High-precision powder x-ray diffraction and ${}^{57}\mathrm{Fe}$ Mössbauer studies up to $20\mathrm{GPa}$ show that magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ undergoes a reversible normal$\leftrightarrow$inverse transition with increasing pressure or decreasing temperature. There is no resolvable change in the spinel-type crystal structure or unit-cell volume at the phase transition. However, the volume of the tetrahedral $\left(A\right)$ site increases (17%) and that of the octahedral $\left(B\right)$ site decreases $(\sim 7\%)$ as electron charge density is transferred from the $B$ to the $A$ site. The corresponding valence changes cause the inverse $\to$ normal transition with increasing pressure: ${\mathrm{Fe}}_A^{3+}\to {\mathrm{Fe}}_A^{2+}$ and ${\mathrm{Fe}}_B^{2.5+}\to {\mathrm{Fe}}_B^{3+}$. There is an intermediate mixed configuration region at pressure-temperature $\left(P\text{−}T\right)$ conditions lying between those at which magnetite is normal, $T<T_{\mathrm{norm}}\left(P\right)$, or inverse, $T>T_{\mathrm{inv}}\left(P\right)$.

• Received 14 July 2006

DOI:https://doi.org/10.1103/PhysRevB.75.020102