London Dispersion Effects on the Structure and Properties of Nonlinear Optical BiB3O6 Crystal

α-BiB3O6 (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 Bi3+ 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 Bi3+ 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-BiB3O6 (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 d22=40 pC/N and largest free EO coefficientsγ 12 T ${{\gamma }_{12}^{T}}$ ,γ 22 T ${{\gamma }_{22}^{T}}$ ,γ 32 T ${{\gamma }_{32}^{T}}$ 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 ofγ 53 S ${{\gamma }_{53}^{S}}$ =-4.17 pm/V,γ b 1 S ${{\gamma }_{{\rm b}1}^{S}}$ =-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.

Lone-pair, electron-dominated, nonlinear optical responses in sulfur clusters and electric tunability properties

Sulfur clusters have abundant lone-pair electrons, tunable geometries, and electronic structures, thus providing a new concept for achieving a tradeoff between transparency and nonlinearity. In this study, the optical properties of the sulfur clusters were investigated through first-principles calculations. Sulfur clusters exhibit broad transmittance bands spanning from the visible to the far-infrared regions, moderate third-order nonlinear optical effects comparable to those of p-nitroaniline, and size-dependent third-order nonlinear anisotropy. Furthermore, this study demonstrated that the nonlinear optical responses of lone-pair electrons are sensitive to the nuclear binding strength. By tuning the nuclear binding strength using an external electric field, the third-order nonlinear optical responses of the sulfur clusters were improved considerably and yielded a moderate red shift in the transparent bands. This investigation of the optical behaviors of sulfur clusters provides profound physical insights into the development of nonlinear optical materials with lone-pair electrons.

Two Noncentrosymmetric Bismuth Phosphates: Rational Design Synthesis and Optical Properties

Developing strategies to rational design noncentrosymmetric structure still attract much interest for their applications in nonlinear optical and piezoelectric materials. Two noncentrosymmetric (NCS) alkaline earth metal bismuth phosphates have been successfully achieved via partial replacement of Bi3+ with Ca2+ or Sr2+ ions. BiCa(H0.5PO4)2 (designated as CaBiPO) and BiSr(H0.5PO4)2 (designated as SrBiPO), together with their solid solution Bi(Sr1-xCax)(H0.5PO4)2 (0 < x ≤ 0.5), crystallize in the NCS space group C2. Both CaBiPO and SrBiPO exhibit ultraviolet nonlinear optical (NLO) properties, and their second-harmonic generation effects belong to type-II phase matching. Meanwhile, they could also act as photoluminescence hosts in which the Eu3+-doping samples SrBiPO:xEu3+ (x = 0.02-0.2) emit orange light. The effect of different radius ions on the derivative structures and the structure-NLO property relationship has also been discussed in detail.

d6versus d10, Which Is Better for Second Harmonic Generation Susceptibility? A Case Study of K2TGe3Ch8 (T = Fe, Cd; Ch = S, Se)

Two acentric chalcogenide compounds, K₂CdGe₃S₈ and K₂CdGe₃Se₈, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K₂CdGe₃S₈ and K₂CdGe₃Se₈ were accurately determined by single-crystal X-ray diffraction and crystallize in the K₂FeGe₃S₈ structure type. K₂CdGe₃S₈ is isostructural to K₂FeGe₃S₈ with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K₂CdGe₃S₈ is 18× K₂FeGe₃S₈ for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K₂CdGe₃S₈ over K₂FeGe₃S₈ are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS₄] tetrahedra within K₂CdGe₃S₈ exhibit a higher degree of distortion and larger volume compared to the [FeS₄] tetrahedra in K₂FeGe₃S₈. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d¹⁰-elements are better for SHG susceptibility than d⁶-elements in this study. K₂CdGe₃S₈ is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS₂, samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS₂), and good stability.

Unprecedented mid-infrared nonlinear optical materials achieved by crystal structure engineering, a case study of (KX)P2S6 (X = Sb, Bi, Ba)

Three superior type-I phase-matching middle infrared nonlinear optical materials (Ea > 4.0 eV, dij > 2× AGS, LDT > 8× AGS) were achieved via crystal structure engineering.

A2P2S6 (A=Ba, Pb): A Good Platform to Study Polymorph Effect and Lone Pairs Effect to Form Acentric Structure

Three ternary thiophosphates α-Ba2P2S6, β-Ba2P2S6, and Pb2P2S6, were synthesized via a high temperature salt flux method or an I2 transport reaction. β-Ba2P2S6 and Pb2P2S6 were previously structurally characterized without investigating...

Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe)

Two new quaternary sulfides K₂TGe₃S₈(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K₂TGe₃S₈(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K₂TGe₃S₈(T = Co, Fe) were grown. K₂CoGe₃S₈ crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. K₂FeGe₃S₈ crystallizes in the K₂FeGe₃Se₈ structure type and the noncentrosymmetric space group P2₁ (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, β = 96.604(2)°. There is a high structural similarity between K₂CoGe₃S₈ and K₂FeGe₃S₈. The larger volume coupled with higher degrees of distortion of the [FeS₄] tetrahedra compared to the [CoS₄] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS₄]_{T= Co or Fe} tetrahedra play a crucial role in controlling the structure and properties of K₂TGe₃S₈(T = Co, Fe). The measured optical bandgaps of K₂CoGe₃S₈ and K₂FeGe₃S₈ are 2.1(1) eV and 2.6(1) eV, respectively. K₂FeGe₃S₈ shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K₂CoGe₃S₈. The magnetic measurements also demonstrate the divalent nature of transition metals in K₂TGe₃S₈(T = Co, Fe).