Massively defective crystalline solutions in fluorite-structure oxides: the systems ThO2-Ln2O3 (Ln=La3+, Gd3+, Yb3+)

Subsolidus phase relations have been defined in three representative rare earth sesquioxide (La₂O₃, Gd₂O₃, Yb₂O₃)-thoria systems over a wide range of compositions by X-ray methods (from phase detection and precision lattice parameter measurements). Characterisation of the point defects in these fluorite structure materials shows good agreement between calculated anion vacancy models and density values measured. Currently accepted crystal chemical generalisations were found to be inadequate to rationalise phase equilibria and crystalline solubility limits in these systems.