why does pressure change crystal structure?
Based upon the extrapolation of the high-pressure data using an  exponential form of the Murnaghan equation of state, the volume of hcp  iron at zero pressure is 6.72±0.06 cm3/mole. The volume change for the bcc-hcp transformation at 130 kbar is -0.34±0.01 cm3/mole.  This value satisfies the triple point conditions for the bcc-hcp-fcc  triple point when published values for the other phase transformations  are used.