Effects of water, cations, and structure on energetics of layer and framework phases, NaxMgyMnO2 center dot nH(2)O

The energetics of 14 synthetic manganese dioxides, including framework phas es with todorokite structure and layer phases with birnessite and buserite structures, were studied using high-temperature solution calorimetry in mol ten 3Na(2)O(4). MoO3 at 977 K. The composition of these phases with a gener al formula of NaxMgyMnO2. nH(2)O was systematically varied. Using the measu red enthalpies of drop solution, the enthalpies of formation from oxides ba sed on 1 mol of NaxMgyMnO2. nH(2)O at 298 K, Delta H-f((oxides))o, were obt ained. The Delta H-f((oxides))o data show no obvious correlation with eithe r content of charge-balancing cations or content of water for the layer and framework phases. Using a new parameter, the hydration number of the charg e-balancing cations (N-(hyd) = the number of water molecules per cation), a ll of the Delta H-f((oxides))o data can be linearly correlated. The linear correlations are Delta H-f((oxides))o = 16.5N((hyd)) - 73.3 for the layer p hases, and Delta H-f((oxides))o = 11.7N((hyd)) - 64.4 for the framework pha ses. The average N-(hyd) values for the birnessite, buserite, and todorokit e phases are 2, 4, and 2, respectively.