The incommensurately modulated structures of the perovskites NaCeMnWO6 and NaPrMnWO6

Abstract

The structures of the doubly ordered perovskites NaCeMnWO 6 and NaPrMnWO 6 , with rock salt ordering of the Mn 2+ and W6+ B-site cations and layered ordering of the Na + and (Ce 3+ /Pr 3+) A-site cations, have been studied by transmission electron microscopy, electron diffraction, neutron and synchrotron X-ray powder diffraction. Both compounds possess incommensurately modulated crystal structures. In NaCeMnWO 6 the modulation vector (with reference to the ideal ABX3 perovskite subcell) is q ≈ 0.067a* (∼58.7 Å) and in NaPrMnWO6 q ≈ 0.046a* (∼85.3 Å). In both compounds the superstructures are primarily the two-dimensional chessboard type, although some crystals of NaCeMnWO 6 were found with one-dimensional stripes. In some crystals of NaPrMnWO6 there is a coexistence of chessboards and stripes. Modeling of neutron diffraction data shows that octahedral tilting plays an important role in the structural modulation