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The electrical conductivity of perovskite-structured KTaO 3 crystals acceptor doped with Co, Cu or Fe was investigated after treatments in oxidizing and reducing atmospheres under both wet and dry conditions. Isotope effect measurements (using HO vs. D0O) show that, after treatments in wet gases of low P(OC), all the crystals are primarily protonic conductors, through a process of proton hopping with an activation energy close to 0.84 eV.Electron hole conduction dominates at high P(O) in the case of Fe and Cu doping. For Co-doped crystals, the conductivity is independent of P(0 2 ) up to 1 atm., indicating that ionic conduction predominates.There is no evidence of oxygen vacancy migration, leading to the conclusion that the activation energy for that process Is relatively high.

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