Synthesis and structural characterization of U-phase, [3Ca2Al(OH)6][Na(H2O)6(SO4)2·6H2O] layered double hydroxide

We have determined the crystal structure of U-phase, which is one of the calcium-aluminate layered double hydroxides (Ca–Al LDHs). Based on X-ray powder diffraction data, we derived the hydrogen-free structural model with space group R3 and hexagonal unit-cell dimensions a ​= ​0.99404(1) and c ​= ​3.00791(4) nm. The location of hydrogen sites was determined by calculation methods. The final structural model, with structural formula [3Ca2Al(OH)6][Na(H2O)6(SO4)2‧6H2O] (Z ​= ​3, a ​= ​0.99372 and c ​= ​2.82658 ​nm), was composed of [Ca2Al(OH)6]+ layers and [Na(H2O)6(SO4)2‧6H2O]3– interlayers. They were alternately stacked in c-axis direction and connected by hydrogen bonds to build the overall structure. The positions of Ca2+ in Ca(OH)64− octahedra were significantly shifted from the centroids of octahedra. Considering the coordination environment of divalent cations in Mn2+-Al LDH and Fe2+-Al LDH, the remarkable displacements of Ca2+ sites in the relevant octahedra would be one of the common structural characteristics of Ca–Al LDHs.