Tilt engineering of Prussian blue analogues towards new multiferroics

Multiferroicity is the rare coexistence of magnetic and electric order, which opens up for applications within e.g. memory storage.

However, one great limitation is the paucity of compounds with magnetic order and symmetries compatible with ferroelectricity.

These two properties have conflicting requirements: Ferroelectricity typically arises in systems with empty d-shells, whereas magnetism requires unpaired electrons.

An alternative mechanism for ferroelectricity is hybrid improper ferroelectricity (HIF), where symmetry-lowering distortions are combined to break the inversion centres.

This does not exclude magnetism and offers potential for the development of novel multiferroics.This project aims to design new multiferroic materials based on Prussian blue analogues (PBAs): cyanide versions of double perovskites with formula AMM´(CN)6 (A = alkali metal, M/M´ = transition metals).

Large A-site cations typically order in an alternating fashion, which can couple to octahedral tilts to drive polarity via HIF.

RbMnCo(CN)6 is a good illustration: it exhibits the desired Rb order and develops polar symmetry under pressure due to octahedral tilts. I will use chemical substitution to explore this further.

First, replacing Rb with a smaller A-site cation (K/Na) may reduce the transition pressure to the polar phase, potentially leading to polarity at ambient conditions. Second, substitution of the transition metals will be used to develop a ferromagnetic PBA with polar instabilities.