Structural Refinement of the High-Pressure Phase of Aluminum Trihydroxide: In-Situ High-Pressure Angle Dispersive Synchrotron X-ray Diffraction and Theoretical Studies

Cluster defects in gibbsite nanoplates grown at acidic to neutral pH

Gibbsite [α-Al(OH)₃] is the solubility limiting phase for aluminum across a wide pH range, and it is a common mineral phase with many industrial applications. The growth mechanism of this layered-structure material, however, remains incompletely understood. Synthesis of gibbsite at low to circumneutral pH yields nanoplates with substantial interlayer disorder. Here we examine defects in this material in detail, and the effects of recrystallization in highly alkaline sodium hydroxide solution at 80 °C. We employed a multimodal approach, including scanning electron microscopy, magic-angle spinning nuclear magnetic resonance (MAS-NMR), Raman and infrared spectroscopies, X-ray diffraction (XRD), and X-ray total scattering pair distribution function (XPDF) analysis to characterize the ageing of the nanoplates over several days. XRD and XPDF indicate that gibbsite nanoplates precipitated at circumneutral pH contain dense, truncated sheets imparting a local difference in interlayer distance. These interlayer defects appear well described by flat Al₁₃ aluminum hydroxide nanoclusters nearly isostructural with gibbsite sheets present under synthesis conditions and trapped as interlayer inclusions during growth. Ageing at elevated temperature in alkaline solutions gradually improves crystallinity, showing a gradual increase in H-bonding between interlayer OH groups. Between 7 to 8 vol% of the initial gibbsite nanoparticles exhibit this defect, with the majority of differences disappearing after 2-4 hours of recrystallization in alkaline solution. The results not only identify the source of disorder in gibbsite formed under acidic/neutral conditions but also point to a possible cluster-mediated growth mechanism evident through inclusion of relict oligomers with gibbsite-like topology trapped in the interlayer spaces.

Insights into the polymorphic transformation mechanism of aluminum hydroxide during carbonation of potassium aluminate solution

Polymorphic transformation from bayerite to gibbsite is discussed kinetically and structurally. Possibility of crossovers in Al(OH)₃ polymorphs stability is investigated for the first time.

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

The high-pressure phase of gibbsite has been studied byin situsingle 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 forη-Al(OH)3areP21/b11 (#14),a= 8.612(3) Å,b= 5.013(2) Å,c= 9.194(5) Å andα= 90.34(6)°, respectively. The crystal structure ofη-Al(OH)3consists of an Al octahedral layer, and the layers are connected via H-bonds. The mechanism of the phase transition from gibbsite toη-Al(OH)3is also discussed in the context of previously reported powder X-ray diffraction data.