STACKING ORDER AND DISORDER IN LAYERED K(3)ME(CN)(6) COMPOUNDS STUDIED BY DIFFUSE-X-RAY SCATTERING - A REALIZATION OF THE ANNNI MODEL

Layered crystalline materials like K(3)Me(CN)(6) with Me = Cr, Mn, Fe, Co may often exist in various polytypic forms, due to a variety of ch oices of layer stacking modes. For cases where the interlayer constell ations can be limited to only two energetically almost equivalent ways , the buildup of the crystal may be described by a spin-1/2 Ising-like model. For the system presently being studied one can rationalize the layer stacking to a four-valued choice (i.e., a 1D 4-state Pelts case ), or use an Ising-like two-sublattice model. Previous diffraction stu dies of K(3)Me(CN)(6) indicated that two long-range ordered structures prevailed, an orthohombic one named MDO(1), with one double layer per repetition unit, and a monoclinic one, MDO(2), with two double-layer units. Our studies reveal a more complex situation: The Fe material is for the most part of the MDO(2) type. But in addition, in some crysta l samples, a hitherto unobserved phase also appears, with six double-l ayer repetition units, in fact a hybrid of MDO(1) and MDO(2). The Co m aterial is for the most part of the MDO(2) type, but contains in addit ion a considerable contribution of stacking disorder, as evidenced by the presence of diffuse X-ray scattering lines. The lines do, however, contain distinct maxima, indicating the presence of several layer sta cking modes with preference of two, three, four, five, and seven doubl e-layer correlations. The findings can be qualitatively discussed in t erms of the ANNNI model.