High‐pressure phase transition in Al(OH)₃: Raman and X‐ray observations

Abstract

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 diamond anvil cells at room temperature. More than 20 Raman‐active modes were observed at the ambient conditions and these modes were predominately distributed in two regions: the low‐wavenumber (LW, 200 to 1200 cm −1 ) and high‐wavenumber regions (HW, 3000 to 3700 cm −1 ). The wavenumber of Raman modes in LW region increases linearly with pressure while the wavenumber of those in the HW region decreases with pressure. The later observation indicates the enhancement of hydrogen bonds between 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‐pressure 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.