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 $\alpha$-Fe${}_2$O${}_3$ to a new ${\alpha }^{\mathrm{\prime }}$-Fe${}_2$O${}_3$ phase at a pressure of 50-62 GPa. The ${\alpha }^{\mathrm{\prime }}$-Fe${}_2$O${}_3$ phase differs from $\alpha$-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 $\alpha$ and ${\alpha }^{\mathrm{\prime }}$ 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.