High pressure behaviour of the magnetic van der Waals molecular framework Ni(NCS)₂

Summary by Madeleine Geers

Selected as Editor’s Suggestion

We investigated what would happen to Ni(NCS)₂ when subjected to the force of an elephant over a square centimetre. Elephant by Dmitry Abramov

Detecting pressure changes plays an essential role in our everyday lives, from biomedical applications such as assessing adhesion of prosthetics, to entering a code on a PIN pad or monitoring gas flow on planes. Often we cannot measure pressure changes directly, so identifying materials whose physical properties, such as magnetism, change when a pressure is applied can be very useful. Two-dimensional, or “van der Waals”, materials are particularly exciting for this kind of application. The atoms in 2D materials are strongly bonded in two directions, whilst having weak “dispersion” or van der Waals forces along the third axis: with graphite being the most famous example. If a material is magnetic, the temperature at which it magnetically orders can be increased quite a lot by applying pressure, for example the 2D magnetic material NiI₂ increases its magnetic ordering temperature by 235 K when compressed to 190 kbar, an increase of +1.6 % kbar^–1. 190 kbar is a considerable pressure: James Cameron in his submarine reached pressures of 1 kbar at the bottom of the Mariana trench and a typical volcano erupts with between 0.2–3 kbar of pressure. It would be useful to have materials which show big changes at more everyday pressures.

Nickel thiocyanate, Ni(NCS)₂, is a van der Waals material like NiI₂, but contains NCS^–, a molecular anion rather than the atomic I^–. The Ni²⁺ cations and NCS^– anions are joined via coordination bonds along two axes, making two-dimensional layers, which are stacked along the third axis via van der Waals forces. At ambient pressure, it is an antiferromagnet (i.e. the magnetic moments cancel) but where the magnetic moments within each layer are ferromagnetic (all the moments are aligned parallel). The magnetic moments on neighbouring layers point in opposite directions, giving the antiferromagnetic structure. In this work, we compressed Ni(NCS)₂ up to pressures of 8 kbar (the working pressure of a water jet cutter), to see how its structure and magnetic properties changed. We found that Ni(NCS)₂ is actually quite a soft compound, meaning that a relatively small amount of pressure produces a big decrease in its volume—in particular because the layers are easily compressed together. Consequently, the magnetic ordering temperature increases from 54 K (-219ºC) at ambient pressure to 64 K (-209ºC) at 8 kbar , an increase of +2.3 % kbar ^–1. This increase in ordering temperature occurs because the magnetic moments on neighbouring layers are forced closer together, but the overall antiferromagnetic structure of the compound doesn’t change. The softness of Ni(NCS)₂ means that the relative increase in ordering temperature per unit of applied pressure is actually one of the larger measured changes for a van der Waals type material, motivating further research into pressure-sensitive magnetic materials which could be implemented in future electronic devices making use of magnetic spins (“spintronics”).