The thermal decomposition reactions of some lanthanide and uranium(IV) nitrates

Abstract

The thermal decomposition of some lanthanide nitrates was studied. Moving down the lanthanide series, an increasing number of reversible processes, occurring during the decomposition reactions, was observed. These temperature-dependent reversible processes could be phase transitions (only in the solid state) or minor structural changes in the lanthanide nitrates. All reversible processes are endothermic, except for the process at 349 °c for Ho(N03) 3. The reversible processes cause changes in the kinetics of the decomposition reactions. The dehydrated lanthanide nitrates decompose to oxynitrates, except Ce(N03) 3, which decomposes directly to the oxide. The temperatures at which decomposition starts, and the reaction enthalpy 3 values at a pressure of 5 x 10 kPa, were used as relative measures of the thermal stability. An increasing instability was found on moving down the lanthanide series. The enthalpy at atmospheric pressure showed an increase in value on moving down the lanthanide series, due to the increasing number of reversible processes occurring during the decomposition reactions. The pressure of 5 x 10 3 kPa seems to inhibit the reversible endothermic processes. The relationship between increasing ionic radii and reaction enthalpy j at 5 x 10 kPa, seems to be of an exponential order. The comparison of the experimental data to the nineteen different kinetic models, seems to have only empirical importance. This could be due to the sample preparation. Decreasing ionic radii in the lanthanide series give rise to greater polarisation of the nitrate ions and therefore greater instability of the lanthanide nitrate. The solid-state chemistry of U(N03) 4 •2L (L = tdpo, tppo and tpyrp) has been investigated. Taking into consideration the decomposition onset temperatures, the enthalpy values, the bond lengths in the molecules and the XPS data, an order of stability was established; From crystal and molecular structure data, it seems that tppo is the strongest electron donor 1 igand, followed by tdpo and then tpyrp. The stronger the electron donor properties of the ligand, the more stable is the uranium (IV) nitrate complex. Due to the stabilizing effect of the stronger donor ligand, the U-O(N02) bonds are also stabilized and strengthened. Activation energy and pre-exponential factor values determined from the nineteen kinetic models for solid state reactions do not seem to be comparable to one another. The values seem to be associated with a definite process, e.g. three-dimensional diffusion, in the solid state.