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, 3Na₂O·4MoO₃, 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 β-cristobalite and amorphous PON, respectively, and -961.88 ± 3.86 kJ mol^-1 for a 9 wt % nitrogen-containing LiNaPON glass. The β-cristobalite energy of amorphization is -15.57 ± 5.97 kJ mol^-1. A linear relation, ΔH(f)/(o)(Li_0.5Na_0.5PO((3-3x/2))N(x)) = ΔH(f)/(o)(Li_0.5Na_0.5PO₃) + 441.7x, was found between the enthalpies of formation of the glasses and their atomic nitrogen content, x (0 ≤ x ≤ 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.