Ca₄Bi₆O₁₃, a Compound Containing an Unusually Low Bismuth Coordination Number and Short Bi•••Bi Contacts

Single crystals and powder samples of a new bismuth(III) calcium oxide, Ca₄Bi₆O₁₃, have been synthesized and studied by X-ray diffraction. This compound crystallizes in the orthorhombic space group C2mm with Z = 2. The absence of a center of symmetry was confirmed by the presence of a second harmonic signal some 60 times that observed for quartz. The cell parameters are a = 5.937 (1), b = 17.356 (4), c = 7.206 (4) Å. A weak superstructure (2 × 3.6 Å), visible in long-exposure rotation and precession photographs, exists along c* due in part to alternation of oxygen and vacancies along the c axial direction. The structure consists of ribbons of edge-linked BiO₅ square pyramids running parallel with the c axis. These chains are linked via a novel three-coordinate Bi atom to form semicylinders stacked along the a axial direction. Sheets of these units are then stacked along the b axial direction and are separated by Ca ions in 7-fold coordination with oxygen. Along the c direction, the three-coordinate Bi atoms form •••BiOBi•••BiOBi••• chains. The Bi•••Bi contacts of these chains are short, 3.341 (2) Å, and the bridging oxygen atoms are displaced by about 0.25 Å from the centers of the Bi-O-Bi bridges in the direction perpendicular to these bridges. Molecular orbital calculations suggest that this displacement of the bridging oxygen atoms reduces the extent of lone pair-lone pair repulsion that occurs in each short Bi•••Bi contact.