Electron transport in FeBO$_3$ ferroborate at ultrahigh pressures

The electrical resistance of FeBO$_3$ crystals at high and ultrahigh pressures (up to $198$ GPa) and low temperatures has been measured using diamond anvil cells. It has found that in the high-pressure phase, $46<P<100\,$GPa, the activation energy $E_{\rm ac}$ decreases gradually from $0.55$ to $0.3$ eV according to a linear law. Its extrapolation to zero gives an estimated value of about $210$ GPa for the pressure at which complete metallization is expected. However, above $100$ GPa, the linear $E_{\rm ac}(P)$E dependence smoothly transforms to a nonlinear one. At the same time, the temperature dependence of the electrical resistance at fixed pressure significantly deviates from the Arrhenius activation law and does not obey the Mott law for the hopping conductivity. Experimental data demonstrate the dependence of the activation energy $E_{\rm ac}$ both on pressure and temperature. At $T = 0$, the gap tends to zero. Theoretical analysis shows that the decrease in $E_{\rm ac}$ upon cooling can be interpreted in terms of the transition of the low-spin FeBO$_3$ phase to the magnetically ordered (antiferromagnetic) state.