Heat capacities and thermodynamics of formation of ε-Keggin MAl12 Selenates (M = Al(III), Ga(III), or Ge(IV))

We report enthalpies of formation from the elements and oxides, ΔfH° = −(22,673.44 ± 30.19) kJ·mol-1 and Δf,oxH = −(869.68 ± 28.75) kJ·mol-1, for [GeO4Al12(OH)24(H2O)12](SeO4)4·12H2O(cr) (GeAl12) measured using high temperature oxide-melt solution calorimetry. This material is the selenate salt of the germanium-substituted polynuclear Al137+ ion in the ε-Keggin structure. We also report heat capacities from temperatures of (2 to 300) K on this GeAl12 selenate and its other heterometal substituted forms, Na[AlO4Al12(OH)24(H2O)12](SeO4)4·12H2O(cr) (AlAl12) and Na[GaO4Al12(OH)24(H2O)12](SeO4)4·12H2O(cr) (GaAl12), measured using a Quantum Design Physical Property Measurement System (PPMS). These measurements were used to calculate entropies and subsequently free energies of formation for these three materials. All three MAl12 selenates, where M = Al(III), Ga(III), or Ge(IV), have similar heat capacities from temperatures (2 to 300) K and similar characteristic Debye temperatures, ΘD, suggesting similar lattice vibrational densities of states.