Li Rukang
Beijing Centre for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190.
China and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China.
Chemphyschem. 2024 Aug 1;25(15):e202400142. doi: 10.1002/cphc.202400142. Epub 2024 Jun 11.
α-BiBO (BiBO) is an important nonlinear optical (NLO) material with high efficiency for applications in harmonic generations and quantum technology. Owing to its low symmetry and cooperative Bi lone pair arrangement, it has also exceptional large piezoelectric and electro-optic coefficients and strong anisotropies on other material characteristics. Previous theoretical calculations on its physical (mainly optical) properties often gave confusing results. It is found here that London dispersion (LD) tends to stabilize structures with closer pack entities like lone pair heavy ion Bi with large polarizabilities, which is ignored in most previous density functional theory (DFT) calculations. Present study shows that without considering the LD effect, the structure of α-BiBO (BiBO) was predicted with an over-estimated (by over 10 %) unique b-axis while underestimates a and overestimates c in a less amount. Consequently it is not possible to use the calculated structure to obtain meaningful properties of this important material. By applying a modified post-DFT LD correction based on linear combination of atomic orbitals (LCAO) and B3LYP functional, the experimental structure is well reproduced with the theoretical optimized one. Many important material property tensors of BiBO crystal are calculated in unprecedented precisions, including: dielectric constants (static and in THz range), elastic and elasto-optic constants, piezoelectric constants, refractive indices, NLO and electro-optic (EO) coefficients. Among them, theoretical calculation of the refractive indices in the THz range by diagonalizing the clamped-ion dielectric constants was firstly achieved at least for BiBO crystal. The calculation also confirms that BiBO has an exceptional large piezoelectric constant d=40 pC/N and largest free EO coefficients , , on the order of 10 pm/V among borate crystals. The calculation also reveals that the large free EO coefficients are largely originated from the piezoelectric induced photo-elastic effect and for practical high speed applications only the clamped-ion EO coefficients take effect. The clamped ion EO coefficient of =-4.17 pm/V, =-2.61 pm/V are obtained for the first time and may be consulted if one seeks to design BiBO crystal as a high-speed EO modulator. Furthermore, full tensor matrix of the elasto-optic constants was obtained on the first time. Together with the calculated elastic constants, it can help to design acoustic optic modulating devices with preferable figure of merits 10 times that of traditional quartz crystal.
α-铋硼酸盐（BiBO）是一种重要的非线性光学（NLO）材料，在谐波产生和量子技术应用中具有高效率。由于其低对称性和Bi孤对电子的协同排列，它还具有异常大的压电和电光系数以及在其他材料特性上的强各向异性。先前对其物理（主要是光学）性质的理论计算常常给出令人困惑的结果。在此发现，伦敦色散（LD）倾向于稳定具有更紧密堆积实体的结构，如具有大极化率的孤对重离子Bi，而这在大多数先前的密度泛函理论（DFT）计算中被忽略。目前的研究表明，不考虑LD效应时，α-铋硼酸盐（BiBO）的结构预测中，其独特的b轴被高估（超过10%），而a轴被低估，c轴被低估的程度较小。因此，不可能使用计算出的结构来获得这种重要材料的有意义的性质。通过应用基于原子轨道线性组合（LCAO）和B3LYP泛函的改进的后DFT LD校正，实验结构与理论优化结构得到了很好的重现。首次以前所未有的精度计算了BiBO晶体的许多重要材料性质张量，包括：介电常数（静态和太赫兹范围内）、弹性和弹光常数、压电常数、折射率、NLO和电光（EO）系数。其中，至少对于BiBO晶体，首次通过对角化钳位离子介电常数实现了太赫兹范围内折射率的理论计算。计算还证实，BiBO具有异常大的压电常数d = 40 pC/N，以及在硼酸盐晶体中约为10 pm/V量级的最大自由EO系数 、 。计算还表明，大的自由EO系数主要源于压电诱导的光弹效应，对于实际的高速应用，只有钳位离子EO系数起作用。首次获得钳位离子EO系数 = -4.17 pm/V， = -2.61 pm/V，如果有人试图将BiBO晶体设计为高速EO调制器，可以参考这些值。此外，首次获得了弹光常数全张量矩阵。连同计算出的弹性常数，它有助于设计品质因数比传统石英晶体高10倍的声光调制器件。
