Structural refinement of the high-pressure phase of aluminum trihydroxide : In-situ high-pressure angle dispersive synchrotron X-ray diffraction and theoretical studies

In-situ high-pressure synchrotron angle-dispersive X-ray diffraction for gibbsite (aluminum trihydroxide) was performed at room temperature up to 20 GPa. A pressure-induced phase transition was observed at 2.6 GPa. The new high-pressure phase can be recovered at ambient pressure. Rietveld refinement shows that the new phase of Al(OH) 3 has an orthorhombic structure, spacegroup Pbca, and the lattice parameters at ambient condition are a = 868.57(5) pm, b = 505.21(4) pm, c = 949.54(6) pm, V = 416.67(6) x 10 6 pm 3 with Z = 8. The compressibility of gibbsite and the high-pressure polymorph was analyzed, and their bulk moduli were estimated as 49.8 ′ 1.8 and 81.0 ′ 5.2 GPa, respectively. First-principles calculations of the high-pressure phase were performed to determine the hydrogen positions and to confirm the structural stability of the new phase.