Combining complex orders in perovskites

Many of the most useful properties of materials arise from the collective ordering of many individual nanoscale components across an entire crystal: for example, ferromagnetism is the result of aligning the magnetic spins within a material and ferroelectricity is the ordering of polar electric distortions. One way to create new properties in materials is therefore to order simultaneously multiple different features at once (e.g. combining ferromagnetic order with ferroelectric order to create magnetic and electric ‘multiferroics’). Making use of this strategy, however, means confronting the general tendency for materials towards disorder.

In this paper, we make a four new compounds, each consisting of a negatively-charged 3D nickel bismuth hexathiocyanate framework containing a different charge balancing positively charged cations in its pores. The chemistry of thiocyanate frameworks produces multiple kinds of order: the difference between the nitrogen and sulfur ends of the thiocyanate ion makes the nickel and bismuth order into an alternating pattern and the shape of the thiocyanate ion distorts the framework through tilting of the metal coordination octahedra. In this paper, we show that the charge balancing ‘A-site’ cations also order - and this order relates directly to the other two distortions. Using a combination of X-ray and neutron diffraction (carried out at the Institut Laue Langevin in Grenoble, France) we were able to determine the structures of these materials. Symmetry analysis and computational chemistry calculations helped us to explain the orderings adopted by the A-site cations in these materials.

These insights allowed us to design a new metal bismuth thiocyanate which we predict will be piezoelectric: i.e. it will be electrically polarised when compressed, like quartz. These results could help researchers synthesise new materials with interesting and useful properties like ferro- or piezoelectricity.

This work was carried out by Jie Yie Lee (a summer student in the group) in collaboration with the Advanced Materials Research Group at the University of Nottingham, the Department of Chemistry at Warwick University and the Institut Laue Langevin.