Discrete Phase Selection Driven by Evaporation-Induced Off-Stoichiometry in Melt-Grown CsPbBr3

We show that halide evaporation during melt growth of CsPbBr3 on polycrystalline FTO under partially open conditions drives discrete phase selection between the line compounds of the CsBr–PbBr2 system, producing a sharp CsPbBr3/CsPb2Br5 bilayer instead of compositional grading. In situ optical imaging shows that solidification begins with nucleation and lateral growth of a planar CsPbBr3 single crystal while the melt layer is still thick enough to average over the FTO relief. As the crystal thickens, the residual melt then becomes inhomogeneous and unstable, producing a buried porous layer of faceted CsPb2Br5 grains with a characteristic in-plane spacing of 1–10μm). This morphology is consistent with a faceted Mullins–Sekerka-type instability under a non-conservative evaporative boundary condition. Beneath the single-crystal cap, the first-formed faceted islands are large and become progressively smaller as the advancing front approaches the FTO pyramids, while elevated ambient halide partial pressure suppresses the instability, consistent with diffusion–capillarity selection under decreasing residual melt thickness and steepening local gradients, modified by evaporative flux. Oxygen associated with microvoids or the oxide substrate enables a secondary reaction–diffusion pathway forming Pb–Br–O crystallites without altering the primary length scale. These results identify evaporation as an active control parameter coupling phase equilibria and interfacial stability in volatile halide melts. In the buried, porous bilayer morphology observed here, the secondary phases and porosity reduce the active CsPbBr3 volume and are expected to degrade scintillation through increased trapping, nonradiative recombination, and light scattering.