Order−Disorder and Direct Evidence of Oxygen Vacancies in a New Family of BICUWOX Compounds

During the synthesis of the compound with nominal composition Bi₄Cu_1/3W_2/3O_7.8Cl, a new family of compounds, which can be described as ordered intergrowth of one Sillén block [Bi₂O₂Cl] and n oxygen deficient blocks [Bi₂Cu_1/(2n+1)W_2n/(2n+1)O_6-δ]_n with δ = 0.18, was produced. Transmission electron microscopy and associated techniques were used to determine the composition, unit-cell symmetry, and space group of the main phase corresponding to the n = 1 member Bi₄Cu_1/3W_2/3O_8-δCl. Structural models for both compounds n = 1 and n = 2 were deduced by exit wave reconstruction in a crystal where these phases were intergrown. The existence of the highest members n = 3 and 4 was confirmed by HREM in several crystallites, in which we have found the compounds with n = 1−4 as disordered intergrowths. Moreover, some crystals with cationic ordering in the octahedral layer were also found. Oxygen atoms and vacancies were imaged using the exit wave reconstruction technique in the structure determination of the first Sillén-BICUWOX intergrowth. The average structure of the main phase Bi₄Cu_1/3W_2/3O_8-δCl was refined from X-ray powder diffraction data using the Rietveld method, yielding a tetragonal unit cell with lattice parameters a = 3.845(9) Å, c = 14.188(7) Å (space group P4/mmm No. 123), which could be described as a Sillén−Aurivillius oxygen deficient intergrowth.