Substitutional and positional disorder in Sr(2.88)Cu(3.12)(PO4)4.

The title compound, a hydrothermally synthesized strontium copper(II) phosphate(V) (2.88/3.12/4), is isotypic with Sr(3)Cu(3)(PO(4))(4), obtained previously by solid-state reaction, but not with Sr(3)Cu(3)(PO(4))(4), obtained previously by the hydrothermal method. A surplus of copper was observed by both structural and chemical analysis, and the formula obtained by the structural analysis is in full agreement with results of the EDX (energy-dispersive X-ray diffraction) analysis. The structure consists of layers of Cu(3)O(12) groups which are linked via the PO(4) tetrahedra. The Cu(3)O(12) groups are formed by one Cu1O(4) and two Cu2O(5) coordination polyhedra sharing corners. The central Cu1 atom of the Cu(3)O(12) group is located at an inversion centre (special position 2a). The unique structural feature of the title compound is the presence of 12% Cu in the Sr1 site (special position 2b, site symmetry 1). Moreover, disordered Sr2 atoms were observed: a main site (Sr2a, 90%) and a less occupied site (Sr2b, 10%) are displaced by 0.48 (3) A along the b axis. Such substitutional and positional disorder was not observed previously in similar compounds.

Reinvestigation of Sr2[Cu(OH)6].

During an investigation of the SrO-CuO-P2O5-H2O system, single crystals of distrontium hexahydroxidocuprate(II), Sr2[Cu(OH)6], were obtained by the hydrothermal method. The blue prismatic crystals of Sr2[Cu(OH)6] adopt the same structure type as Ba2[Cu(OH)6], Sr2[Zn(OH)6] and Ba2[Zn(OH)6]. The Cu atoms, located at (0, 0, 1/2) (site symmetry 1), form mutually isolated and highly elongated Cu(OH)6 octahedra, which are interconnected to slightly distorted Sr(OH)6 trigonal prisms, forming a layered structure. The location of H atoms from difference Fourier maps and their refinement allowed the precise determination of a three-dimensional hydrogen-bonding network in which all hydroxide O atoms are involved. In addition, the hydrogen-bonding topologies in Sr2[Cu(OH)6] and other similar hexahydroxidometallates with the general formulae M1[M2(OH)6], M1(2)[M2(OH)6] and M1(3)[M2(OH)6] were analysed in detail.

Sr, Ba and Cd arsenates with the apatite-type structure.

X-ray diffraction analysis of single crystals of three new arsenates adopting apatite-type structures yielded formula Sr(5)(AsO(4))(3)F for strontium arsenate fluoride, (I), (Sr(1.66)Ba(0.34))(Ba(2.61)Sr(0.39))(AsO(4))(3)Cl for strontium barium arsenate chloride, (II), and Cd(5)(AsO(4))(3)Cl(0.58)(OH)(0.42) for cadmium arsenate hydroxide chloride, (III). All three structures are built up of isolated slightly distorted AsO(4) tetrahedra that are bridged by Sr(2+) in (I), by Sr(2+)/Ba(2+) in (II) and by Cd(2+) in (III). Compounds (I) and (II) represent typical fluorapatites and chlorapatites, respectively, with F(-) at the 2a (0, 0, {1/4}) site and Cl(-) at the 2b (0, 0, 0) site of P6(3)/m. In contrast, in (III), due to the requirement that the smaller Cd(2+) cation is positioned closer to the channel Cl(-) anion (partially substituted by OH(-)), the anion occupies the unusual 2a (0, 0, {1/4}) site. Therefore, Cl(-) is similar to F(-) in (I), coordinated by three A2 cations, unlike the octahedrally coordinated Cl(-) in (II) and other ordinary chlorapatites. Furthermore, in (III), using FT-IR studies, we have inferred the existence of H(+) outside the channel in oxyhydroxyapatites and provided possible atomic coordinates for a H atom in HAsO(4)(2-), leading to a proposed formulation of the compound as Cd(5)(AsO(4))(3-x)(HAsO(4))(x)Cl(0.58)(OH)(0.42-x-(y/2))O(x+(y/2))(y/2).