Structural Aspects

Layered solids are molecular crystals formed by the packing of giant planar macromolecules called layers or lamellae. The bonds between the atoms present in the layer are strong, primarily covalent, while those between the atoms of adjacent lamellae are weak, essentially of the van der Waals type. Thus, layered solids generally exhibit a high anisotropy in their physical properties. The reactivity of layered solids is shown by the intercalation reaction, that is, the reversible insertion of guest species into the interlayer region without appreciable modification of the structure of the lamellae which move apart to accommodate the guest species. Hence, the structural aspects of a layered solid are closely connected with the bidimensional structure of the layers. The Greek letter prefix that often indicates a layered phosphate is related to the layer structure. The structures of the layered phosphates listed in Table 1 will be illustrated with reference to the a-, y- and A-zirconium phosphates, but the phosphates of other elements have similar structures. Geometrical considerations indicate that bidimensional structures can be easily formed by concatenation through the vertices of MO6 octahedra (M being the polyvalent metal) of suitable dimension, and of PO4 tetrahedra. In the present case different concatenation gives rise to different layer structures.

Crystals of a-Zr(HPO4)2 • H2O are monoclinic with a = 9.060(2) A b = 5.297(1) A, c = 15.14(3) A, and P = 101.71(2) A, space group P21/n. The sequence of two layers is shown in Figure 1. Each layer may be described as the concatenation of ZrO6 octahedra and O3POH tetrahedra. Note that each tetrahedron bridges three different octahedra and these, in turn, bridge six tetrahedra. The layer is a planar macromolecule bearing acid P-OH groups on the

Figure 1 Computer-generated representation of the sequence of two layers of a-Zr(HPO4)2 • H2O. (Crystal data from Clearfield A and Smith GD (1969) Inorganic Chemistry 8: 431-436.)

Figure 2 Computer-generated representation of the sequence of two layers of HSb(PO4)2. (Crystal data from Piffard Y, Oyetola S, Courant S and Lachgar S (1985) JournalofSolid State Chemistry 60: 209-213.)

surfaces. The distance between adjacent phosphate groups on one side of the layer is 5.3 A and the 'free area' around each P-OH group is 24 A2. The inter-layer distance is 7.56 A and the arrangement of the pendant phosphate groups creates six-sided cavities, each containing one water molecule, in the interlayer region. This layered structure is common to the other a-layered phosphates and it is very similar to that of HSb(PO4)2, shown in Figure 2.

The second layer structure, in which two different tetrahedral species are used at the same time, is present in the y-compound with formula Zr(IV)(PO4)(H2PO4) • 2H2O. The y-layer consists of two ideal planes containing zirconium atoms bonded by tetrahedral PO4 and H2PO4 groups (see Figure 3). The PO4 group shares all four oxygens with zirconium atoms while the H2PO4 shares two oxygens with two different Zr atoms and points the remaining two OH groups towards the interlayer region. The interlayer distance is 12.2 A, and the free area surrounding the P(OH)2 groups on the surface of the layers is 35 A2.

A third layer structure of great interest can be formed by bridging four different zirconium atoms with a tetrahedral PO4 group in a slightly different manner from y-zirconium phosphate, and then by balancing the residual positive charge and completing the octahedral configuration of each zirconium atom with a monovalent anionic ligand, Cl~ and a neutral monodentate ligand, (CH3)2SO, as

Figure 3 Computer-generated representation of the sequence of two layers of y-Zr(PO4)(H2PO4) • 2H2O. (Crystal data from Christensen A, Andersen EK, Andersen IGK, Alberti G, Nielsen N and Lehman MS (1990) Acta Chemica Scandinavica 44: 865-872.)

illustrated in Figure 4. Note that this structure is essentially the same as that of layered vanadyl phosphate (see Figure 5) and of uranyl phosphate.

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