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Bulk PbSnSe has a two-phase region or miscibility gap as the crystal changesfrom a Van der Waals-bonded orthorhombic 2D layered structure in SnSe-richcompositions to the related 3D-bonded rocksalt structure in PbSe-richcompositions with large contrast in the electrical, optical, and thermalproperties across this transition. With an aim to understand and harness thistransition in thin films devices, we epitaxially integrate PbSnSe on GaAs bymolecular beam epitaxy using an in-situ PbSe surface treatment and show asignificantly reduced two-phase region by stabilizing the Pnma layeredstructure out to Pb$_{0.45}$Sn$_{0.55}$Se, beyond the bulk-limit ofPb$_{0.25}$Sn$_{0.75}$Se. Pushing further, we directly access metastabletwo-phase epitaxial films of layered and rocksalt grains that are nearlyidentical in composition around Pb$_{0.5}$Sn$_{0.5}$Se and entirely circumventthe miscibility gap. We present microstructural evidence for an incompletedisplacive transformation from rocksalt to layered structure in these filmsthat we speculate occurs during the sample cool down to room temperature aftersynthesis. In-situ x-ray diffraction measurement of an as-grownPb$_{0.56}$Sn$_{0.44}$Se rocksalt film under cryogenic cooling reproduces thekey attributes of this transition and validates our hypothesis. Notably, wefind well-defined orientation relationships between the phases forming in theprocess and unconventional strain-relief mechanisms involved in the crystalstructure transformation by transmission electron microscopy. Overall, our workadds a scalable thin film integration route to harnessing metastable layeredcompositions as well as the dramatic contrast in material properties in PbSnSeacross a potentially ultrafast structural transition.

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