Ag 8 SiS 4 Te 2 , a New Thiosilicate Telluride

Three different diffraction images were observed for crystals of the iron-deficient compound TlFe2–xSe2. The crystals of nearly the same composition (x ≈ 0.4) were synthesized by various routes. Different temperature regimes obviously lead to different ordering patterns of the iron atoms and vacancies in this material. Three different crystal types were identified yet: Crystals of type A exhibit a complete statistical distribution of iron atoms and vacancies, their diffraction images contains only Bragg reflections compatible with the ThCr2Si2 type unit cell (space group I4/mmm, lattice parameters a = 389.1(1) pm, c = 1402.7(4) pm). Type B crystals show a commensurate √5 √5 2 superstructure (I4/m, a = 869.6(1) pm, c = 1400.9(9) pm) like the one reported in [1]. Two independent Fe potions are partially occupied in this structure to a significantly different amount. Crystals of type C show a diffraction pattern of a modulated structure (P2(αβ0)0 with α = 0.2, β = 0.6, a = b = 389.9(1) pm, c = 1388.0(2) pm, γ = 90.0(1)°). A partial ordering of vacancies in only one of the two Fe layers per unit cell, as displayed in Figure 1, is found here.