EXPERIMENTAL-MEASUREMENT OF PRE-MELTING AND MELTING OF THORIUM-DIOXIDE

Thorium dioxide samples have been submitted to homogeneous four-beam l aser pulses in an autoclave filled with different gases. Phase transit ions were analysed through the produced thermal arrest stages, which w ere detected by a precise multichannel pyrometer. The spectral emissiv ity of the samples, measured on line during heating up to 4000 K, is a lso reported in the paper. Thoria exhibits a melting point at 3651 K, and a pre-melting transition at 3090 K. These temperatures are weakly influenced by chemical reduction of the sample. Whilst virtually stoic hiometric thoria melts congruently, in the hypostoichiometric oxide, t he liquidus and solidus temperatures could be clearly observed at O/Th = 1.98. The analysis of the measured data and, in particular, of the heat capacity shows that the features of the pre-melting transition ca n be described by a Debye-Huckel oxygen defect model, where co-operati ve effects lead, in the oxygen sublattice, to a critical order-disorde r transition, whose thermodynamic nature depends on the parameters of the defect mean-field coulomb interaction. A comparison of the high te mperature phase diagrams of thoria and urania indicates close analogie s in the pre-melting behaviour of these two compounds.