Hydrothermal growth of facet-tunable fluoride perovskite crystals KMF3 (M = Mg, Mn, Co, Ni and Zn)

Abstract

Perovskite-structured ABX₃ halides are one of the most promising families of materials with many potential applications. However, the controllable growth methods of their crystal facets and the corresponding synthetic chemistry have not yet been resolved. In this paper, we report a ligand substitution strategy for the facet-tunable growth of perovskite structure KMF₃ (M = Mg, Mn, Co, Ni and Zn) fluorides under mild hydrothermal conditions. The competition of the strong ligand of NH₃ and weak ligand of F⁻ in the octahedral crystal fields of M²⁺ results in different facet evolution routes for KMF₃ samples with the M-site of empty-, partially- or fully-filled 3d orbitals. Although X-ray diffraction results indicated that all five KMF₃ compounds are crystalized into the same space group (Pmm, no. 221), the crystal shape of KMF₃ with M = Mg, Mn, Co and Ni evolved from cubic shape to edge-truncated chamfer box shape, while KZnF₃ evolved from cubic shape to corner-truncated cubic shape under the same set of synthetic conditions. The cubic shape habitat morphology of KMF₃ crystals is enclosed by {100} facets, chamfer box {100} and {110} facets, and corner-truncated cube {100} and {111} facets. The crystal shape formation mechanism was analysed based on ligand field theory and the Bravais–Friedel–Donnay–Harker (BFDH) theory. We expect that the shape controllable synthetic method can benefit the growth of other halide material crystals, and the shape tunable crystals can find more important applications.