HIGH-PRESSURE AND HIGH-TEMPERATURE STUDIES ON OXIDE GARNETS

We report high-pressure and high-temperature studies on a series of ox ide garnets of chemical composition A(3)B(2)C(3)O(12). The members of this family investigated are gadolinium scandium gallium garnet (GSGG) , gadolinium gallium garnet (GGG), and yttrium aluminium garnet (YAG). The GSGG adn GGG are doped with both neodymium and chromium while the YAG is doped only with neodymium. Photoluminescence, synchrotron x-ra y-diffraction, and laser heating studies were carried out in a diamond -anvil cell. Variety of optical sensors (ruby, Sm-doped YAG) and x-ray pressure marker (copper) were employed for pressure measurement. Pres sure-induced amorphization was observed in GSGG at 58+/-3 GPa and GGG at 84+/-4 GPa by x-ray-diffraction studies. The photoluminescence stud ies show only gradual broadening of emission bands through the amorphi zation transition. On increasing pressure beyond amorphization, very b road and featureless emission bands were observed in the fluorescence spectra at 77+/-2 GPa for GSGG and at 88+/-2 GPa for GGG. Laser heatin g of the pressure induced amorphous phase in GSGG caused recrystalliza tion to the stable cubic phase. High-pressure x-ray study on YAG shows that it retains cubic phase up to 101+/-4 GPa. A pressure-volume rela tion for each member of the oxide garnet at ambient temperatures is pr esented, structural transformation mechanisms, and application of oxid e garnets as pressure sensors are also discussed.