Chemical and Lattice Stability of the Tin Sulfides.

Jonathan M Skelton,Lee A Burton,Aron Walsh,Fumiyasu Oba

doi:10.1021/acs.jpcc.6b12581

Abstract

The tin sulfides represent a materials platform for earth-abundant semiconductor technologies. We present a first-principles study of the five known and proposed phases of SnS together with SnS2 and Sn2S3. Lattice-dynamics techniques are used to evaluate the dynamical stability and temperature-dependent thermodynamic free energy, and we also consider the effect of dispersion forces on the energetics. The recently identified π-cubic phase of SnS is found to be metastable with respect to the well-known orthorhombic Pnma/Cmcm equilibrium. The Cmcm phase is a low-lying saddle point between Pnma local minima on the potential-energy surface and is observed as an average structure at high temperatures. Bulk rocksalt and zincblende phases are found to be dynamically unstable, and we show that whereas rocksalt SnS can potentially be stabilized under a reduction of the lattice constant the hypothetical zincblende phase proposed in several previous studies is extremely unlikely to form. We also investigate the stability of Sn2S3 with respect to SnS and SnS2 and find that both dispersion forces and vibrational contributions to the free energy are required to explain its experimentally observed resistance to decomposition.

Highlights

The tin sulfides are a technologically-important family of earth-abundant optoelectronic materials comprising tin monosulfide (SnS), tin disulfide (SnS2) and tin sesquisulfide (Sn2S3)
The recently-discovered π-cubic phase is metastable with respect to the orthorhombic Pnma/Cmcm equilibrium
Our calculations show conclusively that the hypothetical zincblende phase is both energetically and dynamically unstable, and we suggest that reports of this phase be reassessed as either of the other two cubic phases

Summary

Introduction

The tin sulfides are a technologically-important family of earth-abundant optoelectronic materials comprising tin monosulfide (SnS), tin disulfide (SnS2) and tin sesquisulfide (Sn2S3). The ground-state orthorhombic Pnma phase,[8] a high-temperature Cmcm phase,[9] and three cubic phases - rocksalt,[10] zincblende[11,12,13] and the recently-reported P213 (“π-cubic”) phase with a 64-atom primitive cell.[14,15,16,17] The sesquisulfide Sn2S3 has been a source of confusion, as it is relatively easy to prepare, and clearly distinguishable from the other tin sulphides,[18] yet is frequently predicted to be unstable with respect to decomposition into SnS and SnS2 in theoretical studies (e.g. as in the current Materials Project[19] entry, mp-150920) Both issues are important for contemporary PV research, since are phase impurities highly likely to play a role in the underwhelming performance of current SnS-based devices,[18, 21] but tin sulfides may form as impurities during the growth of more complex multicomponent semiconductors such as Cu2ZnSnS4 (CZTS).[22]

Methods

Findings

Conclusion