Structure zircon

This is nicely confirmed by a study of some Eu +-activated phosphates and vanadates with zircon structure (33). In Table 6 we give the observed ratio of... 

All the orthovanadates with zircon structure and formula LnV04 where Ln = Ce, Pr, Nd, Sm, Eu, and Gd, which have been studied up to now, have very similar spectra. The vs vibration which is predicted to spUt into two bands from the site symmetry treatment appears in the IR as one strong, broad band with a weak shoulder. On the other hand, Vi appears as a sharp band. CaCr04, which has the same structure, possesses a very similar spectrum. 

Luminescence spectra at low temperature, together with very short decay times are rather characteristic for Cr " luminescence, which was not known until recently, and which is especially similar to the luminescence of CrO " doped YPO4 with zircon structure (Brunold et al. 1997). At 15 K it also consists... 

Another example is the luminescence of in YPO4 with tetragonal zircon structure (Oomen et al. 1988). The emission consists of two bands, one in the UV region and one in the visible part of the spectrum. The intensity ratio of these bands is strongly temperature dependent (Fig. 5.57). 

The rutile and zircon structures are considered together as they have related cation... 

Fig. 39 a, b. A plane of (a) the rutile structure (large circles Ti, small circles O) and (b) the zircon structure (large open circles Zr, large filled circles Si, small circles O)... 

Table 4. Observed and calculated parameters and distances (in A) in the zircon structure...

Zirconia. Zircon (zirconium silicate), the most widely occurring zirconium-bearing mineral, is dispersed in various igneous rocks and in zircon sands. The main deposits are in New South Wales, Australia Travancore, India and Florida in the United States. Zircon can be used as such in zircon refractories or as a raw material to produce zirconia. The zircon structure becomes unstable after about 1650°C, depending on its purity, and decomposes into Zr02 and Si02 rather than melting (see Zirconiumand zirconium compounds). 

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of 3+p5 xv5+o compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO4) have the monoclinic Monazite structure (P2 /n) for A La, Ce-Gd, and the tetragonal Zircon structure (l4]/amd) for A Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO, and the Zircon structure for A = Ce - Lu. When the end members are iso-structural, e.g., LaPO /LaVO, Monazite, YbPC /YbVOA,... 

The zircon structure-type (ZrSi04) [49] is also adopted occasionally by so-called ternary compositions (Table 3) [38], Zircon is isostructural with the xenotime series of phosphates, typified by xenotime-(Y), YPO4 [50, 51], In this structure-type, the eight-coordinate polyhedra (snub disphenoids) share edges to form a framework composed of cross-linked chains that extend along the [100] and [010] directions. The framework of polyhedra exhibits prominent voids along the [111] direction these are occupied by tetrahedra. The tetrahedra share edges with the polyhedra and thus serve to further connect the chains (Fig. 7). 

Zircon-type superstructures apparently occur in actinide arsenates as a result of cation ordering when both monovalent alkali metals and tetravalent actinides occupy polyhedral sites (Table 3). Cation ordering is also a factor in the orthorhombic symmetry of the low-temperature polymorph of CaU(P04)2 [41], shown in Figure 8, and which is closely related to the zircon structure. 

At Ttetragonal symmetry (zircon structure) monoclinic symmetry... 

All the studied compounds have the tetragonal zircon structure (spatial group ... 

The tantalum ion in TaBO4 occurs in eight coordination (zircon structure. Ref. 59). This is exceptional, not only for tantalum, but for the whole class of complexes under study. The luminescence of TaBO4 has been reported The quenching temperature is low (Tq < 80 K). The emission consists of a complicated broad band in the blue-white spectral region. The Stokes shift is very large (19000 cm ). Further details are lacking. 

The structural properties of several mixed RE-orthophosphates were also investigated by Mullica et al. (1986, 1990, 1992, 1996). In the case of 1 1 Gd/Yb orthophosphate (Mullica et al. 1986), the structure was found to be of the tetragonal zircon structure type with a = 6.865(2) A, c = 6.004(2) A, and unit cell volume = 283.0(3) A. Structural refinements were also carried out for Gd/Er-, Gd/Y-, and Gd/Yb-orthophosphates with a 1 1 RE ratio and for (Gd/Yf))P04 and with a 75 25 ratio of Gd toYb (Mullica et al. 1990). The mixed systems 1 1 (Gd/Tb)P04, 3 l(Gd/Tb)P04 and 9 1 (Lu/Tb)P04 were also investigated (Mullica et al. 1992) as were seven mixed compounds in the (Ce/Tb), (Nd/Tb), and (Sm/Tb)P04 families of orthophosphates (Mullica et al. 1996). All of the mixed orthophosphates investigated by Mullica et al. were found to crystallize with the tetragonal zircon structure. At this time, a considerable body of structural and crystal chemical data exists for the synthetic RE, Y, and Sc orthophosphates as evidenced by the previously cited references of Mullica et al., Milligan et al., and Ni et al. to which the reader is referred for details. Additionally the related structural investigations of Chakoumakos et al. (1994) should be noted. In these studies, structural refinements of the zircon-type RE-, Y-, and Sc-orthovanadates are reported. 

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