Solid‐state NMR study on changes of phosphate and proton species in metal pyrophosphate composite (MP2O7–MO2) ceramics

In order to investigate the effects of composite production and the role of the counter cation for metal phosphate conductors, changes in the solid‐state nuclear magnetic resonance (NMR) spectra and magnetic relaxation times caused by the removal of volatile products and water washing were examined for various metal pyrophosphate (MP₂O₇–MO₂; M = Sn, Si, Ti, and Zr). Acidic species could be detected by ¹H MAS NMR spectra for all the composites except ZrP₂O₇–ZrO₂ that had the lowest conductivity. The ³¹P DD‐MAS NMR spectra for MP₂O₇–MO₂ composites showed different signal patterns depending on the counter cations participating in the ion exchange as a result of different microstructures. Combinational analysis of ³¹P DD‐MAS and ³¹P CP‐MAS NMR spectra of the composites indicated that protonic bulk phosphates were observed at slightly lower fields than non‐protonic bulk phosphates in all of the MP₂O₇–MO₂ composites. After water washing, the acidic species and the protonic bulk phosphates of MP₂O₇–MO₂ composites disappeared or were reduced to trace amounts. The T₁H values of the water‐washed composites lengthened because of removal of orthophosphoric acid, although the T₁P values remained almost unchanged. The results of the solid‐state NMR studies suggest that the protonic bulk phosphates of MP₂O₇–MO₂ composites do not generally distribute in the bulk but exist in the interface between excess H₃PO₄ and the bulk. Copyright © 2016 John Wiley & Sons, Ltd.