A-site cations

It is commonly accepted that the B-site ions play a crucial role for various redox reactions and the role of A-site cations is to stabilize the B-0 octahedra as well as modify the chemical environment of the B-site cations. 

Baiker, A Marti, PE Keusch, P Fritsch, E Reller, A. Influence of the A-site cation in AC0O3 (A = La, Pr, Nd, and Gd) perovskite-type oxides on catalytic activity for methane combustion. J. Catal, 1994, Volume 146, Issue 1, 268-276. 

Figure 6.38 Variation of (TJ of various rare-earth manganates with weighted average radius of the A-site cations, (After Mahesh et ai, 1995). The area in the bottom left hand corner represents the ferromagnetic insulator regime.

Thus, the behavior in both the titanium and niobium systems is consistent with the hypothesis that the A-site cation is primarily responsible for variation in flat-band potential while the structure is primarily responsible for variation in optical band gap. Of course, it has been noted elsewhere that other properties such as the magnitude of the quantum efficiency also depend upon structure (10). 

When only a single species of photoactive center is present in a compound, the presence of a non-active, A-site cation produces a characteristic shift in the flat-band potential. A change in structure, however, will in general produce a shift in the optical band-gap energy. This is accompanied by corresponding shifts in any other, higher-energy interband transition, but the qualitative features remain the same, and hence appear to be characteristic of the particular photoactive center. 

Fig. 8 Variation of the ferromagnetic Tc or the metal-insulator transition temperature, Tim, in Lnj.xAxMn03 with weighted average radius of the A site cations, [from Rao (reproduced with permission from ref. 8)].

Figures 8 and 9, plate 3, are lattice images of two compounds (n = 2 and n = 5 respectively) showing evidence of 180° domain walls. The c axes, normal to the dark Bi202 sheets in the plane of the image, are nearly, but not exactly, parallel in the two domains the two component c axes, C and C, are inclined at about 3°. These observations suggest that the domain walls are of 180° type and that the polar axes of both the n = 2 and n = 5 compounds deviate slightly from the Ct or c0 axes. As in Bi4Ti3012, there may be some small monoclinic distortion. Selected area diifraction patterns, nominally taken from both sides of the domain boundary, were identical, but the small size of the domain, about 50 nm across, makes the evidence from selected area diffraction inconclusive. As expected for 180° domains, the domain walls are quite thin since only a small adjustment or relaxation of the [B08] octahedra would be necessary, the structure of the wall is undoubtedly simple. As a consequence, the lines of contrast due to the A site cations are clearly visible in the neighbourhood of the wall (figure 9), with only minor distortions around the boundary (dashed line).

Case Studies. To understand typical scenarios of charge-ordered manganates, it is instructive to examine the properties of two manganates with different sizes of the A-site cations. For this purpose, we choose NdasSr MnOa with a weighted average radius of the A-site cations, (rx), of 1.24 A and Pro,6-Cao,4MnO) with an (rA) of 1.17 A (Shannon radii are used here). 

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