Computer modeling and description of nonstoichiometric apatites Cd5-η/2(VO4)3I1-η and Cd5-η/2(PO4)3Br1-η as modified chimney-ladder structures with ladder-ladder and chimney-ladder coupling

Diffraction patterns from apatite-structure compounds Cd5-η/2(TO4)3X1-η with T=P, V and X=Br, I show sheets of diffuse scattering normal to c* at incommensurate l=q (q=1.63 for Cd-V-I apatite and q=1.78 for Cd-P-Br apatite), because the c repeat of the average unit cell is shorter than two X diameters. The equilibrium X..X spacing along c defines the incommensurate periodicity c/q and stoichiometry (1-η)=q/2. The layers show a honeycomb texture for the Cd-V-I apatite, which is condensed into discrete spots for the Cd-P-Br compound. In both phases, X..X repulsions along 〈100〉 force neighboring rods of X atoms out of phase. In the Cd-P-Br phase, additional 〈210〉 attractions drive incipient formation of a rhombohedral superstructure. Average structure site occupancies and the observation of second-order diffuse layers at both l=2q and l=q+2 imply the existence of strong Cd..X in addition to X..X interactions. A three-dimensional computer model was used to produce finite-temperature structure simulations as a function of X..X interactions along 〈001〉, 〈100〉, and 〈210〉, and X..Cd interactions, from which diffraction patterns were calculated. The experimental patterns were fit and approximate values for the interaction energies obtained (hundreds to thousands of joules per mole). It was apparent that lock-in to commensurability caused by the X..Cd term and the formation of nonprimitive incommensurate modulated structures driven by X..X interactions were mutually antagonistic, and the actual structures are compromises between the two

Structural studies of apatite-type oxide ion conductors doped with cobalt

A series of Co doped lanthanum silicate apatite-type phases, La9.83Si4.5Co1.5O26, La9.66Si5CoO26, La10Si5CoO26.5 and La8BaCoSi6O26, have been synthesised, and neutron diffraction, EXAFS and XANES used to investigate their structures in detail. All compositions were shown to possess the hexagonal apatite structure, and the results confirmed that cobalt can be doped onto both the La and Si sites within the structure depending on the starting composition. The Co doping is shown to cause considerable local distortions within the apatite structure. In the case of Si site doping two compositions showed anisotropic peak broadening, which has been attributed to incommensurate ordering of oxygen within the apatite channels.</p

Synchrotron X-ray absorption spectroscopy and X-ray powder diffraction studies of the structure of johnbaumite [Ca-10(AsO4)(6)(OH,F)(2)] and synthetic Pb-, Sr- and Ba-arsenate apatites and some comments on the crystal chemistry of the apatite structure type in general

The chemical composition oft he natural arsenate-apatite mineral johnbaumite [nominally Ca10(AsO4)6(OH)2] and its alteration product hedyphane [Ca4Pb6(AsO4)6Cl2] have been determined by electron microprobe analysis and the structures ofjohnbaumite and synthetic Sr-, Ba- and Pbarsenate apatites have been studied by As K-edge X-ray absorption spectroscopy and synchrotron X-ray powder diffraction. All samples belong to the holosymmetric apatite space group P63/m with As5+ substituted for P5+ in the tetrahedral structural site. Johnbaumite contains small amounts ofF and Pb (~0.9 and ~4.4 wt.% respectively) and hedyphane has the ideal composition (formula given above); the compositions ofthese coexisting phases define the two limbs ofa solvus occurring between Ca- and Pb-arsenate apatite end members. The unit-cell parameters and cation–oxygen bond lengths for the arsenate apatites studied are discussed alongside published data for end-member Ca-, Sr-, Ba- and Pbphosphate apatite analogues with (OH), F, Cl or Br as the anions at the centres ofthe channels in the apatite structure. This discussion rationalizes the relationships between the two structural sites A(1) and A(2) occupied by divalent cations in terms ofthe size ofthe A�O polyhedra and the distortion ofthe A(1)�O polyhedron as measured by the metaprism twist angle [O(1)�A(1)�O(2) projected onto (001)]