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) ų, 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.