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 - to-bcc phase transition. A room-temperature equilibrium transformation pressure of 353 GPa between the and bcc phases of Zr metal was observed. A negative slope of 395 K/GPa along the -bcc phase boundary observed in this experiment gives experimental evidence indicating thermal contribution to the structural sequences in transition metals and shows that the 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 -to-bcc phase transition. © 1991 The American Physical Society.