The ionic conductivity of hot-pressed samples of undoped Y2O3 was studied by using the emf method in atmospheres with controlled O and water-vapor pressures. The variation in the ionic conductivity was studied as a function of time (7 months at 1200C), temperature (600 to 1300C), water-vapor pressure (3 to 1400Pa) and O pressure (10-10  to 105Pa). The overall conductivity could be divided into contributions from electronic carriers (mainly electron holes), native ionic defects and H defects. The transport of charged H species was dominated by the migration of so-called free protons. The H-ion conductivity was detectable under all conditions, and became the predominant ionic-conductivity contribution at high water-vapor pressures and low temperatures. Proton-diffusion coefficients in yttria were estimated to be described by:

D (cm2/s) = 2 x 10-1 exp[-150(kJ/mol)/RT]

Proton and Native-Ion Conductivities in Y2O3 at High Temperatures. T.Norby, P.Kofstad: Solid State Ionics, 1986, 20[3], 169-84