Heat capacities and thermodynamics of formation of flat-Al13 nitrate - [Al13(OH)24(H2O)24](NO3)15·11H2O

We report enthalpies of formation from the elements and oxides, ΔfH∘ = −(26195.31 ± 34.46) kJ · mol−1 and Δf,oxH = −(1402.09 ± 33.36) kJ · mol−1, for crystalline [Al13(μ3-OH)6(μ2-OH)18(H2O)24](NO3)15·11H2O(cr) (flat-Al13) measured using aqueous solution room temperature calorimetry. This material is a nitrate salt of a polynuclear aluminum hydroxide ion with thirteen aluminum metal centers, Al1315+. Unlike the ε-Al137+ Keggin ion, it contains solely octahedrally coordinated aluminum in a more flat topology. We also report heat capacities from temperatures of (2 to 300) K on this flat-Al13 nitrate using a Quantum Design Physical Property Measurement System (PPMS). These measurements were used to calculate the standard molar entropy and subsequently the free energy of formation for this material. We observed that during the heat capacity measurements, the flat-Al13 sample underwent a second order phase transition near T = 214 K.