Formation and Dehydration Enthalpy of Potassium Hexaniobate

Abstract

The formation energetics of hydrous and dehydrated potassium hexaniobates are investigated using high-temperature oxide melt solution calorimetry. The enthalpies of formation of K4Nb6O17 and K4Nb6O173H2O from oxides are ( – 864.42 10.63) and ( – 899.32 11.48) kJ/mol, respectively. The formation enthalpy of K4Nb6O17 from elements is ( – 7289.64 12.50) kJ/mol, and of K4Nb6O173H2O is ( – 8181.94 13.24) kJ/mol. The enthalpy of dehydration (DHdehy) for the reaction K4Nb6O173H2O (xl, 25°C) = K4Nb6O17 (xl, 25°C) + 3H2O (l, 25°C) is endothermic and is 34.60 7.56 kJ/mol. The DHdehy per mole of water, 11.53 2.52 kJ/mol, indicates the water molecules in K4Nb6O173H2O are not just physically adsorbed, but loosely bonded in the K4Nb6O17 phase, presumably in specific interlayer sites. The loss of this water near 100°C on heating is consistent with the weak bonding of water