Solid-state NMR study of dopant effects on the chemical properties of Mg-, In-, and Al-doped SnP2 O7.

The effects of doped low-valence cations on the properties of the SnP2 O7 proton conductor at ambient temperature are investigated from changes in solid-state NMR spectra and nuclear magnetic relaxation times. Although the T1 H values increased with decreasing acidity as a result of cation exchange, the (1)H chemical shifts moved to lower field in Al- and In-doped materials compared with undoped ones. Furthermore, the shifts changed to higher field in Mg-doped materials, suggesting the existence of different protonic species in those materials. The bulk phosphate chemical shifts in the (31)P dipolar-decoupling MAS NMR spectra were very similar, regardless of the nature and amount of the doping species. On the other hand, by (1)H/(31)P cross-polarization MAS NMR, P2O7 signals interacting with an interstitial proton [Q(1)(proton)] were observed in all the undoped and doped SnP2 O7, while acidic P-OH-type phosphate signals [Q(1)(acid)] were additionally observed in the Mg-doped conductor. The different affinity of the proton with the dopants and phosphates caused lower conductivity and larger activation energy in the Mg-doped materials, compared with those in the In- and Al-doped materials.