Rock-salt ordered double perovskites

Figure 2.1 The octahedral framework of the rock-salt ordered double perovskite structure. The A-site cations are omitted for clarity...

Figure 2.33 A phase diagram indicating the range of stability of B-site ordered double perovskites, denoted as -I—I—I—I—I- rock salt, in terms of differences in ionic radii and formal charge. Reprinted with permission from Anderson eta/., 1993 [80], Copyright (1993) Elsevier...

The symmetry of ordered rock-salt A2(BB X) double perovskites, taking into account octahedral tilting, has been systematically derived via group theoretical analysis. If the stmcture is cubic, with a tilt system aVa , the space group is Ftriitn (225), the new axes run parallel to the simple cubic perovskite axes, and the lattice parameter is double the simple cubic lattice parameter a. Thus, in the case of Ba FeMoO, a=0.81865 nm Relatively small A cations usually lead to octahedral tilting, as... 

Double perovskites are described by the general formula A2B B"06 5, and can be interpreted as a doubling of the regular perovskite ABOa.s- The elements on the B site are different from those on the B" site, and the two sites alternate to form a sublattice with a rock-salt structure. In a perfectly ordered lattice, every B ion would be coordinated by six perovskite unit cells with B" ions and vice versa. This is illustrated in Fig. 3.16 for Sr2MgMo06 g (SMMO) the Mo ion (green) is surrounded by 6 Mg ions (red). The ordering is caused by large differences in the... 

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