Ordered B-Site Vacancies in an ABX3 Formate Perovskite

We report the synthesis and structural characterisation of a series of aliovalently doped metal–formate ABX3 perovskite frameworks [C(NH2)3]Mn2+1–x(Fe3+2x/3,Vx/3)(HCOO)3 (V = B-site vacancy). For sufficiently large x, the vacancies order, lowering the crystal symmetry from orthorhombic Pnna to monoclinic P2/n. This system establishes B-site vacancies as a new type of defect in formate perovskites, and one with important chemical, structural, and functional implications. Monte Carlo simulations driven by a nearest-neighbour vacancy repulsion model show checkerboard vacancy order to emerge for x > 0.6, in accord with experiment.