December 2022 ESRFnews16

FUNCTIONAL MATERIALS

thousands of experiments have been performed, with no apparent success. This year, however, Dominique Laniel at the University of Edinburgh in the UK and colleagues obtained diffraction measurements at the ESRF s ID11 and ID27 beamlines to demonstrate that they had synthesised three different carbon nitrides made of corner-sharing CN4 units in a DAC at temperatures around 2500 K and pressures between 124 and 134 GPa (see Hard science , below). Tantalisingly, the new materials proved to be metastable (arXiv:2209.01968). As the work is not yet reviewed, Laniel cannot comment

on it directly. Nevertheless, he points out that, in general, the main accomplishment of experiments such as these is not necessarily the synthesis itself but the proof of it, using the current state-of-the-art synchrotron instrumentation. I suspect people have made new materials in the past, they ve just been unable to see them, he says. You really need an intense beam to see some of these compounds. Once promising new metastable materials such as

Laniel s carbon nitrides are found, the next step is to try S TE

F C A N D É / E

S R F

Just open to users, the new HPLF will blast samples with a laser of 100 J energy to create new materials.

to make more of them, perhaps using a large hydraulic press, so that their properties can be fully characterised. Presses like these, and the technique of chemical vapour deposition, can both be used for large-scale synthesis once properties have been proved. Diamond, as well as another family of hard materials, the boron nitrides, are commercially synthesised this way. But, as Laniel says, You need something with really amazing properties for it to be industrially viable. Materials for energy storage are another motivation.

Last year, Matteo Ceppatelli at the European Labora- tory for Non-linear Spectroscopy and ICCOM-CNR in Sesto Fiorentino, Italy, and colleagues used X-ray diffraction at the ID27 and ID15B beamlines to observe the first ever synthesis of crystalline arsenic nitride at pressures between 25 and 36 GPa and at temperatures above 1400 K; it also persisted at room temperature and at pressures between 9.8 and 50 GPa (Angew. Chem. Int. Ed. 61 e202114191). As the crystal has a structure similar to a polymerised form of nitrogen that is considered to be one of the most energy-dense materials, but which can be syn- thesised and stabilised only under more extreme pressure and temperature conditions, its discovery could help in the development of propellants for sustainable transport. Nitrides in general are attractive targets for Maxim

Bykov at the University of Cologne in Germany. They are predicted to have impressive physical and chemical properties, they can be catalysts and, as with polymer- ised nitrogen, are billed as potential energy-storage materials. Their downside is that they tend to decom- pose and release their nitrogen easily unless contained via high pressures. Last year, Bykov and his colleagues employed the ESRF s ID15B beamline to study a laser- heated diamond anvil cell in which they synthesised a new polymorph of beryllium nitride. Like graphene,

Carbon nitrides with a structure of corner-sharing CN4 tetrahedra are predicted to be harder than diamond. Laniel and colleagues have shown they can be synthesised from tetracyanoethylene embedded in solid molecular nitrogen, at pressures above 124 GPa. Before laser heating (left), the sample shows no sign of a phase change; after laser heating (middle), it turns transparent, signalling the transition to the carbon nitride C3N4. Recovered at ambient pressure (right), the sample looks the same, proving its metastability.

HARD SCIENCE

D O M IN IQ U E LA

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