Transport and optical properties of iron borate FeBO$_3$ under high pressures

The optical absorption spectra of iron borate FeBO$_3$ were measured in diamond anvil cells at high pressures up to $P=82\,$GPa. The electronic transition with an abrupt jump in the absorption edge from $\sim 3$ to $0.8\,$eV was observed at $P\approx 46\,$GPa. The resistance and its temperature dependence were directly measured for FeBO$_3$ at high pressures up to $140\,$GPa. It was established that the electronic transition at $P\approx 46\,$GPa was accompanied by the insulator-semiconductor transition. In the high-pressure phase, the thermoactivation gap decreases smoothly at $46<P<140\,$GPa approximately from $0.55$ to $0.2\,$eV following the linear law. The extrapolated value of the pressure at which the sample becomes fully metallic is equal to about $210\,$GPa.