Complex pyrochlores

Another group of complex pyrochlores containing rare earth cations might be viewed as solid solutions between R Mf"O7 and A2 M 07 (table 4). 

Complex pyrochlores containing a rare earth cation. 

Concentrated sulfuric acid (97 wt %) at 300—400°C has been used to solubili2e niobium from columbite and pyrochlore (18,19). The exothermic reaction is performed in iron or siUcon-iron cmcibles to yield a stable sulfato complex. The complex is filtered free of residue and is hydroly2ed by dilution with water and boiling to yield niobic acid which is removed by filtration as a white coUoidal precipitate. 

Complex oxides Chromite niobite-tantalite pyrochlore ilmenite wolframite scheelite... 

Pyrimidinethione complexes, osmium, 37 296 Pyriminethionate, as bridging ligands in platinum dimers, 40 202-205 Pyrochlores, 17 108 Pyrococcus furiosus, 38 344-383 aldehyde ferredoxin oxidoreductase, 38 374-381... 

Carter, M. L., Vance, E. R., Mitchell, D. R. G., Hanna, J. V., Zhang, Z. Loi, E. 2002. Fabrication, characterisation, and leach testing of hollandite, (Ba,Cs)(Al,Ti)2Ti6016. Journal of Materials Research, 17, 2578-2589. Chakhmouradian, A. R. Mitciiiill, R. H. 1998. Lueshite, pyrochlore and monazite-(Ce) from apatite-dolomite carbonatite, Lesnaya Varaka complex, Kola Peninsula, Russia. Mineralogical Magazine, 62, 769-782. 

The elements themselves require little comment. Niobium is 10 to 12 times more abundant in the earth s crust than tantalum. The main commercial sources of both are the columbite-tantalite series of minerals, which have the general composition (Fe/Mn)(Nb/Ta)206, with the ratios Fe/Mn and Nb/Ta continuously variable. Niobium is also obtained from pyrochlore, a mixed calcium-sodium niobate. Separation and production of the metals is complex. Both metals are bright, high melting (Nb, 2468°C Ta, 2996°C), and very resistant to acids. They can be dissolved with vigor in an HN03—HF mixture, and very slowly in fused alkalis. 

Recently one of the solutions to overcome this problem has been proposed.This does concern surface modification of the pyrochlore-based oxide.s. It is known that cerium and zirconium chlorides provide vapor phase complexes with aluminum chloride at elevated temperatures.The new surface modification technique utilizes the formation of these vapor complexes to remove and modify the top surface of the pyrochlore ceria-zirconia solid solution. This method is named "chemical filing". Application of the above complexes formation has already been demonstrated for the vapor phase extraction and mutual separation of rare earths based on the so-called chemical vapor transport (CVT). ... 

Figure 3.9 shows a schemalic representation of the chemical filing process. The first step of the process involves the chlorination of the surface of the pyrochlore-based ceria-zirconia sample. The extent of the chlorination can be controlled by the concentration of the chlorine gas and/or chlorination time and the cerium and zirconium chlorides partially formed on the surface are vaporized and transported by the formation of gaseous complexes with aluminum chloride. This chemical filing process is carried out at 1273 K to stabilize the surface modification effects at high temperatures. A similar effect can also be achieved by chlorination with ammonium chloride followed by dominant vaporization of formed zirconium chloride. ... 

Zirconate pyrochlores have a low isomorphic capacity with respect tetravalent actinides. However the amount of isomorphic substitution can be increased by coupled substitution of Ca, according to the scheme 2REE = An" -i- Ca. Production of single phase ceramic can be attained only for actinide waste streams with rather simple chemical compositions. In the case of a complex waste stream composition, extra phases occur [129]. Finally, zirconate matrices require high temperatures for their synthesis. In order to prepare REE-zirconates from oxide mixtures, sintering at 1500-1600 for about 50 hours is required... 

Greegor RB, Lytle FW, Ewing RC, Haaker RF (1984) Ti-site geometry in metamict, armealed and synthetic complex Ti-Nb-oxides by X-ray absorption spectroscopy. Nucl Instr Meth B1 587-594 Greegor RB, Lytle FW, Chakoumakos BC, Lumpkin GR, Ewing RC (1985a) An investigation of metamict and armealed natural pyrochlore by X-ray absorption spectroscopy. Mater Res Soc Symp Proc 44 655-662... 

---