Thermochemistry of phosphorus oxynitrides: PON and LiNaPON glasses

High-temperature solution calorimetry has been very useful in elucidating the energetics of many oxide materials. Recently, a sodium molybdate melt, 3Na(2)0.4MoO(3), has been shown to be very effective for nitride calorimetry. This methodology has now been used to determine the energetics of formation of phosphorus oxynitride PON samples and of a series of LiNaPON oxynitride glasses. Enthalpies of formation from the elements at 298 K are --371.71 +/- 4.45 and -356.14 +/- 3.98 kJ mol(-1) for beta-cristobalite and amorphous PON, respectively, and -961.88 +/- 3.86 kJ mol-l for a 9 wt % nitrogen-containing LiNaPON glass. The P-cristobalite energy of amorphization is -15.57 +/- 5.97 kJ mol(-1). A linear relation, Delta H-f(0)(Li0.5PO(3-3x/2)Nx) = Delta H-f(0)(Li0.5Na0.5PO3) + 441.7x, was found between the enthalpies of formation of the glasses and their atomic nitrogen content, x (0 less than or equal to x less than or equal to 0.57). The magnitude of the energetics of nitrogen/oxygen substitution within PON and LiNaPON glasses has been correctly evaluated by using N-N, O-O, P-N, and P-O bond strengths. The in-situ precipitation of metallic particles from corresponding oxides in LiNaPON glasses has been predicted from high-temperature solution calorimetry results and appropriate thermodynamic cycles. These results constitute the first set of energetic data on nitridophosphates.