Cobaltates
Cobalt oxides have been studied extensively because of their interesting transport and magnetic properties, as well as in connection with a variety of potential applications. For example, La1−x Sr x CoO3 [1, 2] is a candidate material for high-performance
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Cobalt oxides have been studied extensively because of their interesting transport and magnetic properties, as well as in connection with a variety of potential applications. For example, Lal-xSrxCo03 [1,2] is a candidate material for high-performance electrodes in solid-oxide fuel-cells and NaCo 20 4 [3-12] has potential as a thermoelectric material. Recently, superconductivity [13] has been discovered in water-intercalated NaO.35Co02·1.3H20. In these cobalt oxides, a cobalt ion is caged in an octahedron of oxygen ions. Usually, the electronic state of a transition metal ion is fixed, that is to say the total-spin quantum-number and the number of electrons in the 3d orbitals have more or less precise values. In the cobalt oxides, however, the electronic state of a Co ion can have both different valence states and even when the valence is fixed there exist different spin states. In some cases, the ionic state of a Co ion changes with temperature. This transition is accompanied with change of the transport and magnetic properties. The interplay of the properties and the transition of ionic state is a main issue of the cobalt oxides, i.e., the problem of low-, high- and intermediate-spin states.
6.1 Low-, High- and Intermediate-Spin States Let us first focus on an isolated Co0 6 octahedron and examine the electronic properties. Due to the effect of the crystal field, the 3d orbitals split into two fold, eg , and three fold t2g degenerate levels with the splitting defined to be 10Dq. There exist six (five) electrons in the 3d shell of the Co3+ (Co4+) ion. If the crystal field is strong enough, all the electrons go into t 2g orbitals (see Fig. 6.1(a)). Suppose that two of the electrons go into the eg orbitals. In this case, the electronic state costs an energy lODq x 2 and is unstable. The stability of the electronic state also depends on the spin degree of freedom. The Hund's rule coupling J H favors the electronic configurations t~ge~ with S = 2 for Co3+ and t~ge~ with S = 5/2 for Co4+. The Hund's rule coupling and the crystal-field splitting lODq compete with each other. When neither coupling dominates, the electronic configurations t~ge~ with S = 1 and t~ge~ with S = 3/2 may also be possible for Co3+ and Co4+, respectively. It is interesting to note that the states shown in Fig. 6.1 (c) have the same orbital degree of freedom as those described in Chap. 4. The electronic states of S. Maekawa et al., Physics of Transition Metal Oxides © Springer-Verlag Berlin Heidelberg 2004
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6. Cobaltates
(b) HS states
(a) LS states
~ ~ eg
s=o
Co4+e,
_
t!:
t2g ~
t 2g
S=5/2
S= 1/2
co3+ eg
=t=
t 2g
t 2g
t 2g
eg
4
co3+eg
_
(c) IS states
Co4+
--t-
e, _
t2g ~ S= 3/2
Fig. 6.1. Schematic representation of the electronic states of cobalt ions, Co3+ and Co4+. The horizontal lines indicate the energy levels of e g and b g orbitals. The arrow represents a spin of an electron. (a) Low-spin states, (b) high-spin states, and (c) intermediate-spin states. The magnitude of spin S is indicated.
a Co3+ (Co4+) ion wi
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