Microscopic Theory of the Temperature-Pressure Phase Diagram of Iron

The free energy of bcc, fcc, and hcp iron has been calculated as a function of temperature and pressure within the single-site spin-fluctuation theory of band magnetism, which has been developed recently by Hubbard and Hasegawa. The simple theory can account for the occurrence of the $\alpha$, $\gamma$, $\delta$, and $\epsilon$ phases in the $T-P$ phase diagram of iron, the phase transitions being driven by the magnetic contributions to the free energy.