A-site cation ordering in AA′BB′O6 perovskites

Unlike ordering of the octahedral B-site cations, ordering of the larger A-site cations in stoichiometric perovskites is rare. Herein the A- and B-site ordering characteristics of several double perovskites with AABB'O-6 Stoichiometry have been investigated. The compounds investigated include NaLaMgWO6, NaLaMgTeO6, NaLaScNbO6, NaLaScSbO6, NaLaTiO6, and NaLaZrO6. Group theoretical methods are used to enumerate the possible structures of AA'BB'X-6 double perovskites that result from the combination of rock salt ordering of the B-site cations, layered ordering of the A-site cations, and octahedral tilting distortions. This combination results in 12 possible structures in addition to the aristotype. Among the compounds investigated only NaLaMgWO6 and NaLaScNbO6 show significant long-range ordering of the A-site cations, Na+ and La3+. A complete structural characterization is presented for NaLaMgWO6. This compound possesses monoclinic C2/m (#12) space group symmetry, With unit cell dimensions of a = 7.8074(1)angstrom b = 7.81 58(1) angstrom, c = 7.8977(1) angstrom, beta=90.136(1)degrees at room temperature. The results presented here show that ill AA'BB'O-6 perovskites layered ordering of A-site cations creates a bonding instability that is compensated for by a second-order Jahn-Teller distortion of the B' cation. These two distortions are synergistic and the removal of one leads to the disappearance of the other. (c) 2006 Elsevier Inc. All rights reserved.