Structural Behavior of CoB under Extreme Conditions and Its High Incompressibility

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 degrees C. The structural properties calculated from firstprinciple 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.