The BaZrO₃ base perovskites have been known to show a proton conductivity because they are favorably hydrated in humidified atmosphere so as to form the protonic defects as follows.

O_O^×+V_O^••+H₂O⇄2OH_O^•                                                  (Eq. 1)

The pioneer works have predicted the hydride ion defects could also form in the perovskite oxides under relatively high hydrogen partial pressure[1]. Meanwhile, the oxides that are extensively hydrogenated with H₂ gas under ambient pressure have been missing. Recently, we have demonstrated that BaZr_0.5In(III)_0.5O_2.75 (BZI55), which is a member of BaZrO₃ base proton conductors, can be reductively hydrogenated by heating at 800°C in a dried hydrogen atmosphere (p_H2O ~ 1 Pa), transforming to isomorphous oxyhydride BaZr_0.5In(I, III)_0.5O_2.25H_0.5 (H-BZI55) [2]. Neutron diffraction confirms that hydride ion (H^-) occupies both oxygen anion sites and interstitial sites located near the face center of 100 plane in H-BZI55. Pressure-composition-temperature (PCT) measurements indicated that the concentration of hydride ions in H-BZI55 is proportional to p_H2^1/2 at p_H2O below 1 Pa, predicting that the hydride ions in H-BZI55 are in equilibria with hydrogen gases  by the following defect reactions.

1/2H₂+V_O^••+e'⇄H_O^•                                                    (Eq. 2)

1/2H₂+e'⇄H_i'                                                              (Eq. 3)

The H^- ion conductivity would increase with p_H2^1/2 if the main ionic carrier in BZI55 is H_O^•  defects. However, the conductivity would decrease with p_H2^1/2 if the main ionic carrier in BZI55 is H_i'  defects.The main objective of this study was to determined the main carriers in BZI55 through the relationship of the conductivity vs p_H2 under equilibrium.

The lattice contraction by the hydrogenation is less than 0.05%, so that the dense sinters of BZI55 can be converted to the corresponding ones of H-BZI55 without structural collapse by H₂ annealing. Hence the H^- ion/electron mixed conductivity of H-BZI55 can be measured by using the bulk sintered bodies. The electrical conductivity of H-BZI55 was 10^-2 S cm^-1 at T > 500°C, which was much higher than the proton conductivity of BZI55. When the measurement gas was switched from 20%-H₂/Ar to 20%-D₂/Ar, a significant H/D isotope effect was observed, indicating that H⁻ ion conductivity is dominant in BZI55.