Cyanide and magnetism

Both chemists and physicists try to control interactions in materials to produce unusual and useful states of matter, but usually they make use of very different techniques and for very different kinds of materials. In this paper we show that chemists can learn a lot from physicists and hopefully vice versa!

We look at the surprisingly complicated structures of some simple metal cyanide compounds, and show that these structures can be explained by using an analogy to the physics of intrinsically disordered ‘frustrated’ magnets. By changing the chemistry in the material, we were therefore able to explore the physics of these frustrated magnets, including quite unusual magnetic states!

The ‘coinage metal’ (i.e. gold, silver and copper) cyanides all form similar structures, formed from rods of alternating M⁺ CN^-, packed together in to a triangular array. Where the structures of gold and silver cyanide differ is how the MCN rods line up. In gold cyanide, the metals line up, and in silver cyanide, adjacent chains are offset by one third of the repeat distance. The difference between these structures can be explained by the relative strengths of two major interchain forces: the electrical attraction between the positive metal ion and negative cyanide ion and the ‘metallophilic’ attraction between metal atoms. In gold cyanide, the metallophilic interactions win out, in silver, the electrical attraction.

We show that because of the repeating pattern of the rods (M-CN-M-CN) you can think of it behaving a bit like a magnetic spin rotating in 2D. The structure of gold cyanide is thus where the spins line up (‘ferromagnetic’) and silver, where the spins want to point in opposite directions (‘antiferromagnetic’). We show that if you look at the more complex structure of silver gold cyanide, it behaves like a very unusual magnetic material. This suggests that by studying simple chemical systems we might discover other routes to exciting physics - and useful properties!

And if you want to read a more thorough explanation, here’s a great summary by Andrew Cairns on the Goodwin group website.