At room temperature the bismuth strontium cuprate Bi2.osSrl.8aCuO6 [a = 5.3791 (6) pAZ/~, M, = 755.44, Dx = 7.05 g cm -3, V = 711.7 ,/k 3, Z = 4, Cu Ka radiation, = 1.541838/~, /z = 1203 cm-i, F(000) = 1278.24, T = 294 K] exhibits a one-dimensional incommensurate modulated structure with a modulation wavevector q* having components 0.2030 (2), 0, 0.467 (2). Both a displacive modul~ition wave, acting on all atoms and a density modulation wave, governing the occupancy probabilities of the Bi, Sr and apical 0(2) sites are involved in the crystal. 1117 unique hklm reflections with I_> 3o-(1) were used to refine the modulated structure. The final R factors (based on F) of main reflections (292), and first-and secondorder satellite reflections (563 and 262) are 0.061, 0.061 and 0.086 respectively. The modulation is anharmonic for all atoms. Evolution of the Bi, Sr and apical 0(2) site occupancies throughout the crystal is correlated in a spectacular way and shows the existence of Bi and 0(2) vacant sites in some unit cells; moreover, some Sr sites are occupied by Bi atoms. These features give rise to various coordinations i.e. square planar, pyramidal, octahedral for 0108-7681/92/040407-12506.00 copper and from six to nine for strontium. The most likely coordination for bismuth in strontium sites is ninefold coordination; owing to the displacive modulation, both dilute and Bi-condensed regions are found in the crystal. Within the condensed regions, bismuth achieves a classical fourfold coordination. To satisfy the Bi coordination in the Bi-dilute regions it is proposed that extra O atoms have to be introduced either within the Bi---O layers or between two adjacent Bi---O layers.

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Lanthanum octamolybdate [LaM 08014 , a = 11.129 (1), b = 10.000 (1), c = 9.218 (1) A~, basic space group C2ca, superspace group_ pc~, Mr = 1130.4, Dx = 7.32gcm -3, V= 1025.9A 3, Z=4, T= 294K, Mo Ka radiation, A =0.710730 A,, # = 134.5 cm-i, F(000) = 2020] exhibits a onedimensional modulated structure that is found to be commensurate within experimental error with a modulation wavevector q* having components 0,~,0. Both a density modulation wave, governing the occupancy probability of some atomic sites [La(1), La(2), Mo(4) and Mo (5)], and a displacive modulation wave of small but significant amplitude, acting on all atoms, are involved in the crystal. The modulated structure was refined from 3347 unique hklm reflections [I_> 30-(/)] with the REMOS program. The final R factors (based on F) of the (1845) main reflections and the (1502) first-order satellite reflections are 0.038 and 0.090, respectively. Because the q2* component of q* has the particular value and the modulation is harmonic, remarkable relations result between the atomic displacements. Within a classical approach, with use of the spacegroup description, these relations would be masked. The average structure of this crystal appears to be closely related to the structure of LaMo7.7Ot4 crystal, which is not modulated. The main result of this study is the occurrence of four distinct configurations of the Mo8 metallic clusters inside the same crystal. As suggested by the results of electron microscopy investigations, a more or less large deviation from an ideal structural order would be involved in most of the modulated crystals.