Crystal pyrochlore

Perhaps the most important synthesis parameter affecting the crystallization of pyrochlore from alkaline solution is the oxidizing potential within the reaction medium. It is observed that this parameter has dramatic effects on both crystallinity and extent of ruthenium substitution by Pb . For example, a synthesis in which O2 is bubbled into the reaction medium will yield a well crystallized pyrochlore 2-3 times faster than a synthesis where O2 sparging is not provided. Crystalline pyrochlores cannot be obtained under any synthesis conditions (except those that are electrochemically assisted) when N2 sparging is used. The necessity for relatively oxidizing conditions in order to yield crystalline expanded pyrochlores is consistent with the hypothesis that these pyrochlores do... 

The compounds characterized by X Me = 3.5 have a common formula of M2Me205F2 and crystallize either in a pyrochlore [192] or a veberite [229] type structure. According to X-ray powder diffraction patterns, the structure of Na2Nb205F2 can be regarded as a super-structure of pyrochlore, which is made up of octahedrons connected in layers and arranged in the (111) direction. The layers are linked via octahedrons so that each octahedron in one layer shares three vertexes with an octahedron in the adjacent layer. 

Known oxyfluoroniobates (-tantalates) with compositions that correspond to X Me = 3 crystallize in typical structures of ReC>3, pyrochlore, and hexagonal and tetragonal tungsten bronze. Table 35 presents structural parameters of such compounds. 

The compounds of the MMe205F type, where Me = Nb or Ta M = Rb, Cs, Tl, crystallize in cubic symmetry and correspond to a pyrochlore-type structure [235-237]. This structure can be obtained from a fluorite structure by replacing half of the calcium-containing cubic polyhedrons with oxyfluoride octahedrons. 

Figure 6.5 Arrhenius plots of ln(

The gangue composition of the various carbonatite ores varies considerably. Calcite-dolomite content in some ores ranges from 30% (Niobec, Canada) up to 70% (Panda Hills, Africa). From a mineralogical point of view, pyrochlore usually occurs in crystallized form, as well as octahedron form. Pyrochlore occurs in considerable range of colours, varying from translucent white to opaque black appearance with glassy surfaces. The Nb205... 

For our present purposes, simple crystal structures might be segregated into three groups. The first of these is comprised of those few structures with even fewer parameters than independent bond lengths examples already encountered above include the (cubic) perovskite, pyrochlore and Na3Pt04 structures. 

Lumpkin, G. R., Ewing, R. C., Williams, C. T. Mariano, A. N. 2001. An overview of the crystal chemistry, durability, and radiation damage effects of natural pyrochlore. Materials... 

Mazzi, F. Munno, R. 1983. Calciobetafite (new mineral of the Pyrochlore Group) and related minerals from Campi Flegrei, Italy crystal structures of polymignite and zirkelite comparison with pyrochlore and zirconolite. American Mineralogist, 68, 262-276. 

In both cases, the replacement product is usually reported as having a distinctly fibrous or prismatic morphology. The higher level of radiation damage in metaloparite (lucasite ) could be due to a difference in the critical amorphization dose of loparite and the alteration product, provided that the alteration event occurred soon after crystallization of the loparite. For example, Smith et al. (1998) and Lumpkin et al. (1998) have shown that the critical amorphization dose of perovskite structure types may be as much as a factor of five greater than the critical dose of other Nb-Ta-Ti minerals (e.g., pyrochlore and zirconolite). 

Niobium occurs, usually with tantalum, in columbite Fe(NbC>3)2, (80% Nb205). pyrochlore (50% Nb2C>5). samarskne (50% Nb2Oj). chiefly found in western Australia, and South Dakota. Recovered along with tantalum by fusion with potassium bisulfate, and obtained in the residue after subsequent extraction with II20. Niobium and tantalum are separated by fractional crystallization uf the potassium fluorides, niobium concentrating in the mother liquid and tantalum in the crystals. 

The crystal structure of cadmium rhenium(V) oxide, as determined by single-crystal technique,1 is of the face-centered cubic pyrochlore type (a = 10.219 A.). The only positional parameter for the 48 (/) oxygens is x = 0.309 0.007 when rhenium is at the origin. The density, determined pycnometrically, is 8.82 0.03 g./cc., compared with the theoretical value of 8.83 g./cc. for Z = 8. The resistivity between 4.2 K and room temperature is very low (10-3-10-4 J2-cm.) and has a positive temperature coefficient. Over the same temperature range the magnetic susceptibility is low and temperature-independent. These properties indicate that cadmium rhenium(V) oxide exhibits metallic conductivity. 

Because the mineral scheelite and the related pyrochlore (Section 6.3.9) are important laser host materials for rare earth metal ions, many such compounds and their doped crystals have been investigated intensively. Compounds with the scheelite structure are given in Table 6.8. 

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