Conductivity Mechanisms in Acceptor Doped KTaO 3 Crystals
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CONDUCTIVITY MECHANISMS IN ACCEPTOR DOPED KTaO,
CRYSTALS
T. SCHERBAN, S.Q. FU AND A.S. NOWICK Henry Krumb School of Mines, Columbia University, New York, NY 10027
ABSTRACT The electrical conductivity of perovskite-structured KTaO3 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(02 ) 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. INTRODUCTION Potassium tantalate, KTaO3 , is an ideal perovskite oxide in several respects. It has the cubic structure and does not undergo a ferrolelectric or other phase transition (from its melting temperature to -1K) that makes the study of BaTiO, and SrTiO, more complex. In spite of this simplicity, the nature of defects and the electrical conductivity behavior of KTaO, are only partially understood. In this paper we confine ourselves to crystals of KTaOs doped with transition-metal ions Cu2+, Co2 + and Fes+. Consideration of ionic radii indicates that the transition-metal ions reside at the Tas+ site and this has been confirmed for Fes+ [1]. The dopants are, therefore, acceptors and are known to be charge compensated by oxygen-ion vacancies, V'. But it has also been shown that by heating in water vapor, 0 protons enter the structure, making the material a protonic conductor [2]. The reaction involved is:
H2O(g)
+
V;" _-, Ox + 2H'
(1)
the intersitial proton forming a strong OH- bond with an oxygen ion in the lattice. This class of protonic conductors differs significantly from other classes in that hydrogen is present as a minor constituent and proton migration occurs through a simple hopping mechanism [3]. It is also possible for the oxygen vacancies to be replaced as charge compensators by electron holes upon oxidizing treatment: 1/2 02 (g)
+
V-"
-->
Ox
+
2h'
(2)
It has been shown through quantitative EPR measurements [4] that, rather than forming free holes in the valence band, the oxidation acts to change the valence of the dopant. To be specific, if we denote the acceptor In reduced form as A... (i.e. a dopant of valence +2) and in oxidized form as ATa, we have the reaction: 1/2 02 (g) + V-
+ 2A'
00
O
+ 2A'' Ta
Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
(3
664
The acceptor dopant in the oxidized state may, however, release a hole to the valence band, according to the ionization reaction: A'' .>A''' h*() Ta --Ta + h (4) thus making possible electron hole conduction as well as ionic conduction. In the present work
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