The non-centrosymmetric borate oxides, MBi2B2O7 (M=Ca, Sr)

Two novel noncentrosymmetric borates oxides, MBi2B2O7 or MBi2O(BO3)2 (Mdouble bond; length as m-dashCa, Sr), have been synthesized by solid-state reactions in air at temperatures in the 600–700 °C range. Their crystal structures have been determined ab initio and refined using powder neutron diffraction data. CaBi2B2O7 crystallizes in the orthorhombic Pna21 space group with a=8.9371(5) Å, b=5.4771(3) Å, c=12.5912(7) Å, Z=4, Rwp=0.118, χ2=2.30. SrBi2B2O7 crystallizes in the hexagonal P63 space group with a=9.1404(4) Å, c=13.0808(6) Å, Z=6, Rwp=0.115, χ2=4.15. Large displacement parameters suggest the presence of disorder in SrBi2B2O7 as also revealed by diffuse 2×a superstructure reflections in electron diffraction patterns. Both structures are built of identical (001) neutral layers of corner-sharing BO3 triangles and MO6 trigonal prisms forming six-membered rings in which Bi2O groups are located. Adjacent layers are stacked in a staggered configuration and connected through weak Bi–O bonds. A moderate efficiency for second harmonic generation (SHG) has been measured for a powder sample of CaBi2B2O7 (deff=2deff(KDP)).