Weberite structure

Figure 29 (a) Kagome net formed by [M Fe] (bright) and [M "Fg] (dark) octahedra (b) O-I weberite structure of Na2NiAlp7. View along the M" octahedral chains, Na grey spheres... 

Na2NiAlp7 300- -100 3.1 0p= —25 K Parad weberite structure one-dimensional anti- 73H8,... 

Figure 20.30 A schematic representation of weberite structure of M2Sb207 (M = Ca, Sr) and two kinds of SbOg octahedra...

The MRVjOy (M = alkali metal) compounds have two structure types pyro-chlore and weberite (Chincholar, 1972). The pyrochlore structure is formed with lithium and for smaller rare earths also with sodium. The potassium compounds and sodium compounds with larger rare earth ions seem to have weberite structure, which is an orthorhombic distortion of cubic pyrochlore (Gabe and Chincolkar, 1979). The corresponding pentavalent niobates and tantalates have similar structural behaviour to the vanadates. 

Pyrochlore- and RbNiCrFg-Type Structures. Closely related to the weberite variants are pyrochlores AA M2Fv (A = Na, A = Ca, Cd M = Mg, Co, Ni, Cu, Zn). Here, all octahedra are equivalent and the cubic 3D network can be described as intersecting systems of Kagome nets with planes parallel to the faces of a tetrahedron. Another approach is the description as cross-linked octahedral chains where at the crossing points typical tetrahedra of comer-connected octahedra are generated (Figure 31). 

Recent work by Allpress and Rossell (1978) and Rossell (1978) has shown that all rare earths other than scandium form niobates and tantalates with an O/M ratio of 1.75. These compounds are superstructures of fluorite they are orthorhombic wRh probable space groups Cmcm for the La compound and C222 for those containing Nd, Gd, Ho and Y. Both structures are very similar to each other and to both pyrochlore and weberite they are related to fluorite by a = [200]f, b = [011]f, c = [0T1]f. While the fluorite cation array is again intact, the anion arrangement results in the formation of slabs within which the cations all have cubic or... 

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