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 approximately 1 GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K0, 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:t3 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.

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We report high-pressure x-ray diffraction and magnetization measurements combined with ab-initio calculations to demonstrate that the high-pressure optical and transport transitions recently reported in TiOCl, correspond in fact to an enhanced Ti Interaction among different degrees of freedom provides a way for opening up a gap for the collective excitations, and different types of long-range order become then possible.Among the latter, a magnetoelastic coupling in S=1/2 chains results in a spin-Peierls phase transition to a low temperature nonmagnetic dimerized structure, 3,4 which is a localized counterpart of a conventional Peierls transition for itinerant electrons. 5 This effect was found in CuGeO 3 , 6 for a long time the only inorganic system with a pure spin-Peierls distortion. Recently, the possibility that TiOCl shows a low temperature spin-Peierls dimerization has been suggested, although the behavior of this material is

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