Study of group 5B transition metal monoborides under high pressure

We report the structural stability and compressibility behavior of 5B transition metal (TM) monoborides VB and TaB under high pressure. For the study, VB was synthesized in single phase by arc melting method followed by repeated annealing treatments and intermediate grinding; and TaB was procured. High pressure X-ray diffraction studies were carried out using synchrotron X-ray radiation up to 37.5 and 40.5 GPa for VB and TaB, respectively. The ambient orthorhombic lattices were stable for the monoborides in the pressure range studied. The bulk moduli were estimated to be 301(5) and 367(4) GPa, respectively. The axial compressibility was highest along the a direction for both VB and TaB due to predominant presence of metallic TM-TM bonds and absence of covalent B-B bonds along this direction. However, the least compressibility occurred along the c axis for TaB, and along the b axis for VB. The experimental results were substantiated with ab initio electronic structure calculations and the electronic and elastic properties were studied. Both VB and TaB were metallic due to the metallic nature of TM-TM bonds. Study of the density of states (DOS) and charge density distribution revealed that the TM-B bonds exhibited both ionic and covalent nature. The charge transfer from Ta to B was greater than that from V to B. The higher bulk modulus of TaB can be attributed to the stronger B-B covalent bond and enhanced charge transfer between Ta and B. Presence of a pseudogap at Fermi level in both their DOS plots inferred high structural stability. The cause of pseudogap formation was discussed. The study of elastic properties showed that both VB and TaB were mechanically stable in the pressure range studied. Small Pugh and Poisson's ratio indicated that VB and TaB were not ductile or malleable in nature.