Fergusonite structure

As far as we know the tetrahedral niobate group occurs only in the fergusonite structure of YNbO4 which is a distorted version of the scheelite structure of CaMo04 This luminescence is bright blue and has a relatively high Tq, viz. 500 jfg decay time is 15 MS at 11 which is short in comparison with the plateau value for the molybdate tetrahedron. It is very unfortunate that no more data exist on this complex with its efficient luminescence. 

The fergusonite structure is the normal low-temperature form found for the rare earth orthoniobates (except Sc ) and for the orthotantalates of the... 

Fig. 75. The crystal structure of fergusonite- 8-(Y). The coordination polyhedra of Nb atoms are drawn as tetrahedra (left) and octahedra (right), respectively.

Two phases, a high-temperature (fergusonite-(Y)) T-phase with tetragonal symmetry and a low temperature M-phase (fergusonite-)S-(Y)) with monoclinic symmetry, are known for fergusonite. Komkov (1959) determined the crystal structures of both phases. Later, the crystal structure of the M-phase was refined independently by Weitzel and Schrocke (1980) and by Trunov et al. (1981). Their unit cells were different from those of Kinzhibalo et al. (1982) a = 7.037, b = 10.945, c = 5.298 A, i = 134.07° (Trunov et al. 1981). 

The eight-coordination polyhedra include a bicapped octahedron (f), a cube (g), a Djd (or d)-dodecahedron (h), a square antiprism (i) or a bicapped trigonal prism (j). The bicapped octahedron (f) is found in the structures of vitusite-(Ce) and lucasite-(Ce). Fergusonite- -(Y), euxenite-(Y), fersmite and the zirconolite-group minerals have distorted cubic coordinations of the rare earth sites in their structures. The cube (g) can be derived from a distorted bicapped octahedron (f) by flattening the six pairs of triangles to squares. The D2d (or J)-dodecahedron (h) is one of the most... 

Minerals with crystal structures containing anionic groups of octahedral ions for example, fergusonite-(Y), euxenite-(Y). 

A monoclinic form of scheelite has been described by Komkov (1959). This structure is known as fergusonite and belongs to space group 12. As a distorted form of scheelite its lattice parameters are similar to those of scheelite for YTa04 these are a = 5.34 A, b = 10.94 A, c = 5.07 A, /3 = 95.3° (Ferguson, 1957). Wolten and Chase (1967) have described this phase in terms of cation nets perpendicular to the scheelite c-axis. In scheelite these nets are square and flat, in fergusonite they are puckered and distorted. 

Fig. 28.67. Projection of the structure of M -fergusonite along [010] showing its relationship to wolframite. Different cation types are indicated by filled and open circles and different heights of these by shading of the polyhedra. Two rows of MOe octahedra and one row of MO4 tetrahedra are shown.

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