Spin crossover and the magnetic $P-T$ phase diagram of hematite at high hydrostatic pressures and cryogenic temperatures

The magnetic properties of the $\alpha$-Fe$_2$O$_3$ hematite at a high hydrostatic pressure have been studied by synchrotron Mössbauer spectroscopy (nuclear forward scattering (NFS)) on iron nuclei. Time-domain NFS spectra of hematite have been measured in a diamond anvil cell in the pressure range of $0$–$72$ GPa and the temperature range of $36$–$300$ K in order to study the magnetic properties at a phase transition near a critical pressure of $\sim50$ GPa. In addition, Raman spectra at room temperature have been studied in the pressure range of $0$–$77$ GPa. Neon has been used as a pressure-transmitting medium. The appearance of an intermediate electronic state has been revealed at a pressure of $\sim48$ GPa. This state is probably related to the spin crossover in Fe$^{3+}$ ions at their transition from the high-spin state (HS, $S= 5/2$) to a low-spin one (LS, $S=1/2$). It has been found that the transient pressure range of the HS-LS crossover is extended from $48$ to $55$ GPa and is almost independent of the temperature. This surprising result differs fundamentally from other cases of the spin crossover in Fe$^{3+}$ ions observed in other crystals based on iron oxides. The transition region of spin crossover appears because of thermal fluctuations between HS and LS states in the critical pressure range and is significantly narrowed at cooling because of the suppression of thermal excitations. The magnetic $P-T$ phase diagram of $\alpha$-Fe$_2$O$_3$ at high pressures and low temperatures in the spin crossover region has been constructed according to the results of measurements.