A phase transition in doped U4Osub(9-y). I

Heat capacities of U4Osub(9-y) doped with Th4+ and Nb5+ ions as impurities in the temperature range from 200 to 505 K by means of adiabatic calorimetry and the effect of doping on the phase transition of U4Osub(9-y) was investigated. The lambda-type peak in the heat capacity curve becomes broadened on doping, resulting from U4+ and U5+ sites being disordered by the presence of impurity ions. The enthalpy change during the transition is proportional to the transition temperature for both doped and undoped samples, but at the same enthalpy change the transition temperature of doped samples is higher than that of undoped samples. The entropy changes during the transition are interpreted in terms of the average valence of the uranium. (Auth.)