Transition Metal Ions cubic perovskites with

The same analysis can be applied to compounds with a more complex formula. For example, the oxide LaCoCL, which adopts the cubic perovskite structure, usually shows a large positive Seebeck coefficient, of the order of +700 jjlV K-1, when prepared in air (Hebert et al., 2007). This indicates that there are holes present in the material. The La ions have a fixed valence, La3+, hence the presence of holes must be associated with the transition-metal ion present. Previous discussion suggests that LaCo03 has become slightly oxidized to LaCoCL+j, and contains a population of Co4+ ions (Co3+ + h or Coc0)- Each added oxygen ion will generate two holes, equivalent to two Co4+ ... 

Ordered Perovskite-type Compounds, A2(BB )06 Systems Cubic Fmim A feature of the perovskite structure is that, with the proper substitutions, many types of ordered structures can readily be formed. This can be accomplished by the substitution of two suitable metal ions (with different oxidation states) in the octahedral sites of the structure. In this case the unit ceil is doubled along the three cubic axes to generate an 0.8 A unit ceil (Figure 15). Partial substitution of different transition metal ions in the octahedral sites is also possible the general formulation for these compounds would be A2(B2 xB x)06. The parentheses in this formulation enclose atoms occupying the octahedral sites in the structure. 

We now examine results concerning ferroelectric-like (FE) distortions in a pseudocubic perovskite phase, in which the oxygen sublattice is kept fixed in its undistorted cubic coordinates, and only the transition metal ion M displaced within its octahedron. The pseudocubic is another idealised phase, for which no direct comparison with experiment can be made, but it enables us to compare different functionals in describing structural distortions. For each combination of composition and Hamiltonian reported in Table 3, we examined an FE distortion, in which the M ion is displaced along the [001] direction. Each phase is based on the equilibrium lattice parameter for the cubic phase (Table 1). This idealised distortion makes reference to the tetragonal phase of FE materials such as BaTi03 and KNb03 [99]. 

Cubic Perovskites with Transition Metal Ions ... 

The ideal perovskite structure is cubic and adopted by oxides with ABOj stoichiometry (e.g., CaTiOj), where the A metal sits on the corners of the cube and is 12-fold coordinated by oxygen, which are located on the faces. The B cation, which is usually a transition metal ion, occupies the center of the cube (Figure 15.2c). 

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