16c site

The structure of YCu4.74Pbo.26 is shown as an example in Figure 19. The copper atoms on the 16c site build up a three-dimensional network of corner-sharing tetrahedra. This network leaves voids of coordination number 16 which are filled by the rare earth and Pb/Cu atoms on the four-fold sites. The large coordination number of this void readily explains the higher lead content on this site. The atoms occupying the 16c site (mainly copper in the RCus-xPb series) have coordination number 12 in the form of a distorted icosahedron. For more crystal chemical details on the Laves phases and related compounds we refer to review articles (Simon, 1983 Nesper, 1991 Johnston and Hoffmann, 1992 Nesper and Miller, 1993). 

Spinels. The A[M2]04 spinels have a cubic-close-packed array of oxygen atoms with M-site cations in half of the octahedra, the 16d sites of space group Fd3m of Fig. 3.15. The [M2]04 array represents a strongly bonded framework with A atoms in the interstitial tetrahedral sites 8a. The empty 16c octahedral sites share faces with the 8a sites to form, with the 8a sites, a 3D-interconnected interstitial space. On LF-ion insertion into an empty 16c site, the coulombic repulsive force between the LF-ion and the near-neighbor A-site cations pushes all the A-site cations in a cascade into the 16c sites [18],... 

In the case of magnetite, Fe[Fe2]04, insertion of one Li per formula unit produces an ordered rock-salt array with Fe remaining on aU the 16d sites and Li and Fe randomly distributed over the 16c sites. Further insertion of Li extrudes Fe reversibly... 

For Ln = Sm to Lu, LnaTiaOy crystallizes in the pyrochlore/cubic structure in bulk. In this structure, the Ln cations occupy the 16d site and are coordinated with 8 oxygen ions, while Ti occupy the 16c site and are located at the center of the distorted octahedra (using Wyckoff notation). The pyrochlore/cubic structure is a superstructure of the ideal fluorite with 1/8 anion defect. The lattice parameter evolves from = 10.231(1) A [59,60] to 10.024 A [61] for Sm to Lu, respectively. 

Figure 2.37 The topology of the A (16d) and B (16c) sites in the pyrochlore structure. Both form a three-dimensional array of comer-sharing tetrahedra...

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