Reduction of the bulk modulus at high pressure in CrN

Abstract

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.

Epitaxial suppression of the metal-insulator transition in CrN

“…This suggests that the thermal expansion coefficient may change at the critical temperature for the phase transition ~275 K, even in the absence of an actual structural phase transition. Such a conclusion supports the recent report on a reduction in the bulk modulus, 16 however, it cannot be made with any certainty, since the error bars of the individual data points allow equally well the fitting with a straight line for the whole temperature range, as shown in the plot. The data clearly shows a 0.14% increase of lattice constant at 270-284 K, indicating a phase transition for the polycrystalline CrN layer.…”

“…3,4 Electronic transport studies report controversial results for CrN, including (i) values for the resistivity ρ at room temperature range over more than two orders of magnitudes, from 1.7×10 -3 to 3.5×10 -1 Ωcm, 3,[5][6][7][8][9][10] even when only considering the most reliable data for single crystal CrN layers; (ii) the temperature dependence of ρ shows metallic behavior with dρ/dT > 0 in some studies, 6,7,11 but an increase in ρ with decreasing temperature in other reports, 3,5,9,12 which has been attributed to the presence of a band gap 5 or carrier localization due to crystalline defects 13 or N-vacancies; 14 (iii) some studies report a discontinuity in ρ(T) at 260-280 K, 3,6,7,14 which is associated with a magnetic and structural phase transition from a paramagnetic NaCl structure at room temperature to a low-temperature antiferromagnetic orthorhombic P nma phase 11,15 with a 0.56-0.59% higher density, 11 and a 25% lower bulk modulus, 16 while other reports show no evidence for a phase transition in the ρ(T)-curves. 5,7,9 Electronic structure calculations suggest that magnetic stress relief couples magnetic ordering with the structural phase transition, 17 and that CrN exhibits a band gap if the Hubbard Coulomb interaction term is sufficiently large.…”

“…The distortion of the cubic to the orthorhombic phase causes a 1.2 % increase and a 1.2 % decrease of the two body diagonals in the original cubic unit cell, corresponding to the 011 and 201 lattice spacings in the new orthorhombic structure. 16 Fig. 1(c) shows exemplary x-ray diffraction ω-2θ scans, including the corresponding Lorentzian fitting, from a polycrystalline CrN layer.…”

Observation of the negative pressure derivative of the bulk modulus in monoclinic ZrO2

“…Up to now, reduction of the bulk modulus at high pressure has been reported in CrN, But the behavior was accompanied by a cubic-to-orthorhombic structure transition. 27 On the other hand, from our first-principles calculations optimized lattice parameters for the M phase at normal pressure were a = 5. Fig.…”

Predicting high temperature mechanical properties of CrN and CrAlN coatings from in-situ synchrotron radiation X-ray diffraction

“…Rivadulla et al [32] Samples made with CrN coating on M2 steel will be denoted as CrN-M2ST and samples made with CrxAl1-xN coating on M2 steel will be denoted as CrAlN-M2ST.…”