Temperature dependence of the ω-bcc phase transition in zirconium metal

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Abstract
High-pressure, high-temperature energy-dispersive x-ray diffraction studies were carried out on the group-IV transition-metal zirconium (Zr) to 36 GPa using a synchrotron-radiation source to investigate the entropy contribution in the ω- to-bcc phase transition. A room-temperature equilibrium transformation pressure of 35±3 GPa between the ω and bcc phases of Zr metal was observed. A negative slope of 39±5 K/GPa along the ω-bcc phase boundary observed in this experiment gives experimental evidence indicating thermal contribution to the structural sequences in transition metals and shows that the ω phase of Zr metal (a phase of group-IV transition metals) has lower entropy than its pressure-induced bcc phase (isostructural with group-V transition metals). Extrapolation of the present data indicates that the previous observation of a phase transition in shock-wave experiments at 26 GPa and 540 K is due to this ω-to-bcc phase transition.