Low-dimensional quantum magnetism in CuNCS2

Structure and magnetic properties of copper(II) thiocyanate

Structure and magnetic properties of copper(II) thiocyanate. (a) The structure of Cu(NCS)2. The polymeric chains go into the screen. (b) The low temperature magnetic structure of Cu(NCS)2.

In most magnetic materials the magnetic spins stop moving and freeze into an ordered array when cooled down. For some materials, for example iron, this happens at hundreds of degrees Celsius; for others, it can be a fraction of a degree above absolute zero (-273 ºC). However, in spin liquids, the spins never freeze due to the combination of their geometrical arrangement and interactions. These spin liquid phases have many strange properties, for example ‘fractionalisation’, where electrons in the crystal behave as if their fundamental properties can be separated into distinct quasiparticles, like ‘holons’, which have charge but no spin, and ‘spinons’, magnetic but uncharged quasiparticles.

Low-dimensional magnetic materials (where the magnetic interactions are confined to chains or planes) showing spin liquid behaviour have been widely studied because reducing the dimensionality not only tends to prevent the magnetic order that would destroy the spin liquid, but also makes the theory much more mathematically manageable. One big challenge in this area is to find materials which have the right combination of interactions and lattice to show these unusual properties. In this paper we report a new quasi-one-dimensional quantum antiferromagnet, copper(II) thiocyanate (Cu(NCS)2). Cu(NCS)2 is atypical for a magnetic material, as the magnetic interactions between copper atoms are carried by a multiatom molecular anion (NCS), rather than either directly between the two metal atoms or via a single atom (e.g. O2–). Magnetic communication through multiple atoms is usually weak, and resultantly these molecular framework magnets are comparatively unexplored. Unexpectedly, we found that Cu(NCS)2 has reasonably strong magnetic connections between metal atoms (>100K). Despite these strong interactions, it only orders at 12K because its quasi-one-dimensional structure tends to suppress magnetic order. To work out the ordered spin structure below 12K we used neutron diffraction measurements carried out at the ISIS spallation source in Oxfordshire. The unusually strong magnetic interactions suggest we might be able to find other exotic magnetism in these molecular framework compounds.

There is a second, more chemical interest to this paper: we report the structure of Cu(NCS)2 for the first time. This might be surprising to chemists reading this, as the compound has a very simple composition: it is only made from two ions (copper(II) and thiocyanate) and both are very easily available. Indeed, the first reported synthesis of Cu(NCS)2 is from Karl Ernst Claus, 170 years ago!1 The reason why nobody has studied its properties or solved its structure before is probably because (black) Cu(NCS)2 slowly decomposes in water to (white) Cu(NCS), meaning it is hard to grow single crystals or to make pure. Despite these difficulties, with the benefit of modern instruments and computers, we were able work out the structure. I think Karl Ernst Claus would have had a tough time of it!2

  1. C. Claus, Journ. F. Prakt. Chemie XV, 401 (1838).
  2. Dr Claus would have struggled to determine the crystal structure of Cu(NCS)2 for a number of reasons, not least because X-rays weren’t discovered until 30 years after he died, and even then X-ray crystallography took another 20 years to be invented. Claus is an interesting figure in his own right. He discovered the element ruthenium, and made many other contributions to the chemistry of second- and third-row transition metals. He was known to have a negligent attitude to his health, and tasted many of his compounds. This included osmium tetroxide, a notoriously toxic compound now infamous amongst chemists for the severe eye damage its vapours cause. As a result of his tasting experiments he was at one point forced to stop his experiments for two weeks. Incidentally, he reported that osmium tetroxide has a ‘sharp, pepper-like’ flavour, which I assume is the taste of your tastebuds being oxidised. There’s more about Karl Claus at Wikipedia and in Chemistry and Chemists, pg 31-32 (in Russian - спасибо Google Translate).

Paper

Low-dimensional quantum magnetism in Cu(NCS)<sub>2</sub>, a molecular framework material

M J Cliffe, J Lee, J A M Paddison, S Schott, P Mukherjee, M W Gaultois, P Manuel, H Sirringhaus, S E Dutton and C P Grey

Phys Rev B, 97, 144421 (2018).

The accepted version of this article is available on the arXiv
Open access link.
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