Spontaneous Superlattice Formation in the Doubly Ordered Perovskite KLaMnWO6

Electron, neutron, and synchrotron X-ray diffraction together with transmission electron microscopy studies reveal the spontaneous formation of a complex superlattice in bulk samples of the perovskite KLaMnWO6. The superlattice structure, which possesses P (4) over bar 2m space group symmetry with a = 40.0637(7) angstrom and c = 8.1306(3) angstrom, results from a two-dimensional compositional modulation of the A-site cations (K+ and La3+), combined with a complex pattern of tilts involving the corner connected octahedra. The basic pattern of octahedral tilting involves out-of-phase tilts of neighboring octahedra about the pseudocubic a and b axes (a(-)a(-)c(0) tilting). Unexpectedly, the out-of-phase tilting is disrupted in both directions by an in-phase tilt once every five octahedra. The occurrence of regularly repeating, well-separated in-phase tilts helps to alleviate strains that arise from formation of the compositionally modulated chessboard superlattice.