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) Å3, 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, Al25.44Ca55.96Ti282.20Mn53.72Fe17.24Zr15.00Ho36.64O823, is in good agreement with the empirical formula calculated from electron-microprobe data, Al23.02Ca52.85Ti284.10Mn54.31Fe17.59Zr14.83Ho38.04O823. 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.

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