Phonon vibrational and transport properties of SnSe/SnS superlattice at finite temperatures

TL;DR

This study investigates the finite-temperature phonon and transport properties of SnSe/SnS superlattices, revealing a novel phase transition, enhanced thermoelectric and photovoltaic potential, and the role of anharmonicity in stability and thermal conductivity.

Contribution

It introduces a new P4/nmm phase in SnSe/SnS superlattices and analyzes its stability, phonon behavior, and improved electronic and thermal properties at finite temperatures.

Findings

Discovery of a new P4/nmm phase at finite temperatures.

The P4/nmm phase exhibits extremely low thermal conductivity.

Enhanced electronic and optical properties in the P4/nmm phase.

Abstract

The structural stability and phonon properties of SnSe/SnS superlattices at finite temperatures have been studied using machine learning force field molecular dynamics and the anharmonic phonon approach. The vertical SnSe/SnS superlattice undergoes a phase transition from the Pnma phase to a novel P4/nmm phase at finite temperatures, which is different from the high-temperature Cmcm phase of the SnSe and SnS systems. The stability of P4/nmm phase is determined by molecular dynamics trajectories and anharmonic phonon dispersion relations. The imaginary modes of TO modes at the q=M(1/2,1/2,0) point of the P4/nmm phase in harmonic approximation become rigid at elevated temperatures. An analysis of phonon power spectra upon temperature also confirms the dynamic stabilization. The P4/nmm phase has higher symmetry than the Pnma phase, and the phase transition between them is accompanied by competition between the Jahn-Teller effect and phonon anharmonicity. Unlike the anisotropic distribution of Sn-Se/S bonds in the Pnma phase, the P4/nmm phase forms chemical bonds with similar bond lengths both in-plane and interlayer, and their resonance effect can significantly enhance phonon scattering. The calculated phonon density of states and lifetime is strongly temperature dependent, demonstrating the heavy anharmonicity in the SnSe/SnS system. The P4/nmm phase has an extremely low lattice thermal conductivity, close to the experimental values of SnSe and SnS. Moreover, with the reduction of band gap and the enhancement of band degeneracy near the Fermi level, the P4/nmm phase exhibits superior electronic transport properties and significantly enhanced response to infrared and visible light. This makes it show great potential in thermoelectric and photovoltaic applications.