THERMAL-STABILITY AND VAPOR-PRESSURE STUDIES ON UTE3O9(S) AND UTEO5(S)

UTe3O9 and UTeO5 were prepared by the solid state reaction route and c haracterized by X-ray diffraction and thermal methods. Thermal and X-r ay studies indicated that UTe3O9 and UTeO5 vaporize incongruently acco rding to the reactions: UTe3O9(s) --> UTeO5(s) + 2TeO(2)(g); 3UTeO(5)( s) --> U3O8(s) + 3TeO(2)(g) + 1/2O(2)(g). Vapour pressures of TeO2(g) over the mixture of UTe3O9(s) + UTeO5(s) in the temperature range 888- 948 K and UTeO5(s) + U3O8(s) in the temperature range 1063-1155 K were measured by employing the Knudsen effusion mass loss technique. From the partial pressures of TeO2(g) measured, the standard Gibbs energy o f formation (Delta G(f)(0)) of UTe3O9(s) and UTeO5(s) were obtained wh ich could be represented by the following relations: Delta G(f)(0) UTe 3O9(s) +/- 30 kJ/mol = -2318 + 0.79817 (K) (888-948 K); Delta G(f)(0) UTeO5(s) +/- 25 kJ/mol = -1616 + 0.4006T (K) (1063-1155 K). The kineti cs of decomposition of UTe3O9 and UTeO5 under isothermal heating condi tions in flowing air were studied to determine the reaction mechanisms , rate constants and activation energies. (C) 1998 Elsevier Science B. V.