CR3SI2O7-CENTER-DOT-1 4MX (MX = NACL, NABR, KCL, KBR) - A CAGE STRUCTURE BUILT FROM [(CRO4)-O-II] AND [SI2O7] UNITS/

The new compounds Cr3Si2O7 .(1)/4MX (MX = NaCl, NaBr, KCl, KBr) form a series of chromous disilicates hosting alkali-metal halides in a fram ework structure. purple crystals with edge lengths up to 1 mm were pre pared by reacting mixtures of Cr2O3, Cr, SiO2, and the respective alka li-metal halide in evacuated silica tubes at similar to 1620 K for 2 d ays. The structures of Cr3Si2O7 .(1)/(NaCl)-Na-4, CT3Si2O7 .(1)/(NaBr) -Na-4 Cr3Si2O7 .(1)/4KCl and Cr3Si2O7 .(1)/Br-4 (I4/mmm (No. 139), Z = 8, a similar to 10.3 Angstrom, c similar to 12.97 Angstrom) have been solved and refined from X-ray single-crystal data. The framework cons ists of two crystallographically independent Cr-II in square planar ox ygen coordination. Three of those squares form a bent trimer [Cr3O4O4/ 2]. Four trimers are connected through vertices forming a [Cr12O24] ca ge wherein the halide ion is sited. By the linking of individual [Cr12 O24] units, disilicate groups form a second type of cage which is occu pied by Na+ or K+. The disilicate groups are in eclipsed conformation with a bridging angle angle(Si,O1,Si)similar to 125 degrees. Magnetic measurements indicate strong low-dimensional antiferromagnetic interac tions among the Cr-II ions. The UV/vis spectrum of Cr3Si2O7 .(1)/(NaBr )-Na-4 is reported.