Anhydrous Silicophosphoric Acid Glass: Thermal Properties and Proton Conductivity.

Anhydrous silicophosphoric acid glass with an approximate composition of H 5 Si 2 P 10 O 32 was synthesized and its thermal and proton-conducting properties were characterized. Despite exhibiting a glass transition at 192 °C, the supercooled liquid could be handled as a solid up to 280 °C owing to its high viscosity. The glass and its melt exhibited proton conduction with a proton transport number of ~1. Although covalent O-H bonds were weakened by relatively strong hydrogen bonding, the proton conductivity (4 × 10 -4 S cm -1 at 276 °C) was considerably lower than that of phosphoric acid. The high viscosity of the melt was due the tight cross-linking of phosphate ion chains by six-fold-coordinated Si atoms. The low proton conductivity was attributed to the trapping of positively charged proton carriers around anionic SiO 6 units (expressed as (SiO 6/2 ) 2- ) to compensate for the negative charges.