| Nonlinear optical materials are the basic equipment in application of laser technology and have been widely used in military and medical. Up to now, it is still a challenge to get nonlinear optical materials used in short ultraviolet（UV） or deep UV. Tetrahedron-contained borates have received lots of attention for their various anionic groups, changing connection types, short UV cut-off edges and relative strong second harmonic generation（SHG） response. Recently, tetrahedron-contained nonlinear optical materials with excellent nonlinear optical（NLO） properties have been constantly emerging, while the relationship between tetrahedron and optical properties is not clearly. For the purpose of exploring new nonlinear optical materials, in this work, the DFT calculations are used to study on the role of basic groups in optical properties response in the known Ge/Si-contained borates. The study will have a guiding significance in designing or synthesizing new NLO borate materials. Main contents and results are as follow:（1） By analyzing the structures, we find that in Ge/Si-contained borates, the B-O group change from BO3 plane and BO4 change to only contain BO4 group as the increase of R ratio, meanwhile, the B-O apt to form chains or isolated B-O groups from rings or chains.（2） Results imply that for Ge/Si-contained alkaline, alkaline earth and rare earth metal borates containing BO3 groups, BO3 groups are not the only contributor of strong SHG effect, BO4 and RO4 groups also have a considerable contribution. By analyzing PDOS of O atoms in these compounds, it is found that the nonbonding p orbitals of bridge oxygen in tetrahedra are more nearly to the valence-band maximum than that in BO3. This study can provide a new idea in designing tetrahedron-contained deep UV nonlinear optical materials.（3） In addition, the relationship between electronic structure and optical properties of LiBGeO4, a structure only containing tetrahedral groups with strong SHG response has been studied. Comparing the band structures of Li BGe O4 and BPO4, we find that the dispersion of band structures is the reason why those two compounds have strong SHG response. This work suggests the tetrahedral anion groups possessing relative shorter cut-off edge can also have excellent SHG performance. |
