Growth, Structure, and Optical Properties of a Nonlinear Optical Niobium Borate Crystal CsNbOB 2 O 5 with Distorted NbO 5 Square Pyramids.

In this paper, a revised structure determination of an already known compound CsNbOB 2 O 5 with new structural insights was performed and the detailed characterization of its optical properties was reported for the first time. CsNbOB 2 O 5 was synthesized by spontaneous crystallization. It crystallizes in the Pmn 2 1 space group, and the unit lattice parameters are a = 7.5220(7) Å, b = 3.9881(4) Å, c = 9.7167(9) Å, and Z = 2. In the structure of CsNbOB 2 O 5 , a [NbO 5 ] square pyramid and [B 2 O 5 ] unit are linked to constitute an infinitely extended two-dimensional ∞ 2 [NbOB 2 O 5 ] layer via sharing oxygen atoms. Between these two-dimensional layers, there are no covalent bonds perpendicular to their planes based on Mulliken bond order analysis. CsNbOB 2 O 5 has a wide band gap (4.52 eV) and a large second-harmonic-generation (SHG) response (1.2 × KDP) and demonstrates type-I phase-matchable behavior. First-principles simulations reveal that the birefringence is approximately 0.10. Moreover, SHG-weighted charge density analysis shows that the primary source of the nonlinearity of the title compound is the distorted NbO 5 square pyramids.