THE MODULATED STRUCTURE OF THE COMMENSURATE MISFIT-LAYER COMPOUND (BISE)1.09TASE2

Single-crystal X-ray diffraction (Mo Kalpha radiation, lambda = 0.7107 3 angstrom) was used to determine the structure Of the commensurate in organic misfit-layer compound (BiSe)alpha0TaSe2, alpha0 = 12/11 congru ent-to 1.09. The structure is described as a (3 + 1)-dimensional inter growth compound with two subsystems and with symmetry according to the superspace group P:Fm2m(alpha0,0,0)111BAR. The TaSe2 subsystem has a1 1 = 3.421 (1), a12 = 5.970 (1) and a13 = 24.34, (7) angstrom, with sub system space group Fm2m and subsystem superspace group P.Fm2m(alpha0,0 ,0)111BAR. The BiSe subsystem has lattice parameters a21' = 3.135 (1), a22' = 2.984 (1) and a23' = 12.174 (4) angstrom, with subsystem space group Pm2m and subsystem superspace group P:Pm2m(alpha0(-1),1/2,1/2)1 11BAR. The (a(nu2),a(nu3)*) reciprocal-lattice plane is common to the two subsystem lattices, nu = 1, 2. Refinements on 1614 unique reflect ions converged smoothly to R = 0.050. The modulation was divided into a large block-wave occupational modulation describing the Bi/Se orderi ng and a relatively small displacement modulation. An interpretation i s given of the effect of the latter using the bond-valence method. The structural features are compared to the incommensurate misfit-layer c ompounds, in relation to the inter-subsystem bonding.