Thermochemistry of rare earth oxyhydroxides, REOOH (RE = Eu to Lu)

As intermediate products of the dehydration of hydroxides or the hydration of oxides, rare earth oxyhydroxides (REOOH) have important implications for synthesis and potential applications in luminescent devices. Using high temperature oxide melt solution calorimetry and thermal analysis, thermodynamic properties of REOOH (RE ​= ​Eu to Lu), including enthalpies of dehydration and formation, were determined. The exothermic enthalpies of formation from oxides and endothermic enthalpies of dehydration demonstrate that oxyhydroxides are not only synthetic but also thermodynamic intermediates in rare earth oxide - water systems. The linear relationship between enthalpies of formation from oxides and ionic radius of RE³⁺ confirms the key role of cation size in the stability of REOOH, and reveals that REOOH with lighter rare earth elements are easier to form and harder to dehydrate, both thermodynamically and kinetically.