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The molar enthalpies of dissolution of a number of well-characterized hexagonal hydroxycarbonates Ln(OH)CO 3 (hex) (Ln = La, Nd, Sm, Eu) in 6.00 mol dm -3  HCl were measured at 298.15 K. A new sealed solution micro-calorimeter was developed for this purpose. It was made of an 18-carat gold alloy in order to improve the performances of a calorimeter previously built in our laboratory. The following standard molar enthalpies of formation, Δ f  H ° m  [Ln(OH)CO 3 , hex], in kJ mol -1 , were calculated: -(1627.8±1.6), -(1614.8±1.9), -(1613.4±1.6), and -(1523.0±3.0), for the La, Nd, Sm, and Eu compounds, respectively. These results allowed an extrapolation to Δ f  H ° m  [Eu(OH)CO 3 ·0.5H 2 O, orth]=-(1653.4±3.6)kJ mol -1  and to Δ f  H ° m  [Am(OH)CO 3 , hex]=-(1552.5±3.3)kJ mol -1 . Using auxiliary data and estimated entropies, the solubility products of the hexagonal hydroxycarbonates were calculated. They are compared here with values deduced from solubility and calorimetric measurements for the corresponding orthorhombic hydroxycarbonates. Our approach generally leads to values similar to those deduced from solubility studies. The orthorhombic form is found to be metastable with respect to the hexagonal form.

Thermochemistry of selected trivalent lanthanide and americium compounds: orthorhombic and hexagonal hydroxycarbonates