Reduction of the bulk modulus at high pressure in CrN

Nitride coatings are increasingly demanded in the cutting- and machining-tool industry owing to their hardness, thermal stability and resistance to corrosion. These properties derive from strongly covalent bonds; understanding the bonding is a requirement for the design of superhard materials with improved capabilities. Here, we report a pressure-induced cubic-to-orthorhombic transition at ≈1 GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K 0 , of about 25% in the high-pressure (lower volume) phase. Our combined theoretical and experimental approach shows that this effect is the result of a large exchange striction due to the approach of the localized Cr:t 3 electrons to becoming molecular-orbital electrons in Cr–Cr bonds. The softening of CrN under pressure is a manifestation of a strong competition between different types of chemical bond that are found at a crossover from a localized to a molecular-orbital electronic transition. Chromium nitride is very incompressible, making it ideal for industrial coatings. However, it is now shown that the material softens at high pressure and low temperature in connection with a phase transition from cubic to orthorhombic structure. The results could be fundamental in designing ways to improve the mechanical properties of superhard CrN.