Oxysulfides formed by a rare earth and a second metal II. Shear structures

Many of the oxysulfides containing two metals have layered structures of ( RO) sheets and ( M_xS_y) sheets ( R = rare earth, M = Cu, Ag, Ga, In, Ge, Sn, As, Sb, Bi). In the new compounds (La ₄O ₃)(AsS ₃) ₂ and (La ₅O ₃)In ₆S ₁₇, parallel ribbons of stoichiometry (La ₄O ₃) and (La ₅O ₃) substitute for the (LaO) sheets. The formation of ribbons is explained by shear mechanisms involving two different orientations of the periodic shear planes in the tetragonal (LaO) sheet. In a third compound La ₃GaS ₅O or (La ₂O)LaGaS ₅, the formation of (La ₂O) ribbons involves similar shear mechanism. All these shear structures preserve the main characters of the layered oxysulfides: oxygen is only bound to lanthanum, and the second metal is only bound to sulfur.