Electronic and structural transitions in NdFeO$_3$ orthoferrite under high pressures

The effect of high pressure up to 65 GPa on the crystal structure and optical absorption spectra of NdFeO$_3$ orthoferrite single crystals is studied in diamond anvil cells. At $P\sim37.5\,$GPa, an electronic transition at which the optical absorption edge jumps from $\sim2.2$ to $\sim0.75\,$ eV is observed. The equation of state $V(P)$ is studied on the basis of the $X$-ray diffraction data obtained under pressure. This study reveals a first-order structural phase transition at $P\sim37\,$GPa with a jump of $\sim4\%$ in the unit cell volume. It is shown that the phase transition observed in rare-earth orthoferrites at 30-40 GPa is a transition of the insulator-to-semiconductor type.