Double Perovskites and Related Ordered Structures

When several different ionic species are present in which the charges or the ionic sizes differ enough, lattice energy can often be reduced by ordering some or all of the ions over the available crystallographic sites. The most widely encountered 

Perovskites Structure-Property Relationships, First Edition. Richard J. D. TQley. 2016 John Wiley Sons, Ltd. Published 2016 by John WUey Sons, Ltd. 

The commonest ordering pattern encountered corresponds to the two different cations, B and B, arranged in a chessboard pattern, often called rock-salt ordering (Figiure 2.1). 

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 

Bond valence sums are often calculated for these phases to clarify aspects of valence and stmcture - for example, bond valence sums for the double perovskite Ca MgOsOg are Ca—O, 2.03 Mg—O, 2.15 and Os—O, 6.49. These suggest that assigning the ionic states Mg and Ca is reasonable, but the Os cation is significantly overbonded, suggesting that there may be a considerable departure from the formal Os state in the OsO. units. 

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