Investigation in the variation of Gibbs energy of formation of RE6UO12 (RE= La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu) along the 4f series

Abstract The RE 6 UO 12 (s) (RE= La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu) compounds were synthesized by citrate nitrate combustion method and characterized by X-Ray Diffraction (XRD) technique. Standard molar Gibbs energy of formation of RE 6 UO 12 (s) (RE= La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu) was determined from the equilibrium electromotive force (e.m.f) data obtained from solid-state Galvanic cell with oxide ion conducting yttria-stabilized-zirconia (YSZ) electrolyte. The cells were operated under high pure argon atmosphere. E.m.f. measurements were carried out in the temperature range of 850-1150 K. Phase transition in Sm 6 UO 12 (s) and Yb 6 UO 12 (s) were detected at 1125 K and 1065 K, respectively. The transition temperatures were confirmed from high temperature XRD technique. The standard enthalpy of formation and entropy at temperature of 298.15 K for RE 6 UO 12 (s) was calculated from the Gibbs energy data using second and third law analysis and the data were compared with the corresponding data reported for RE 2 O 3 (s). Similar trend in enthalpy as well as entropy was observed for RE 6 UO 12 (s) and RE 2 O 3 (s) along the series of 4f elements.