Heat Capacity and Absolute Standard Entropy of the High Temperature Polymorph of Calcium Boranate and Thermodynamic Calculations Regarding its Decomposition and Rehydrogenation

Calcium boranate (Ca(BH₄)₂) is synthesized using wet chemistry metathesis reactions resulting in mixtures of both α- and β-Ca(BH₄)₂, with the β phase being the main component. The drying procedure reveals high kinetic stability of the high temperature β polymorph, which is in contrast to the expectations based on the literature. The molar heat capacity function of β-Ca(BH₄)₂ is determined between 2 and 525 K using different calorimeters, a Physical Property Measurement System applying the relaxation method in the low temperature range and a Calvet-DSC for the high temperature range. From these values the absolute standard entropy at 298.15 K for β-Ca(BH₄)₂ is calculated as S^°(298.15 K) = (117.4 ± 4.1) J mol^-1 K^-1. Taking the value of the enthalpy of formation from the literature, the Gibbs energy functions are calculated and the decomposition and rehydrogenation behavior of the compound is discussed.