Octahedral tilting in Prussian blue analogues

TL;DR

This study explores how octahedral tilting occurs in Prussian blue analogues, revealing the influence of composition, pressure, and water content on structural phase transitions relevant for designing functional materials.

Contribution

It provides the first detailed analysis of octahedral tilting in PBAs, linking structural distortions to composition and external stimuli, aiding rational material design.

Findings

Tilts favored by high A-site cation content (Na, K).

Hydrostatic pressure induces tilt transitions regardless of stoichiometry.

Interplay of water content and stimuli affects phase transitions.

Abstract

Octahedral tilting is key to the structure and functionality of perovskites. Here we show how these distortions manifest in the related Prussian blue analogues (PBAs): cyanide versions of double perovskites with formula A$_x$M[M$^{\prime}$(CN)$_6$]$_{1-y}\Box _y\cdot n$H$_2$O (A = alkali metal, M and M$^{\prime}$ = transition metals, $\Box$ = 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 can induce 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 \textit{per se} does not lead to tilts. Implications for rational design of critical functionality -- including improper ferroelectricity and electrochemical performance -- are discussed. The results are important for a fundamental understanding of phase transitions as well as for the development of functional materials based on PBAs.