Structural behavior of CoB under extreme conditions and its high incompressibility

Abstract

We report the structural behavior of CoB under extreme pressure and temperature conditions using a diamond anvil cell. The high-pressure XRD spectra at room temperature indicate that no phase transition occurs up to the highest pressure of 26 ​GPa studied herein. The orthorhombic structure is also stable under simultaneous application of pressure and temperature up to 8 ​GPa and 400 ​°C. The structural properties calculated from first-principle calculations using revised GGA-PBE potentials agree well with that of the experiments. The high bulk modulus of 294 ​GPa indicates that the structure is highly incompressible. Compared with other 3d mono-borides of group VIIIB, CoB is the least compressible. Its low compressibility and high stability regime are due to a prominent pseudo gap in its calculated electronic density of states. Furthermore, enhanced charge sharing between cobalt and boron leads to its high bulk modulus among these borides. In situ high-pressure high-temperature experiments reveal a monotonic decrease in the bulk modulus of CoB with temperature.