Acta Cryst. (2006). B62, 228-235 [ doi:10.1107/S0108768106001510 ]
Abstract: The structures of two specimens of the (K1 - yNay)2F - 2(SrxBa1 - x)2 - FNb2O6 family (KNSBN) have been solved and refined (xI = 0.6, yI = 0.45, FI = 1.07 and xII = 0.72, yII = 0.45, FII = 1.08). The KNSBN compounds appear to be incommensurately modulated, as described in a five-dimensional superspace approach with the tetragonal P4bm (pp1/2, p - p1/2) superspace group. Their modulation wavevectors are almost independent of the composition. The description of the modulation is restricted to first-order harmonics in order to limit the number of refined parameters. The modulation mainly affects the positions of the O atoms and their displacements depend on the Sr/Ba ratio. The structural models for the two compounds are not only almost identical, but also analoguous to the structural model of the SrxBa1 - xNb2O6 parent compound: doping with K+ and Na+ cations has no significant influence on the incommensurate modulation, contrary to the strong dependence observed for the physical properties.
Keywords: modulated structure; first-order harmonics; photorefractive properties.
 |   Structure factor file (CIF format) (900.8 kbytes) |
| Structure factor file (CIF format) (408.0 kbytes) |
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