CHARGE DENSITY STUDY OF NONLINEAR OPTICAL CRYSTALS: LBO, BBO AND KB5. Hwo-Shuenn Shen^l,2, Chih-Chieh Wang^l, Chi-Rung Lee^l, Yu Wang^l,2*, Alex Darovsky³, Robert Bolotovsky^3, Philip Coppens³, ¹Department of Chemistry, National Taiwan University, Taiwan, R.O.C., ²Synchrotron Radiation Research Center, Hsinchu, Taiwan, R.O.C., ³X3, National Synchrotron Light Source, BNL, Upton, New York, U.S.A.

Borate Compounds have attracted great attention recently because of its excellent UV-NLO (nonlinear Optical) properties. KB5(KB₅O₁₀H₄^.2H₂O) is the first developed borate NLO crystal, while BBO ([[beta]]-BaB₂O₄) and LBO (LiB₃O₅) are the best materials so for developed. All the three crystals are crystallized in noncentric space group of Pna21, R3c, and Aba2 for LBO, BBO, and KB5 respectively. Accurate single crystal diffraction data were measured at room temperature using MoK[[alpha]] and at ~50K using synchrotron radiation using 0.394Å for LBO; and at room temperature and 125K using MoK[[alpha]] for BBO and KB5 respectively. Charge density distribution studies were made in order to investigate the correlation between the electron density distribution and NLO property. The NLO properties of LBO, BBO and KB5 are believed to be mainly contributed from their anionic group, (B₃O₇)^5-, (B₃O₆)^3- and [B₅O₆(OH)₄]^1-. The distorted anionic groups are revealed from their bond distances and angles of BO₃ and BO₄. Charge density distribution maps of the three title compounds are derived using the multipole model. The uneven charge density distribution of these anionic groups are clearly represented. A slight difference between the two 3-coordinated B of LBO is found. Highly polarized and delocalized of (B₃O₆)^3- of BBO is clear detected. These uneven charge density distribution and polarization may correlate to their NLO property. The comparison on different wavelengths and different temperatures of the three compounds will be discussed.