Magnetic collapse and the change of electronic structure of FeBO$_3$ antiferromagnet under high pressure

The effect of high pressures up to 60  GPa on single-crystal and polycrystalline samples of iron borate $^{57}$FeBO$_3$ was studied by Mössbauer absorption spectroscopy ($^{57}$Fe nuclei) in a diamond anvil cell. Magnetic field $H_{\rm hf}$ at the$^{57}$Fe nuclei increases with pressure but abruptly drops to zero at $46\pm2\,$GPa, indicating the crystal transition from the antiferromagnetic to nonmagnetic state. This is accompanied by an abrupt change in the isomer shift and quadrupole splitting. Their values in the high-pressure phase are evidence for the transition of Fe$^{3+}$ ions from a high-spin ($S = 5/2$, $^6$А$_{1g}$) to low-spin ($S=1/2$, $^2$T$_{2g}$) state (spin crossover). This correlates with an abrupt decrease in the unit-cell volume (by $\sim9\%$) and optical gap. The change of the magnetic and electronic structures is explained by Mott's transition with rupturing of strong $d-d$-electron correlations.