Unusual Mobility of Cesium via a Reversible Topotactic Dehydration Reaction in a New Hydroxygallophosphate with an Intersecting Tunnel Structure

The investigation of the system Cs−Ga−P−O by hydrothermal synthesis has allowed a new hydrated hydroxygallophosphate with an intersecting tunnel structure, Cs2Ga6(OH)2(PO4)6·1.55H2O, to be synthesized. It crystallizes with a monoclinic symmetry in the P21/a space group with a = 10.2190(4) Å, b = 13.9565(15) Å, c = 17.260(2) Å, and β = 90.193(5)° (V = 2461.6(4) Å3, Z = 4). The detailed analysis of the structure shows that it consists of [Ga2(OH)P2O11] layers interconnected through [GaPO6] chains forming the host lattice [Ga6(OH)2(PO4)6], built up of PO4 and GaO4 tetrahedra and GaO4(OH) trigonal bipyramids. The structure presents a pseudo-orthorhombic symmetry, its monoclinic symmetry being imposed only by the configuration of the [GaPO6] chains and the positions of the Cs+ cation and H2O molecules sitting at the tunnels intersection. The electron microscopy and X-ray diffraction study versus temperature shows that this hydrated phase exhibits a topotactic dehydration toward Cs2Ga6(OH)2(PO4)6. The latter phase presents a Pcab orthorhombic symmetry and its framework is isotypic to that of (CH3NH3)2Ga6(OH)2(PO4)6. A remarkable displacement of one of the Cs+ cations, by about 3 Å, is observed during the dehydration. This reaction is reversible:  placed in water, the dehydrated phase gives back the hydrated one.