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)
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
References
[1]
E. R. Vance, C. J. Ball, M. G. Blackford, D. J. Cassidy, and K. L. Smith, “Crystallisation of zirconolite from an alkoxide precursor,” Journal of Nuclear Materials, vol. 175, no. 1-2, pp. 58–66, 1990.
[2]
A. E. Ringwood, S. E. Kesson, N. G. Ware, W. Hibberson, and A. Major, “Immobilisation of high level nuclear reactor wastes in SYNROC,” Nature, vol. 278, no. 5701, pp. 219–223, 1979.
[3]
G. R. Lumpkin, “Alpha-decay damage and aqueous durability of actinide host phases in natural systems,” Journal of Nuclear Materials, vol. 289, no. 1-2, pp. 136–166, 2001.
[4]
I. E. Grey, W. G. Mumme, T. J. Ness, R. S. Roth, and K. L. Smith, “Structural relations between weberite and zirconolite polytypes—Refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7,” Journal of Solid State Chemistry, vol. 174, no. 2, pp. 285–295, 2003.
[5]
P. P？ml, T. Geisler, and R. J. M. Konings, “High-temperature heat capacity of zirconolite (CaZrTi2O7),” Journal of Chemical Thermodynamics, vol. 38, no. 8, pp. 1013–1016, 2006.
[6]
R. W. Cheary and A. A. Coelho, “A site occupancy analysis of zirconolite ,” Physics and Chemistry of Minerals, vol. 24, no. 6, pp. 447–454, 1997.
[7]
International Atomic Energy Agency, “Design and operation of high level waste. Vitrification and storage facilit,” Technical Report Series 176, IAEA, Vienna, Austria, 1977.
[8]
A. E. Ringwood, Safe Disposal of High Level Nuclear Reactor Waste; A New Strategy, Australian National University Press, Canberra, Australia, 1978.
[9]
A. E. Ringwood, “Disposal of high-level nuclear wastes: a geological perspective,” Mineralogical Magazine, vol. 49, no. 2, pp. 159–176, 1985.
[10]
A. E. Ringwood and S. E. Kesson, Radioactive Waste Forms for the Future, vol. 235, Amsterdam, The Netherlands, 1988.
[11]
G. Leturcq, T. Advocat, K. Hart, G. Berger, J. Lacombe, and A. Bonnetier, “Solubility study of Ti, Zr-based ceramics designed to immobilize long-lived radionuclides,” American Mineralogist, vol. 86, no. 7-8, pp. 871–880, 2001.
[12]
R. W. Cheary, “An analysis of the structural characteristics of hollandite compounds,” Acta Crystallographica B, vol. 42, pp. 229–236, 1986.
[13]
S. E. Kesson and T. J. White, “Synroc-type hollandites, part I: phase chemistry,” Proceedings of the Royal Society of London A, vol. 405, pp. 73–101, 1986.
[14]
R. W. Cheary and R. M. Squadrito, “A structural analysis of barium magnesium hollandites,” Acta Crystallographica B, vol. 45, pp. 205–212, 1989.
[15]
E. R. Vance, C. J. Ball, R. A. Day et al., “Actinide and rare earth incorporation into zirconolite,” Journal of Alloys and Compounds, vol. 213-214, pp. 406–409, 1994.
[16]
P. Tropper, D. Rhede, and F. Bernhard, “Trace element mobility in contact metamorphic rocks from the Austroalpine basement: baddeleyite-zirconolite (-zircon) veins in marbles from the Stubenberg granite contact aureole (Styria, Austria),” European Geophysical Society, vol. 5, no. 2, article 236, 2003.
[17]
E. R. Vance, G. R. Lumpkin, M. L. Carter et al., “Incorporation of uranium in zirconolite (CaZrTi2O7),” Journal of the American Ceramic Society, vol. 85, no. 7, pp. 1853–1859, 2002.
[18]
O. P. Shrivastava and R. Srivastava, “Synthesis, characterization and leach rate study of polycrystalline calcium strontium titanate ceramic powder,” Progress in Crystal Growth and Characterization of Materials, vol. 45, no. 1-2, pp. 103–106, 2002.
[19]
O. P. Shrivastava and R. Shrivastava, “Synthesis and crystallochemical interaction of titania and zirconia based ceramic precursors,” Journal of the Indian Chemical Society, vol. 80, no. 4, pp. 373–378, 2003.
[20]
O. P. Shrivastava, N. Kumar, and I. B. Sharma, “Solid state synthesis and structural refinement of polycrystalline LaxCa1-xTiO3 ceramic powder,” Bulletin of Materials Science, vol. 27, no. 2, pp. 121–126, 2004.
[21]
O. P. Shrivastava, N. Kumar, and I. B. Sharma, “Synthesis and structural refinement of polycrystalline ceramic powder Pr0.1Ca0.9TiO3,” Materials Research Bulletin, vol. 40, no. 5, pp. 731–742, 2005.
[22]
Powder diffraction file no. 84-0163, compiled by JCPDS, International Center for Diffraction Data USA, 2000.
[23]
C. H. MacGillavry and G. D. Rieck, Eds., International Tables for X-Ray Crystallography, vol. 3, Kynoch Press, Birmingham, UK, 1974.
[24]
R. Gieré, C. T. Williams, and G. R. Lumpkin, “Chemical characteristics of natural zirconolite,” Schweizerische Mineralogische und Petrographische Mitteilungen, vol. 78, no. 3, pp. 433–459, 1998.
[25]
L. W. Coughanour, R. S. Roth, S. Marzullo, and F. E. Sennet, “Solid-state reactions and dielectric properties in the system magnesia-lime-tin oxide-titania,” JournaI of Research of the National Bureau of Standards, vol. 54, no. 3, pp. 191–199, 1955.
[26]
B. M. Gatehouse, I. E. Grey, R. J. Hill, and H. J. Rossell, “Zirconolite, CaZrxTi3？xO7; structure refinements for near-end-member compositions with and 1.30,” Acta Crystallographica B, vol. 37, part 2, pp. 306–312, 1981.
[27]
A. Bohre and O. P. Shrivastava, “Diffusion of lanthanum into single-phase sodium zirconium phosphate matrix for nuclear waste immobilization,” Radiochemistry, vol. 55, no. 4, pp. 442–449, 2013.
