Cr(3)Si(2)O(7).(1)/(4)MX (MX = NaCl, NaBr, KCl, KBr): A Cage Structure Built from [Cr(II)O(4)] and [Si(2)O(7)]...

The new compounds Cr(3)Si(2)O(7).(1)/(4)MX (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 Cr(2)O(3), Cr, SiO(2), and the respective alkali-metal halide in evacuated silica tubes at approximately 1620 K for 2 days. The structures of Cr(3)Si(2)O(7).(1)/(4)NaCl, Cr(3)Si(2)O(7).(1)/(4)NaBr, Cr(3)Si(2)O(7).(1)/(4)KCl and Cr(3)Si(2)O(7).(1)/(4)KBr (I4/mmm (No. 139), Z = 8, a approximately 10.3 Å, c approximately 12.97 Å) have been solved and refined from X-ray single-crystal data. The framework consists of two crystallographically independent Cr(II) in square planar oxygen coordination. Three of those squares form a bent trimer [Cr(3)O(4)O(4/2)]. Four trimers are connected through vertices forming a [Cr(12)O(24)] cage wherein the halide ion is sited. By the linking of individual [Cr(12)O(24)] 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 angle(Si,O1,Si) approximately 125 degrees. Magnetic measurements indicate strong low-dimensional antiferromagnetic interactions among the Cr(II) ions. The UV/vis spectrum of Cr(3)Si(2)O(7).(1)/(4)NaBr is reported.