Sr0.735Ba0.265Cu2(BO3)2 and Sr0.661Ca0.339Cu2(BO3)2

The crystal structures of Sr 0.735 Ba 0.265 Cu 2 (BO 3 ) 2 and Sr 0.661 Ca 0.339 Cu 2 (BO 3 ) 2 , which are isostructural with SrCu 2 (BO 3 ) 2 , have been refined from single-crystal X-ray diffraction data. The structures consist of slightly puckered layers with the composition [CuBO 3 ] − stacked along the c axes, with Sr, Ba or Ca atoms eight-coordinated between the layers. The interlayer separation is directly affected by the substitution of smaller Ca or larger Ba atoms in place of Sr. In both structures, each Cu atom is coordinated by four O atoms with Cu-O distances ranging from 1.91 to 1.94 A. The borate groups are distorted from D 3h to C 2v symmetry