| Résumé | Two novel noncentrosymmetric borates oxides, MBi2B2O7 or MBi2O(BO3)2 (MCa, 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)??, b=5.4771(3)??, c=12.5912(7)??, Z=4, Rwp=0.118, [chi]2=2.30. SrBi2B2O7 crystallizes in the hexagonal P63 space group with a=9.1404(4)??, c=13.0808(6)??, Z=6, Rwp=0.115, [chi]2=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)). |
|---|
