Solid-state NMR study on changes of phosphate and proton species in metal pyrophosphate composite (MP₂ O₇ -MO₂ )...

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 (MP₂ O₇ -MO₂ ; M = Sn, Si, Ti, and Zr). Acidic species could be detected by ¹ H MAS NMR spectra for all the composites except ZrP₂ O₇ -ZrO₂ that had the lowest conductivity. The ³¹ P DD-MAS NMR spectra for MP₂ O₇ -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 ³¹ P DD-MAS and ³¹ P 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 MP₂ O₇ -MO₂ composites. After water washing, the acidic species and the protonic bulk phosphates of MP₂ O₇ -MO₂ composites disappeared or were reduced to trace amounts. The T₁ H values of the water-washed composites lengthened because of removal of orthophosphoric acid, although the T₁ P values remained almost unchanged. The results of the solid-state NMR studies suggest that the protonic bulk phosphates of MP₂ O₇ -MO₂ composites do not generally distribute in the bulk but exist in the interface between excess H₃ PO₄ and the bulk. Copyright © 2016 John Wiley & Sons, Ltd.