Synthesis of gadolinium-doped thorium dioxide via a wet chemical route: Limitations of the co-precipitation method

A wet chemical route (oxalate precipitation) was chosen to prepare gadolinium-doped thoria powders as a feed for pellet fabrication. The oxalate and derived oxide powders were characterized by thermogravimetric analysis coupled to evolved gas analysis, BET, XRD and SEM. Dilatometry was used to study the sintering behavior. From homogeneous solutions containing selected mixtures of gadolinium (III) and thorium (IV) nitrates solid precipitates were obtained, that were composed of two phases. Calcination at 700 °C or 1000 °C was insufficient to obtain single phases for any of the compositions. Rietveld analysis of calcined powders confirmed the presence of both a Th-rich phase (Th x Gd 1−x O 2− x /2 ) and a Gd-rich phase ((Gd y Th 1− y ) 2 O 3+(1− y ) ) in which limited substitution of the foreign element (Gd or Th, respectively) occurred. A fluorite-type solid solution formed during sintering at 1750 °C. The unit cell parameters of sintered pellets were determined for Th x Gd 1− x O 2−(1− x )/2 mixed oxides in the composition range: 0 ≤ x ≤ 0.30 and agree with reported values.