In this paper, we report the structural and compostional variation of Ce 2 Zr 2 O 7 pyrochlore under a controlled oxidation/reduction process. On oxidation Ce 2 Zr 2 O 7 transforms to Ce 2 Zr 2 O 8 via an intermediate lattice at Ce 2 Zr 2 O 7.5 . Crystal structures of the Ce 2 Zr 2 O 7 , Ce 2 Zr 2 O 7.5 , and Ce 2 Zr 2 O 8 have been determined accurately from the neutron diffraction data of polycrystalline samples. The retention of cubic pyrochlore-type arrangements is observed even up to the fully oxidized Ce 2 Zr 2 O 8 composition. The unit cell parameters systematically decreased from Ce 2 Zr 2 O 7 (10.6924(3) A ˚) to Ce 2 Zr 2 O 8 ( 10.5443(2) A ˚). All three compositions retain the original pyrochlore-type cation ordering without any intermixing Ce and Zr atoms. All three structures can be explained as cubically coordinated metal atoms, which share the edge of their AO 8 polyhedra similar to that in the fluorite lattice. The transformation from Ce 2 Zr 2 O 7 to Ce 2 Zr 2 O 8 systematically lowers the symmetry from Fd3m to P2 1 3. A complete oxidation of Ce 3þ to Ce 4þ is observed, in contrast to the kinetically and sterically hindered oxidation proposed earlier. The deep crystallographic insights associated with the oxygen intercalation and deintercalation process in Ce 2 O 3 /CeO 2 -ZrO 2 will have immense importance in the development and understanding of new oxygen storage capacitors.

No abstract

Physico-chemical changes accompanying the irradiation of ammonium molybdophosphate (AMP) with electrons up to a dose of 200 MGy have been studied by X-ray di †raction, SEM, EDX, thermal analyses, EPR and NMR measurements. The ion exchange characteristics of the control and irradiated AMP with respect to Cs`have been studied using simulated acidic nuclear waste solutions in a column. Apparent equilibrium constants for the Cs`ÈH`exchange have been calculated from breakthrough curves. Structural breakdown occurred on irradiating AMP to a dose of 100È200 MGy and was partially converted to and MoO 3 ammonium molybdate. Although the useful exchange capacities of the irradiated AMP were reduced, the apparent ion exchange selectivity for Cs`(with respect to H`) improved as compared to the control AMP.

Double perovskite type compounds of the formula BaA'LaTeO6 (A' = Na, K, Rb) were synthesized by solid state route and their crystal structures were determined by Rietveld analysis using powder X-ray diffraction and neutron diffraction data. Na compound crystallizes in the monoclinic system with P2₁/n space group whereas, K and Rb compounds crystallize in Fm3m space group. All the three compounds show rock salt type ordering at B site. Crystal structure analysis shows that La ion occupies A site in Na compound whereas, it occupies B site in K and Rb compounds according to the general formula of AA'BB'O6 for a double perovskite type compound. Effect of this crystallographic site swapping of the La ion was also observed in the photoluminescence study by doping Eu(3+) in La(3+) site. The large decrease in the intensity of the electric dipole ((5)D0-(7)F2) transition in the Rb compound compared to the Na compound indicates that Eu(3+) ion resides in the centrosymmetric octahedral environment in the Rb compound.

Double perovskite antimonates of the type BaLaMSbO 6 (M=Mg, Ca) were synthesized by a standard solid-state route. The compounds were characterized by X-ray crystallography and the structures were refined using Rietveld method. BaLaMgSbO 6 and BaLaCaSbO 6 crystallized in monoclinic space groups (I2/m) and (P2 1 /n), respectively. In both compounds, La occupied the A-site of perovskite, which is 12-coordinated as compared to Ba 2 LaSbO 6 where La ion shifts to the B-site octahedral coordination due to the larger size of Ba as compared with Mg and Ca. The samples were further characterized using FTIR and the frequency of the octahedral vibration is correlated to the electronegativity of the B-site ions. Photoluminescence study of the title compounds and Ba 2 LaSbO 6 was carried out upon doping with 2 atom% Eu 3 + ion, which confirmed that Eu 3 + occupies distorted 12-coordinated A-site in BaLaMSbO 6 (M=Mg, Ca) and symmetrical octahedral B-site in Ba 2 LaSbO 6 . Furthermore, the emission spectrum corresponding to each Eu 3 + ion at different crystal site was successfully isolated through a TRES study. This site selective occupancy of Eu 3 + ion also has a direct impact on the light emission, which was found to change from orange to red in a dark room in the order Ba 2 LaSbO 6 : Eu! BaLaCaSbO 6 : Eu!BaLaMgSbO 6 : Eu. Such an outcome will have high impact in designing new commercial Eu 3 + ion doped phosphor materials and tailoring of their optical properties.