Order-disorder in R₃TaO₇ (R=rare earth) phases

A series of 3R₂O₃·Ta₂O₅ (R₃TaO₇, R=rare earth) samples were synthesized in various conditions; i.e. solid-state reactions at 1350°C or 1700°C, melting and cooling, rapid quenching of the melt and annealing at 1700°C, etc. According to powder X-ray diffraction, the crystal structure of R₃TaO₇ is more symmetric and disordered for the smaller ionic radius of the R³⁺ ion, changing from weberite-type orthorhombic to pseudo-cubic to pyrochlore-type cubic to fluorite-type cubic. We prepared a series of R₃TaO₇ single crystals by a floating-zone method for further investigation of R₃TaO₇ structures. Although the powder X-ray diffraction patterns of R₃TaO₇ showed a cubic fluorite structure for the smaller R³⁺ ions, single-crystal X-ray diffraction gave diffraction spots forbidden by the fluorite face-centered-cubic structure. This observation shows that the R³⁺ and Ta⁵⁺ ions are ordered in R₃TaO₇ even for the smaller R³⁺ ions.