Phase transitions of Fe$_2$O$_3$ under laser shock compression

We present in-situ x-ray diffraction and velocity measurements of Fe$_2$O$_3$ under laser shock compression at pressures between 38-116 GPa. None of the phases reported by static compression studies were observed. Instead, we observed an isostructural phase transition from $α$-Fe$_2$O$_3$ to a new $α^\prime$-Fe$_2$O$_3$ phase at a pressure of 50-62 GPa. The $α^\prime$-Fe$_2$O$_3$ phase differs from $α$-Fe$_2$O$_3$ by an 11% volume drop and a different unit cell compressibility. We further observed a two-wave structure in the velocity profile, which can be related to an intermediate regime where both $α$ and $α^\prime$ phases coexist. Density functional theory calculations with a Hubbard parameter indicate that the observed unit cell volume drop can be associated with a spin transition following a magnetic collapse.