Structural and electronic transitions in gadolinium iron borate GdFe3(BO3)4 at high pressures

The optical properties and structure of gadolinium iron borate GdFe₃(BO₃)₄ crystals are studied at high pressures produced in diamond-anvil cells. X-ray diffraction data obtained at a pressure of 25.6 GPa reveal a firstorder phase transition retaining the trigonal symmetry and increasing the unit cell volume by 8%. The equation of state is obtained and the compressibility of the crystal is estimated before and after the phase transition. The optical spectra reveal two electronic transitions at pressures ∼26 GPa and ∼43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to ∼2.25 eV. Upon the second transition at P=43 GPa, the optical gap deceases down to ∼0.7 eV, demonstrating a dielectric-semiconductor transition. By using the theoretical model developed for a FeBO₃ crystal and taking into account some structural analogs of these materials, the anomalies of the high-pressure optical spectra are explained.