Room temperature reaction between uranyl nitrate hexahydrate and rubidium nitrate and polymerisation during denitration of rubidium uranyl nitrate at elevated temperatures

The interaction between uranyl nitrate hexahydrate (UNH) and rubidium nitrate in solid state at elevated temperatures has been studied for several compositions using simultaneous thermal analysis techniques (TG–DTA–EGA), X-ray diffraction and IR spectral measurements. The formation of rubidium uranyl nitrate has been observed in rubidium nitrate rich mixtures even at ambient temperature during mixing of the two nitrates in solid state. The reaction at room temperature was incomplete in mixtures containing higher than 30 mol% uranyl nitrate hexahydrate. Thermal and XRD data on equimolar mixture indicate complete dehydration of uranyl nitrate hexahydrate by 300 °C to form rubidium uranyl nitrate, Rb[UO2(NO3)3], which decomposes on further heating to the polymerised structures, Rb2[(UO2NO3)2(O)2] and Rb4[(UO2)3(O)4(NO3)2] above 360 °C and 460 °C, respectively. Rb4[(UO2)3(O)4(NO3)2] reacts with UO3 above 550 °C to form rubidium diuranate, Rb2U2O7, when the initial nitrate mixture is prepared from compositions containing UNH at 50 mol% or lower levels. In the case of initial nitrate mixtures containing a higher amount of UNH, however, Rb2U4O13 and U3O8 are formed above 550 °C.