Polysomatism and structural complexity. Structure model for murataite-8C, a complex crystalline matrix for the immobilization of high-level radioactive waste

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) Aa, V = 59799(32) Aa³, 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, Al_25.44Ca_55.96Ti_282.20Mn_53.72Fe_17.24Zr_15.00Ho_{36 .64}O₈₂₃, is in good agreement with the empirical formula calculated from electronmicroprobe data, Al_23.02Ca_52.85Ti_284.10Mn_54.31Fe_17.59Zr_14.83Ho_{38 .04}O₈₂₃. 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.