CALCULATION OF STRUCTURAL PARAMETERS OF PYROCHLORINE PHASES FROM PREDICTABLE INTERATOMIC DISTANCES

Pyrochlores with the general formula A2B2X6Y (space group Fd-3m) form one of the largest and most compositionally flexible families of complex oxides, fluorides and chalcogenides, including phases of interest for the immobilization of radioactive waste. Their structure is described by only two parameters: the cubic lattice parameter a and the free coordinate x of the anion X at the crystallographic 48f site. All B–X and A–X bond lengths depend on these parameters, yet the coordinate x is difficult to refine reliably from laboratory X-ray diffraction data because of the weak scattering power of oxygen. In this work, analytical formulas are derived for calculating a and x from predicted (e.g., within the framework of the bond valence model) or experimentally determined interatomic distances. Two possible scenarios are considered: one in which the parameter a is known and the coordinate x is unknown, and one in which both structural parameters are unknown and only predicted or experimentally determined distances are available. The formulas were tested against published structural data and are recommended for building starting models and restraints for refinement of pyrochlore structures. At the same time, the choice of formulas should take into account the reliability of the predicted or experimentally determined distances used in the calculations.