Irreversible electronic transition with possible metallization in Y$_3$Fe$_5$O$_{12}$ at high pressure

The effect of high pressure up to $70$ GPa on the optical absorption spectra in yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ single crystals was studied in diamond anvil cells. In the pressure range $40\div50\,$GPa, an electronic transition with drastic decrease of optical gap from $\sim2.3\,$eV to nearly zero value was observed implying possible metallization. Final drop of the optical gap to zero is extrapolated at about $55$ GPa. It is shown that the phase transition in yttrium iron garnet observed at $40$–$55$ GPa is possibly a transition of the insulator-metal type. At decompression, back transition to insulating state is not completely reversible. Final spectrum at ambient pressure is essentially different from initial one and corresponds to the optical gap of about $1.8$ eV while in the initial state the gap value is $\sim2.6\,$eV.