The structure of the fluorite-related phase Ca2Hf7O16

The atomic arrangement in the fluorite-related phase, Ca₂Hf₇O₁₆, has been determined by powder X-ray diffraction. The unit cell is rhombohedral, $\text{R}\text{3}$, with a = 9.5273Å, α = 38.801°, and Z = 1, and its volume is $\text{2}\text{1}\text{4}$ times that of the fluorite subcell from which it is derived. The cations are ordered on the cation sites of the fluorite structure with the calcium ions segregated into discrete layers parallel to the (111) fluorite plane: there is some evidence that the formal anion vacancies are also ordered.