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 first-order 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 similar to26 GPa and similar to43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to similar to2.25 eV. Upon the second transition at P = 43 GPa, the optical gap deceases down to similar to0.7 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. (C) 2004 MAIK "Nauka/Interperiodica".