Title: Expanded stability of layered SnSe-PbSe alloys and evidence of displacive phase transformation from rocksalt in heteroepitaxial thin films

Abstract: Bulk PbSnSe has a two-phase region or miscibility gap as the crystal changes from a Van der Waals-bonded orthorhombic 2D layered structure in SnSe-rich compositions to the related 3D-bonded rocksalt structure in PbSe-rich compositions with large contrast in the electrical, optical, and thermal properties across this transition. With an aim to understand and harness this transition in thin films devices, we epitaxially integrate PbSnSe on GaAs by molecular beam epitaxy using an in-situ PbSe surface treatment and show a significantly reduced two-phase region by stabilizing the Pnma layered structure out to Pb$_{0.45}$Sn$_{0.55}$Se, beyond the bulk-limit of Pb$_{0.25}$Sn$_{0.75}$Se. Pushing further, we directly access metastable two-phase epitaxial films of layered and rocksalt grains that are nearly identical in composition around Pb$_{0.5}$Sn$_{0.5}$Se and entirely circumvent the miscibility gap. We present microstructural evidence for an incomplete displacive transformation from rocksalt to layered structure in these films that we speculate occurs during the sample cool down to room temperature after synthesis. In situ temperature-cycling experiments on a Pb$_{0.58}$Sn$_{0.42}$Se rocksalt film reproduce characteristic attributes of a displacive transition and show a modulation in electronic properties. We find well-defined orientation relationships between the phases formed and reveal unconventional strain relief mechanisms involved in the crystal structure transformation, using transmission electron microscopy. Overall, our work adds a scalable thin film integration route to harnessing the dramatic contrast in material properties in PbSnSe across a potentially ultrafast structural transition.