Iron's crystal structure at Earth's core conditions is one of geophysics' fundamental questions. At ambient pressure, iron is BCC (body-centered cubic). Under the extreme pressure of the inner core (~360 GPa), the stable phase is supposed to be HCP (hexagonal close-packed), with FCC (face-centered cubic) as a competitor. BCC iron is considered dynamically unstable at these pressures — phonon calculations show imaginary frequencies, meaning the lattice would spontaneously distort. Case closed: the core is HCP.
Zhang, Panjwani, Xiao, Ghosh, Ogitsu, Ping, and Hu (arXiv:2501.00524) reopen the case by including thermal and electronic excitation effects. At the extreme temperatures of Earth's core (~6000 K), elevated electron temperatures stabilize BCC iron's phonons — the imaginary frequencies become real. The potential energy surface develops a plateau and local minimum that didn't exist at zero temperature. BCC iron becomes thermodynamically competitive with HCP.
The structural insight: the conventional phase diagram was computed at equilibrium, but the conditions at Earth's core are not equilibrium in the usual sense — the electron temperature is so high that thermal excitation of electronic states changes the bonding character of iron. The same atoms, the same pressure, but different electronic occupation produces a different energy landscape. The instability that ruled out BCC was a property of the cold calculation, not of the hot reality.
The phase that “can't exist” at core pressure can exist at core temperature. The diagram was right about the pressure and wrong about the temperature, and in the core, you don't get one without the other.