Modular nature of the polysomatic pyrochlore-murataite series

Synthetic analogues of murataite, a very rare mineral—a complex oxide of REE, actinide, Ti, Fe, and other elements,—are of great interest as confinement matrices of radioactive wastes. They are produced by sintering (1200–1300°C) and melting (1500–1600°C) with subsequent crystallization of the melt. Four structural varieties of murataite distinguished by unit-cell parameters have been established by TEM study. All these varieties are derivatives of the fluorite structure designated as murataite 3C, 5C, 7C, and 8C depending on repetition factor of a parameter of the fluorite subcell. The structural features of the synthetic murataite varieties are analyzed in this paper based on data obtained from high-resolution electron microscopy, microdiffraction, and X-ray refinement. The hypothesis of a modular structure of the members of polysomatic pyrochlore-murataite series has been confirmed. At the same time, the structural modules are zero-dimensional rather than two-dimensional as had previously been suggested. The combinations of zero-dimensional modules in 3D space create the entire structural diversity of the polysomatic series.