Ag2Cs[B15O24]:: An anhydrous quaternary silver(I)-borate with a new helical borate anion

Single crystals of Ag2Cs[B15O24] were prepared by using a B2O3 flux technique with Ag2CO3 and Cs2CO3 in a closed silver tube at 650 degrees C. X-Ray investigations led to a new crystal structure in the space group P 2(1)2(1)2 (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.