Abstract. Simulated annealing approach was successfully applied to solve three unknown molecular structures from X-ray laboratory powder data using a priory known structural fragments. Some possible developments of the method are discussed.

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The results of x-ray diffraction measurehens of a lead magnoniobate single crystal-PbMglpNb2/jO3, performed on a four-circle diffractometer with MO Kci radiation. are described. It is'confirmed that even at relatively high temperatures lead ions are shifted from the highaymetry position. It is shown that these shifts are random both in length and direction. A spherical layer model is proposed for the adequate description of the experimental data. The correspondence between the crystal structure and the physical properties of the crystal is discussed.

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A structural study of Pb3MgNb2O9 and ordered Pb2ScTaO6 single crystals is made by X‐ray diffraction at high temperatures. Displacements of the Pb atom from the special site are clearly observed. Disordered atomic positions are uniformly distributed over a sphere with radius 0.2–0.3 Å and with center at the special position. These atomic displacements are probably caused by local fields due to random statistical distribution of the (Mg, Nb) atoms in Pb3MgNb2O9 and, partially, the (Sc, Ta) atoms in Pb2ScTaO6. Computer modeling, based on the total‐energy‐minimization method, gives satisfactory agreement with experimental results.