Octahedral tilting in Prussian blue analogues†
Octahedral tilting is key to the structure and functionality of perovskites. We present a metastudy of published literature showing how these distortions manifest in the related Prussian blue analogues (PBAs): cyanide versions of double perovskites with formula AM[M′(CN)6]1−y□y·nH2O (A = alkali metal, M and M′ = transition metals, □ = vacancy/defect). Tilts are favoured by high values of x if A = Na or K, whereas the transition metals play a less important role. External hydrostatic pressure induces tilt transitions nearly irrespective of the stoichiometry, whereas thermal transitions are only reported for x > 1. Interstitial water can alter the transitions induced by a different stimulus, but (de)hydration per se does not lead to tilts. Finally, the implications for rational design of critical functionality—including improper ferroelectricity and electrochemical performance—are discussed. The results are integral for a fundamental understanding of phase transitions and for the development of functional materials based on PBAs.
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