Investigation in thorium phosphate by NMR II-phosphorus dipolar networks

With through space and through bond experiments in two-dimensional NMR we analyze the transformation from the thorium phosphate-hydrogen phosphate hydrate (TPHPH) to the β form of the thorium phosphate diphosphate (β-TPD) in relation with the phosphorus networks. These techniques are complementary: the through space coupling gives an insight on the dipolar phosphorus networks while the through bond coupling is particularly efficient in the detection of the P2O7 groups. With these experiments we show that in a first step, by heating the precursor TPHPH above 250 °C, it transforms into an α form of TPD. This transformation is due to the complete condensation of hydrogen phosphate groups HPO4 into P2O7 entities. By heating α-TPD above 950 °C it transforms into its well-known β form. The α form is characterized by a hygroscopic behavior: some water molecules are present near the P2O7 groups that makes non-equivalent their phosphorus nuclei. PO4 dipolar networks are always present in the α form. The main effect of these PO4 and P2O7 units is to give the system a channel structure and the water enters in them.