Abstract
The new chromous oxo-halides Cr4(Si2O7)X2(X= Cl, Br) which adopt a hitherto unknown structure type were obtained as small, deep blue crystals from mixtures of Cr2O3, Cr, SiO2, and CrCl3or CrBr2, respectively, in sealed silica tubes at elevated temperatures (Cr4(Si2O7)Cl2: 1000°C; Cr4(Si2O7)Br2: 1220°C) in the presence of a mineralizer (NH4Cl; excess of CrBr2). They show remarkable stability against oxidation. The crystal structures of both Cr4(Si2O7)Cl2(Cr4(Si2O7)Br2) have been determined and refined from X-ray single crystal data (PcNo. 7),Z= 4,a= 6.3853(7) Å (6.414(3) Å),b= 12.707(2) Å (12.829(6) Å),c= 10.448(1) Å (10.540(6) Å), β = 92.37(1)° (91.61(4)°)). Four circle diffractometer data were used for the refinement of the chloride structure while imaging plate data were used for the bromide (conventional residualR1 = 0.016 (0.053) for 4761 (3491) Fo > 4σ, 4945 (3911) independent reflections, 273 (273) parameters). The structure consists of eight crystallographically different Cr2+with c.n. five or six and mixed coordination by oxygen and halogen. All coordination polyhedra are highly distorted as a result of the Jahn–Teller effect. Two independent disilicate groups exhibit almost eclipsed conformation with bridging angles (Si–O–Si) of 145.8° and 138.4° for the chloride and 146.3° and 140.8° for the bromide.
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