A-site cation ordering in AA′ BB′O 6 perovskites

Abstract

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 AA′ BB′O 6 stoichiometry have been investigated. The compounds investigated include NaLaMgWO 6, NaLaMgTeO 6, NaLaScNbO 6, NaLaScSbO 6, NaLaTi 2O 6, and NaLaZr 2O 6. 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 NaLaMgWO 6 and NaLaScNbO 6 show significant long-range ordering of the A-site cations, Na + and La 3+. A complete structural characterization is presented for NaLaMgWO 6. This compound possesses monoclinic C2/ m (#12) space group symmetry, with unit cell dimensions of a = 7.8 0 7 4 ( 1 ) Å , b = 7.8 1 5 8 ( 1 ) Å , c = 7.8 9 7 7 ( 1 ) Å , β = 9 0.1 3 6 ( 1 ) ° at room temperature. The results presented here show that in 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.