Experimental and ab initio studies of two UV nonlinear optical materials

Abstract

Two new acentric polyborates, Ba₃(OH)(B₉O₁₆)[B(OH)₃] and Ba_2.16Pb_0.84(OH)(B₉O₁₆)[B(OH)₃] have been synthesized hydrothermally and their structures have been determined by single-crystal X-ray diffraction. Both crystals are isostructural and crystallize in the trigonal space group P31c (no. 159). Their structures feature a three-dimensional (3D) B₉O₁₉ framework with 6-membered-ring tunnels, in which the Ba or Pb/Ba cations and [B(OH)₃] groups reside. Powder second-harmonic-generation (SHG) measurements reveal that Ba₃(OH)(B₉O₁₆)[B(OH)₃] and Ba_2.16Pb_0.84(OH)(B₉O₁₆)[B(OH)₃] are type-I phase-matchable, with SHG responses of 1.1× and 1.2× KH₂PO₄, respectively. UV-Vis-NIR diffuse reflectance analysis indicates that Ba₃(OH)(B₉O₁₆)[B(OH)₃] and Pb_0.29Ba_2.71(OH)(B₉O₁₆)[B(OH)₃] have band gaps of 5.11 and 4.65 eV, respectively. In addition, first-principles calculations were employed to elucidate the origin of the NLO properties and the relationship of structure–properties.