Structural sequences of group IV transition metals under ultrahigh pressure.

摘要

Crystal structures of many elemental metals tend to have certain structural sequences when viewed as functions of atomic number. Transition metals are known to have hcp - bcc - hcp - fcc structural sequence when viewed as a function of atomic number. This thesis work gives the first experimental evidence of a pressure-induced structural sequence in transition metals and, fro ma crystal structure point of view, the first experimental evidence of a pressure-induced phase transformation from a group IV transition metal to a group V transition metal. High pressure energy dispersive X-ray diffraction (EDXD) studies were carried out on group IV transition metals hafnium (Hf) to 252 GPa, zirconium (ZR) to 36 GPa, and titinium (Ti) to 209 GPa at room temperature by using synchrotron X-ray source. all the three group IV transition metals have a hcp structure at room pressure and have an omega phase in an intermediate pressure range. At ultra high pressure a new structural phase transformation between the omega phase and a bcc phase (isostructural with group V transition metals) was observed in Hf and Zr. Equilibrium transformation pressures for the hcp to omega, the omega to bcc phase transformations at room temperature are 38 ;High pressure/high temperature energy dispersive x-ray diffraction studies were also carried out on Zr metal to 36 GPa to investigate the thermal (entropy) contribution to the structural sequences in transition metals. A negative slope of 39