Breakthrough in synthesis of superhard materials to rival diamond

Scientists unlock the potential of carbon nitrides, which have potential for technological advancements in fields such as materials science, electronics and optics.

Predictions

Ever since the seminal paper by Liu and Cohen in 1989, carbon nitrides have been a holy grail of materials science. Liu and Cohen's predictions of a fully saturated polymeric C₃N₄ solid with exceptional mechanical properties potentially surpassing diamond in hardness, fuelled years of research, yet no credible claims of such materials were reported.

Now, a multinational team of scientists, led by Dr Dominique Laniel from the Centre for Science at Extreme Conditions of the University of Edinburgh, and including researchers from the University of Bayreuth (Germany) and the University of Linköping (Sweden), has finally achieved the unfulfilled promise of Liu and Cohen's vision.

Subject to extreme condition

The researchers subjected various carbon nitrogen precursors to incredibly high pressures between 70 and 135 gigapascals (GPa), with 100 GPa corresponding to 1,000,000 times the atmospheric pressure, combined with temperatures above 2000 K achieved in laser-heated diamond anvil cell experiments. The samples were then characterized by single-crystal X-ray diffraction at three particle accelerators, the European Synchrotron Research Facility (ESRF, France), the Deutsches Elektronen-Synchrotron (DESY, Germany) and the Advanced Photon Source (APS, United States). From these measurements, the synthesis of four carbon nitrides could be evidenced: oP8-CN, tI14-C₃N₄, hP126-C₃N₄, and tI24-CN₂, featuring the necessary building blocks for ultra-incompressibility and superhardness, i.e. fully saturated C and N atoms, forming corner-sharing C(CN₃) or CN₄ tetrahedra. The crystal structure of these compounds are shown in the figure below. Remarkably, all four compounds can be recovered to ambient pressure and temperature.

Properties and applications

With experimental incompressibility between 365 and 419 GPa and calculated superhardness values between 78.0 and 86.8 GPa, these carbon-nitrogen compounds exceed the hardness of cubic boron nitride (c-BN) and closely approach that of diamond. Further calculations and experiments suggest additional remarkable properties, including photoluminescence, high energy density, piezoelectricity, and non-linear optical properties.

The potential applications of these ultraincompressible carbon nitrides are vast, positioning them as ultimate engineering materials akin to diamond. Their impact spans across numerous natural sciences fields, from materials science to electronics and optics, with use as high-endurance ‘smart’ cutting tools, protective coatings (e.g. for spaceships), and optoelectronic devices (e.g. solar cells and photodetectors).

The research team’s work has been published in Advanced Materials.