Single crystals of Ag2Cs[B15O24] were prepared by using a B2O3 flux technique with Ag2CO3 and Cs2CO3 in a closed silver tube at 650 °C. X-Ray investigations led to a new crystal structure in the space group P 21212 (Nr. 18) with lattice parameters a = 1778.7 (4); b = 2219.2 (4); c = 512.2 (1) pm, Z = 4. The silver atoms are part of distorted AgO4 tetrahedra and planar AgO3 units while the caesium atoms are eightfold coordinated by oxygen. The compound contains an as yet unknown borate anion [B15O24] with twelve BO3- and three BO4 units per formula. The BO3 units are connected to eight cords twisted to helices which are combined via BO4 tetrahedra to a three dimensional framework with large tunnels occupied by the cations.

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Single crystals of AgSr [B70 , 2] were obtained by using a B20 3 flux-technique and Ag2C 0 3 and SrC 03 as starting materials in a silver ampoule at 800 °C. X-ray investigations on single crystals led to the space group C 2/c with a = 664.2(5); b = 1153.6(3); c = 2198.0(6) pm; ß = 92,52(3)°; Z = 8. Ag+ is fourfold coordinated by oxygen atoms and forms distorted A g04 tetrahedra while Sr2+ has nine nearest neighbours. The compound contains a hitherto unknown two dimensional borate anion [B70 i 2]3-which contains four trigonal planar B 0 3 groups and three B 0 4 tetrahedra per unit. Einleitung

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Ag 2 Cs [B 15 O 24 ]: An Anhydrous Quaternary Silver(I)-Borate with a New Helical Borate Anion.-Single crystals of the title borate are obtained from Ag 2 CO 3 , Cs 2 CO 3 and B 2 O 3 using a flux technique. X-ray studies furnish evidence of a quite novel crystal structure (space group P2 1 2 1 2 1 , Z = 4). The Cs atoms are eightfold coordinated by oxygen, while the Ag atoms are part of somewhat rare distorted AgO 4 tetrahedra as well as crystallographically independent, more unusual planar AgO 3 units. Moreover, the compound contains as yet unknown borate anions [B 15 O 24 ] 3− with twelve BO 3 -and three BO 4 -units per formula. The BO 3 units are connected to eight cords twisted to helices which are combined via BO 4 tetrahedra to a three-dimensional framework having large tunnels occupied by the cations. -(WIESCH, A.; BLUHM, K.; Z. Naturforsch., B: Chem. Sci. 53 (1998) 2, 157-160; Inst. Anorg.