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 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.
Rivadulla
Francisco
F
Physical-Chemistry Department, University of Santiago de Compostela, 15782-Santiago de Compostela, Spain. f.rivadulla@usc.es
Bañobre-López
Manuel
M
Quintela
Camilo X
CX
Piñeiro
Alberto
A
Pardo
Victor
V
Baldomir
Daniel
D
López-Quintela
Manuel Arturo
MA
Rivas
José
J
Ramos
Carlos A
CA
Salva
Horacio
H
Zhou
Jian-Shi
JS
Goodenough
John B
JB