Coordination cluster shared octahedra

Some lower oxidation state oxoniobates contain the NbeO cluster shown in Fig. 10.1.4(b) as a basic structural unit. An O atom located above an edge of the Nb6 octahedron may belong to the cluster or connect with another cluster. Up to six O atoms can be attached to the apices of the Nb6 octahedron to constitute an outer coordination sphere such additional atoms may be exclusive to the cluster or shared between clusters. 

If two Type-I clusters exist in adjacent portions of the lattice then further vacancy ordering can occur around another octahedrally coordinated site that shares four faces with tetrahedral positions that are fully occupied in Li2ZnGe04. Two of these tetrahedra contain vacancies and the cations are displaced in a manner that suggests that they are not occupied simultaneously with the central octahedron. This arrangement of a central occupied octahedron linked via shared faces with two full and two empty tetrahedral interstices is a Type-II cluster. 

A plot of the raw data and the best fit in k and R space for the wider range has been included in Fig 5. The first peak in the Fourier transform may be attributed to the six Ti-0 bonds of the basic octahedron, as already predicted from the XANES data. The distance is the same that the short bond of the distorted octahedra in anatase. The peaks between 3 and 6 A are a complex result of the overlap of four different features. The first one, Ti-Ti at 3.09 A, is very similar to the distance observed in anatase between two octahedra sharing edges (3.04 A), while the next one, Ti-O at 3.78 A, is very close to the distance of the oxygen atoms in the second octahedron (3.86 A). The shell Ti-0 would correspond to oxygen atoms in a third octahedron in an anatase-like structure. The shell Ti-0 has no correspondence in a cluster of anatase structure including four octahedra. In relation with the similarities with the anatase strucutre in the other four shell, it has to be pointed out the low coordination number of the Ti-Ti bond at 3.09 A, as well as the lack... 

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