%0 Journal Article
%T Synthesis, Characterization, and Crystal Structure Refinement of Lanthanum and Yttrium Substituted Polycrystalline 2M Type Zirconolite Phases: Ca1-xMxZrTi2O7 (M = Y, La and x = 0.2)
%A Ashish Bohre
%A Kalpana Avasthi
%A O. P. Shrivastava
%J Journal of Powder Technology
%D 2014
%I Hindawi Publishing Corporation
%R 10.1155/2014/902317
%X Solid phases of zirconolite-2M with composition Ca0.8M0.2ZrTi2O7 (M = La, Y) have been synthesized through ceramic route and their structures refined to a satisfactory convergence using Rietveld analysis. Zirconolites crystallize in space group C2/c with Z = 8. The powder diffraction data of Ca0.8Y0.2ZrTi2O7 (CZTY) and Ca0.8La0.2ZrTi2O7 (CZTLa) have been subjected to General Structural Analysis System software to arrive at a satisfactory structure fit with Rp = 0.1128 and Rwp = 0.1805 for CZTY and Rp = 0.1178 and Rwp = 0.1874 for CZTLa, respectively. The unit cell parameters are a = 10.1708 (6) £¿, b = 6.2711 (4), and c = 11.2779 (6) £¿ for CZTY and a = 11.2548 (6) £¿, b = 6.2601 (4), and c=11.2606 (7) £¿ for CZTLa. Calculated interatomic distances and bond angles are in good agreement with their standard values. Particle size along prominent reflecting planes calculated by Scherrer¡¯s formula ranges between 67 and 107£¿nm. The polyhedral (CaO8, ZrO7, and TiO6/TiO5) distortions and valence calculation based on bond strength analysis have been reported. The compositions of the zirconolites were determined using energy dispersive X-ray (EDAX) analysis. Cation site occupancies were determined by applied compositional constraints which were found consistent with the expected zirconolite-2M cation site occupancies. 1. Introduction Borosilicate glass has long been the first choice of material for the immobilization of high-level radioactive wastes (HLW) due to its good glass-forming ability, chemical durability, radiation stability, and so forth. However, there is a potential risk that the conventional borosilicate glass waste forms partially crystallize either during annealing or during long-term storage in deep geological environment [1, 2]. Another proposed method for the long-term storage of high-level radioactive waste is the encapsulation of the waste in a ceramic matrix, followed by storage in an underground repository. One such ceramic, which has many of the properties thought to be favorable for waste storage, is zirconolite (CaZrTi2O7). Zirconolite is a rare accessory mineral found in a wide range of rock types and geological environments. The chemical composition of natural zirconolite can vary extensively, with the main substitutions involving lanthanides (Ln), actinides (Act), Nb, and Fe. Zirconolite is one of the three major phases in the synthetic ceramic ¡°Synroc¡± which is evaluated as a host for high-level radioactive waste [3]. Ceramics made from pyrochlore, titania, and zirconia have been suggested as potential waste forms because they seem to be
%U http://www.hindawi.com/journals/jpt/2014/902317/