Effects of Water, Cations, and Structure on Energetics of Layer and Framework Phases, Na_xMg_yMnO₂·nH₂O

The energetics of 14 synthetic manganese dioxides, including framework phases with todorokite structure and layer phases with birnessite and buserite structures, were studied using high-temperature solution calorimetry in molten 3Na₂O₄·MoO₃ at 977 K. The composition of these phases with a general formula of Na_xMg_yMnO₂· nH₂O was systematically varied. Using the measured enthalpies of drop solution, the enthalpies of formation from oxides based on 1 mol of Na_xMg_yMnO₂·nH₂O at 298 K, ΔH_f^o_(oxides), were obtained. The ΔH_f^o_(oxides) data show no obvious correlation with either content of charge-balancing cations or content of water for the layer and framework phases. Using a new parameter, the hydration number of the charge-balancing cations (N_(hyd) = the number of water molecules per cation), all of the ΔH_f^o_(oxides) data can be linearly correlated. The linear correlations are ΔH_f^o_(oxides) = 16.5N_(hyd) - 73.3 for the layer phases, and ΔH_f^o_(oxides) = 11.7N_(hyd) - 64.4 for the framework phases. The average N_(hyd) values for the birnessite, buserite, and todorokite phases are 2, 4, and 2, respectively.