The Incommensurately Modulated Structures of the Perovskites NaCeMnWO₆ and NaPrMnWO₆

The structures of the doubly ordered perovskites NaCeMnWO₆ and NaPrMnWO₆, with rock salt ordering of the Mn²⁺ and W⁶⁺ <i>B</i>-site cations and layered ordering of the Na⁺ and (Ce³⁺/Pr³⁺) <i>A</i>-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 NaCeMnWO₆ the modulation vector (with reference to the ideal <i>ABX</i>₃ perovskite subcell) is <i>q</i> ≈ 0.067<i>a</i>* (∼58.7 Å) and in NaPrMnWO₆ <i>q</i> ≈ 0.046<i>a</i>* (∼85.3 Å). In both compounds the superstructures are primarily the two-dimensional chessboard type, although some crystals of NaCeMnWO₆ 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