High-Resolution Solid-State 31P NMR of Alkali Phosphates

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Abstract
High-resolution solid-state 31P NMR spectra have been measured for inorganic alkali phosphates with different condensations in the PO4 units, and the isotropic chemical shift and anisotropy have been determined. The isotropic chemical shifts in alkali orthophosphates do not correlate directly with the cation size. The hydration effects have been carefully examined by means of a 1H decoupling experiment and by the heat treatment of the materials, accompanied by thermogravimetric analysis. Hydration causes an upfield shift in Na3PO4, K3PO4, and Na4P2O7, while it causes a downfield shift in Rb3PO4 and Cs3PO4. All the five peaks observed in the K4P2O7 sample can be ascribed to anhydrous species; this is in contrast to the findings of previous works. The field-dependence experiments definitely demonstrate that the splittings in K4P2O7 and (KPO3)n are caused by inequivalent sites.