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.

The hollandite‐related structure La1.16Mo8O16, Mr = 1184.66, tetragonal, , a = 9.983 (1), c = 2.8890 (5) Å, V = 287.9 Å3, Z = 1, Dx = 6.83 g cm−3, (Mo Kα) = 0.71069 Å, μ = 126 cm−1, F(0000) = 530, room temperature, R = 0.041 for 1145 unique reflections with I3σ(I). At room temperature the compound exhibits a one‐dimensional incommensurate modulated structure with a modulation wavevector q* = 0.608 (1)c*. Both a displacive modulation wave, acting on La, Mo and O atoms, and a modulation wave governing the occupancy probability of La sites inside the tunnels are involved in the crystal. Within the superspace group , the final R values of main reflections (477) and first‐ and second‐order satellite reflections (523 and 145) are 0.030, 0.050 and 0.133 respectively. The more spectacular modulation features are the occurrence of La—La pairs in the tunnels and the formation of Mo3 triangular clusters in the double chains of edge‐sharing octahedra. Contrary to previous descriptions based upon rigid tunnels in structures of the hollandite type, the tunnels in the crystal studied are distorted in a periodic way along [001] (λ = 4.75 Å), giving rise to alternate contractions and expansions. The distortion of the double octahedral chains is considerable and probably created via the La—O(1) bonds by insertion of La atoms inside two adjacent tunnels.

NH4)3H(SO4)2, M,.=247.2, monoclinic, C2/c, a= 15.4353 (18), b =5.8647 (11), c= 10.1696(8) A, fl = 101.829 (8) °, V= 901.5 (4) ,~3, Z=4, Dx = 1.82 Mg m -a, A(Mo Ka) =0"71069 ,~, /~ =0"61 mm -1, F(000)=520, T=293K, R=0.029 for 1266 reflections. NH~ tetrahedra are rather regular and linked to SO42-ions by hydrogen bonds. Neighbouring SO 2-ions are linked by O-H...O hydrogen bonds. Periodic bond chains made up of three kinds of bonds run through the structure. The protonic conductivity is explained in the literature by chains which are like one of the kinds found here. Unfortunately they do not exist in the direction of highest conductivity. Relations between structure and morphology show that the commonly occurring forms {100}, {001}, {110}, {117}, {111}, {311}, {101} are F faces according to Hartman theory.

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Potassium dithioacetate KCH3CS2 was obtained by action of dithioacetic acid on potassium carbonate in aqueous solution. KCH3CS2 crystallizes orthorhombic within the space group Cmcm. The cell dimensions are: a = 7.689, b = 10.565, and c = 6.490 Å, Z = 4. The coordination number of potassium is 8. Structural refinement by least‐squares method came to a final R = 0.097.