Polysomatism and structural complexity: structure model for murataite-8C, a complex crystalline matrix for the immobilization of high-level radioactive waste
Pakhomova, Anna S.; Krivovichev, Sergey V.; Yudintsev, Sergey V.; Stefanovsky, Sergey V.
European Journal of Mineralogy Volume 28 Number 1 (2016), p. 205 - 214
published: Mar 1, 2016
ArtNo. ESP147052801018, Price: 29.00 €
Murataite-8C, a prospective synthetic material for the long-term immobilization of high-level radioactive waste and a member of the pyrochlore–murataite polysomatic series, was investigated by means of single-crystal X-ray diffraction. The crystal structure (cubic, F-43m, a = 39.105(12) Å, V = 59799(32) Å 3, Z = 4) is of outstanding complexity and contains forty symmetrically independent cation sites. Its crystal-chemical formula determined on the basis of the crystal-structure refinement, Al25.44Ca55.96Ti282.20Mn 53.72Fe17.24Zr15.00Ho 36.64O823, is in good agreement with the empirical formula calculated from electron-microprobe data, Al23.02Ca 52.85Ti284.10Mn54.31Fe 17.59Zr14.83Ho38.04O823. The crystal structure is based upon a three-dimensional octahedral framework that can be described as an alternation of murataite and pyrochlore modules immersed into a transitional substructure that combine elements of the crystal structures of murataite-3C and pyrochlore. The obtained structural model confirms the polysomatic nature of the pyrochlore–murataite series and illuminates the chemical and structural peculiarities of crystallization of the murataite-type titanate ceramic matrices. The high chemical and structural complexity of the members of the pyrochlore–murataite series is unparalleled in the world of crystalline materials proposed for the high-level radioactive waste immobilization, which makes it unique and promising for further technological and scientific exploration.