HIGH-PRESSURE PHASE-TRANSITION IN AL(OH)(3) - RAMAN AND X-RAY OBSERVATIONS

High-pressure Raman spectra and X-ray diffraction patterns of gibbsite have been investigated up to 23.3 GPa and 8.7 GPa, respectively in di amond anvil cells at room temperature. More than 20 Raman-active modes were observed at the ambient conditions and these modes were predomin ately distributed in two regions: the low-wavenumber (LW, 200 to 1200 cm(-1)) and high-wavenumber regions (HW, 3000 to 3700 cm(-1)). The wav enumber of Raman modes in LW region increases linearly with pressure w hile the wavenumber of those in the HW region decreases with pressure. The later observation indicates the enhancement of hydrogen bonds bet ween octahedral layers in the lattice upon compression. A significant change in the diffraction patterns and Raman spectra was observed for Al(OH)(3) above 3 GPa where a phase transition takes place. The high-p ressure phase of Al(OH)(3) is quenchable and is tentatively determined as nordstrandite, a triclinic polymorph of gibbsite, on the basis of its diffraction pattern and Raman spectrum.