P2, Mg16SiSn4P3, and Mg15Si4Sn4P were calculated. Thermoelectric properties, including the Seebeck coefficient, electrical conductivity, and electronic thermal conductivity were computed after solving a semi-empirical Boltzmann transport model through the BoltzTrap software. To get the total thermal conductivity, lattice thermal conductivity was calculated using phonopy software. The formation energy, cohesive energy, and elastic constants obtained from the study indicate that Mg16Si4Sn4, Mg16Si4Sn3P, Mg16Si3Sn4P, Mg16Si2Sn4P2, and Mg15Si4Sn4P can all exist stably in the system, however, Mg16SiSn4P3 cannot. In the P-doped Mg16Si4Sn4 lattice, P atoms preferentially replace Si atoms and have the best alloying capacity after doping. The energy band structure analysis shows a reduction of the band gap for Mg16Si4Sn4 with P dopants at the substitutional Si-sites. The density of state analysis indicates that the electrons of s and p orbitals of the constituent atoms are the main factor for maintaining the stability of the Mg16Si3Sn4P phase. The impurity atom doping both increases the carrier concentration and reduces the total thermal conductivity. Consequently, the thermoelectric performance of the P doped Mg16Si4Sn4 alloy is improved.