THE MODULATED STRUCTURE OF LAMO8O14

Lanthanum octamolybdate [LaMo8O14, a = 11. 129 (1), b = 10.000 (1), c = 9.218 (1) angstrom, basic space group C2ca, superspace group P111BAR (C2ca), M(r) = 1130.4, D(x) = 7.32 g cm-3, V = 1025.9 angstrom3, Z = 4 , T = 294 K, Mo Kalpha radiation, lambda = 0.710730 angstrom, mu = 134 .5 cm-1, F(000) = 2020] exhibits a one-dimensional modulated structure that is found to be commensurate within experimental error with a mod ulation wavevector q having components 0,1/3,0. Both a density modula tion wave, governing the occupancy probability of some atomic sites [L a(1), La(2), Mo(4) and Mo(5)], and a displacive modulation wave of sma ll but significant amplitude, acting on all atoms, are involved in the crystal. The modulated structure was refined from 3347 unique hklm re flections [I greater-than-or-equal-to 3sigma(I)] with the REMOS progra m. 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, res pectively. Because the q2 component of q* has the particular value an d the modulation is harmonic, remarkable relations result between the atomic displacements. Within a classical approach, with use of the spa ce-group description, these relations would be masked. The average str ucture of this crystal appears to be closely related to the structure of LaMo7.7O14 crystal, which is not modulated. The main result of this study is the occurrence of four distinct configurations of the Mo8 me tallic clusters inside the same crystal. As suggested by the results o f electron microscopy investigations, a more or less large deviation f rom an ideal structural order would be involved in most of the modulat ed crystals.