Effects of Water, Cations, and Structure on Energetics of Layer and Framework Phases, NaxMgyMnO2·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 3Na{sub 2}O{sub 4}{center_dot}MoO{sub 3} at 977 K. The composition of these phases with a general formula of Na{sub x}Mg{sub y}MnO{sub 2}{center_dot}nH{sub 2}O was systematically varied. Using the measured enthalpies of drop solution, the enthalpies of formation from oxides based on 1 mol of Na{sub x}Mg{sub y}MnO{sub 2}{center_dot}nH{sub 2}O at 298 K, {Delta}H{degree}{sub f (oxides)}, were obtained. The {Delta}H{degree}{sub f (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{sub (hyd)} = the number of water molecules per cation), all of the {Delta}H{degree}{sub f (oxides)} data can be linearly correlated. The linear correlations are {Delta}H{degree}{sub f (oxides)} = 16.5N{sub (hyd)} {minus} 73.3 for the layer phases, and {Delta}H{degree}{sub f (oxides)} = 11.7N{sub (hyd)} {minus} 64.4 for the framework phases. The average N{sub (hyd)} values for the birnessite, buserite, and todorokite phases are 2, 4, and 2, respectively.