Effective transference numbers and water incorporation in glass-ceramic La(PO3)3-Ca(PO3)2 in oxidizing atmospheres

► La(PO3)3-Ca(PO3)2 exposure to water vapor in excess of 4.2 kPa disproportionates. ► Proton transport dominates the conductivity in the examined temperature range. ► The measurements suggest that oxygen ions also participate in conduction. ► At 550 °C, the proton transference number of ~ 0.9, agrees with the isotope effect.