Cr₃Si₂O₇·¹/₄MX (MX = NaCl, NaBr, KCl, KBr): A Cage Structure Built from [Cr^IIO₄] and [Si₂O₇] Units¹

The new compounds Cr3Si2O7·1/4MX (MX = NaCl, NaBr, KCl, KBr) form a series of chromous disilicates hosting alkali-metal halides in a framework structure. Purple crystals with edge lengths up to 1 mm were prepared by reacting mixtures of Cr2O3, Cr, SiO2, and the respective alkali-metal halide in evacuated silica tubes at ∼1620 K for 2 days. The structures of Cr3Si2O7·1/4NaCl, Cr3Si2O7·1/4NaBr, Cr3Si2O7·1/4KCl and Cr3Si2O7·1/4KBr (I4/mmm (No. 139), Z = 8, a ∼ 10.3 Å, c ∼ 12.97 Å) have been solved and refined from X-ray single-crystal data. The framework consists of two crystallographically independent CrII in square planar oxygen coordination. Three of those squares form a bent trimer [Cr3O4O4/2]. Four trimers are connected through vertices forming a [Cr12O24] cage wherein the halide ion is sited. By the linking of individual [Cr12O24] units, disilicate groups form a second type of cage which is occupied by Na+ or K+. The disilicate groups are in eclipsed conformation with a bridging angle ∠(Si,O1,Si) ∼ 125°. Magnetic measurements indicate strong low-dimensional antiferromagnetic interactions among the CrII ions. The UV/vis spectrum of Cr3Si2O7·1/4NaBr is reported.