DFT Calculation of High Pressure Phase in Sn/Ge Monochalcogenide

  Long Nguyen-Truong  ,  Guy Makov  
Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva

Tin and germanium chalcogenides, Sn-X and Ge-X (X = S, O) are candidates for a future generation of functional materials for energy conversion and photovoltaic semiconductor technology. They can provide low-cost, environmentally friendly, abundant, and efficient p-type semiconductors for industrial applications. Sn-X and Ge-X electronic properties are strongly affected by temperature and pressure conditions, because of the metastable layered structure of the layered phase and the high-hole mobility. The pressure-induced phase transition of SnS/GeS and SnO are discussed by both experiments and theoretical approaches [1-3]. Compression experiments found new high-pressure phases, monoclinic P21m in SnS and two phases in GeS (P21c and Cmcm). However,  DFT calculations in both SnS and GeS predict only the Cmcm phase. In addition, several experiments also suggest that the shear stress in non-hydrostatic compression  of SnO can induce the tetragonal-orthorhombic transition which is not present under  hydrostatic conditions  However, DFT calculations in Sn-X and Ge-X at high pressure focus only on pure hydrostatic compression. Hence, it is difficult to compare the result between the experiment and DFT calculations and also limits the relevance of the calculations in high pressure phase study.

            Recently, the application of machine-learning techniques has advanced computational applications in materials science. Among them, genetic algorithm, global and swarm optimizations combined with DFT calculations exhibit promise as a powerful material prediction tool. Our study examines the effect of high pressure on the layered structure of Sn-X and Ge-X by combining two methods: Conventional DFT calculations to reveal the hydrostatic compression effect and Genetic Algorithm (GA) approach to identify other meta-stable phases that can be obtained in non-hydrostatic conditions. Thermodynamic relations and electronic properties are carefully examined to clarify the mechanism of these pressure-induced phase transitions. We found that the tetragonal phase of SnO is the stable high-pressure phase under hydrostatic conditions. However, under non-hydrostatic conditions, we also obtained a distorted orthorhombic/monoclinic structure. In SnS/GeS, the Pnma-Cmcm transition under hydrostatic compression agrees with previous DFT calculations. Moreover, we predict several metastable phases in pressure range of P > 50GPa. The electronic properties and phonon stability of the new phases are discussed.

 [1] H. Giefers, F. Porsch and G. Wortmann, Structural study of SnO at high pressure,  Physica B 373, 76 (2006).

 [2] L Ehm et. al., Pressure-induced structural phase transition in the IV–VI semiconductor SnS, J. Phys.: Condens. Matter 16, 3545 (2004).

 [3] R. P. Dias, M. Kim and C. S. Yoo, Structural transitions and metallization in dense GeS, Phys. Rev. B 93, 104107 (2016).