Crystal structures of high-pressure phases in the alumina-water system: I. Single crystal X-ray diffraction and molecular dynamics simulation of η-Al(OH)₃

The high-pressure phase of gibbsite has been studied by in situ single crystal X-ray diffraction method and molecular dynamics (MD) simulation at 3.0 GPa. The crystal structure of the high-pressure phase, η-Al(OH)₃, was successfully determined by direct methods based on the intensities of X-ray diffraction. The space group and lattice constants for η-Al(OH)₃ are P2₁/b11 (#14), a = 8.612(3) Å, b = 5.013(2) Å, c = 9.194(5) Å and a = 90.34(6)°, respectively. The crystal structure of η-Al(OH)₃ consists of an Al octahedral layer, and the layers are connected via H-bonds. The mechanism of the phase transition from gibbsite to η-Al(OH)₃ is also discussed in the context of previously reported powder X-ray diffraction data.