Ordered B-Site Vacancies in an ABX₃ Formate Perovskite

Abstract

We report the synthesis and structural characterisation of a series of aliovalently doped metal-formate ABX3 perovskite frameworks [C(NH2)3]Mn 2+ 1−x (Fe 3+ 2x/3 , x/3)(HCOO)3 (= B-site vacancy). For sufficiently large x, the vacancies order, lowering the crystal symmetry from orthorhombic P nna to monoclinic P 2/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 nearestneighbour vacancy repulsion model show checkerboard vacancy order to emerge for x > 0.6, in accord with experiment.

Static disorder in a perovskite mixed-valence metal–organic framework

“…5,[13][14][15][16][17] Doping and substitutions on the metal cation sites are known for many families of cation-templated metal formates. 13,14,[18][19][20][21][22][23][24][25][26] For instance, dimethylammonium (DMA) iron formates can be synthesised with different iron oxidation state ratios that influence the topologies observed and their physical properties. In particular, they can form in the perovskite (with the Schläfi notation 4 12 •6 3 ), 27 niccolite (4 9 •6 6 )(4 12 •6 3 ), 28 or related (4 9 •6 6 ) 2 (4 12 •6 3 ) topologies, 29 depending on the ratio of the Fe 2+ and Fe 3+ cations.…”

Designing Disorder into Crystalline Materials

“…90,91 The tendency for like B-site cations to avoid occupying neighbouring sites automatically partitions the underlying cubic lattice into its two constituent interpenetrating fcc lattices: one of which is occupied exclusively by the majority B-site cation, and the other occupied by both types in a 1:2 ratio (again frustrated). 92 Hence, Prussian Blue analogues of general formula M II [M III (CN) 6 ] 2/3 1/3 ( = vacancy) are also of this same structure type and show a similar degree of structural complexity that now affects the distribution and connectivity of vacancies. 93 In this particular family, as in the disordered rocksalt cathode materials discussed above, there are now signs that the type of local order present in the disordered state might be tuned rationally by varying chemical composition and/or synthesis approach-an appealing kind of defect engineering.…”

Correlated disorder in metal–organic frameworks

“…50 And in formate perovskites, cation vacancies again interact by avoiding neighbouring sites; this leads to a disorder/order transition in [GUA]Mn 2+ 1−x Fe 3+ 2x/3 x/3 (HCOO) 3 (GUA + = guanidinium cation) at x ord 0.6. 51…”