Dependence of the Second-Harmonic Generation Response on the Cell Volume to Band-Gap Ratio

Crystal structure, electronic structure, and optical properties of the novel Li4CdGe2S7, a wide-bandgap quaternary sulfide with a polar structure derived from lonsdaleite

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4–II–IV2–VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2–II–IV–VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

Material research from the viewpoint of functional motifs

As early as 2001, the need for the ‘functional motif theory’ was pointed out to assist the rational design of functional materials. The properties of materials are determined by their functional motifs and by how they are arranged in the materials. Uncovering the functional motifs and their arrangements is crucial in understanding the properties of materials and rationally designing new materials of desired properties. The functional motifs of materials are the critical microstructural units (e.g. constituent components and building blocks) that play a decisive role in generating certain material functions, and could not be replaced with other structural units without losing or significantly suppressing the relevant functions. The role of functional motifs and their arrangements in materials with representative examples was presented. These examples could be classified into six types of material microscopic structures on a length scale smaller than ∼10 nm with maximum subatomic resolution, i.e. the crystal, magnetic, aperiodic, defect, local, and electronic structures. The method of functional motif analysis could be employed in the function-oriented design of materials, as elucidated by taking infrared nonlinear optical materials as an example. Machine learning is more efficient in predicting material properties and screening materials with high efficiency than high-throughput experimentation and high-throughput calculations. In extracting the functional motifs and finding their quantitative relationships, developing sufficiently reliable databases for material structures and properties is imperative.

Three-in-One Strategy Constructing a Series of Hybrid Tetrahedral Motif-Based Selenides with Balanced Second-Order Nonlinear Optical Performance

Concurrently achieving suitable second harmonic generation (SHG) effect and high laser-induced damage threshold (LIDT) is challenging for infrared nonlinear optical (NLO) materials. Here, a series of pentanary infrared NLO materials CsM^IIIM^IVSnSe₆ (M^III = Ga, In; M^IV = Si, Ge) have been obtained by a three-in-one strategy, viz. three kinds of elements (M^III, M^IV, and Sn) in one position, which is first adopted to design NLO materials. Their three-dimensional structures are constructed by the MQ₄ (M denotes M^III, M^IV, and Sn) tetrahedral units. They exhibit promising hybrid NLO properties, witnessed by their moderate/large SHG effects of 0.52, 0.98, 1.05, and 1.12 × AgGaS₂, and high powder LIDT values of 6.9, 4.1, 8.1, and 5.4 × AgGaS₂, respectively. These NLO properties are well verified by the DFT calculation results. The three-in-one strategy of designing high-performance infrared NLO materials will stimulate more investigations in this field.

The partition principles for atomic-scale structures and their physical properties: application to the nonlinear optical crystal material KBe2BO3F2

In implementing the materials genome approach to search for new materials with interesting properties or functions, it is necessary to find the correct functional motif. To this end, it is common to partition an extended structure into various building units and then partition its properties to find the appropriate functional motif. We have developed the general principles for partitioning a structure and its properties in terms of a set of atoms and bonds by analyzing the differential cross-sections of neutron and X-ray scattering phenomena and proposed the procedures with which to partition an extended structure and its properties. We demonstrate how these procedures work by analyzing the nonlinear optical crystal KBe2BO3F2. Our partitioning analysis of KBe2BO3F2 leads to the conclusion that the second harmonic generation response of KBe2BO3F2 is dominated by the ionically bonded metal-centered groups.

Second harmonic generation responses of KH2PO4: importance of K and breaking down of Kleinman symmetry

The second harmonic generation (SHG) responses of the paraelectric and ferroelectric phases of KH₂PO₄ (KDP) were calculated by first-principles density functional theory (DFT) calculations, and the individual atom contributions to the SHG responses were analyzed by the atom response theory (ART). We show that the occurrence of static polarization does not enhance the SHG responses of the ferroelectric KDP, and that the Kleinman symmetry is reasonably well obeyed for the paraelectric phase, but not for the ferroelectric phase despite that the latter has a larger bandgap. This is caused most likely by the fact that the ferroelectric phase has lower-symmetry local structures than does the paraelectric phase. The contribution to the SHG response of an individual K⁺ ion is comparable to that of an individual O^2- ion. The contributions of the O^2- and K⁺ ions arise overwhelmingly from the polarizable parts of the electronic structure, namely, from the valence bands of the O-2p nonbonding states and from the conduction bands of the K-3d states.

Key Factors Controlling the Large Second Harmonic Generation in Nonlinear Optical Materials

The second harmonic generation (SHG) responses of nonisostructural nonlinear optical (NLO) compounds, β-BaB₂O₄, LiB₃O₅, CsB₃O₅, CsLiB₆O₁₀, KBe₂BO₃F₂, and LiCs₂PO₄, were examined by density functional theory (DFT) calculations, and the contributions of their individual cations and anions were determined by performing atomic response theory analyses. In all of these compounds, the contribution of all metal cations lies in the range of ∼9.3 to ∼29.7% of the total SHG responses, and that of all anions lies between ∼57.4 and ∼72.3%. However, in terms of individual atom contribution, a large metal cation can contribute more than does an anion to the total SHG response. Our work shows that the SHG contribution of an individual anion (e.g., O^2-) is weakened when it forms covalent bonds with its surrounding cations (e.g., O-B). The contribution of an individual cation is further affected by the homogeneity of its surrounding anion distribution and also by how strongly the polarizabilities of its surrounding anions are weakened by covalent bonding with other cations. The SHG response increases with α_sum/(NE_g), an important parameter useful in searching for new NLO materials, where α_sum is the sum of the polarizabilities of all of the ions in the primitive unit cell, N is the total number of atoms per primitive cell, and E_g is the band gap.

Physical Properties of Molecules and Condensed Materials Governed by Onsite Repulsion, Spin-Orbit Coupling and Polarizability of Their Constituent Atoms

The onsite repulsion, spin-orbit coupling and polarizability of elements and their ions play important roles in controlling the physical properties of molecules and condensed materials. In celebration of the 150th birthday of the periodic table this year, we briefly review how these parameters affect the physical properties and are interrelated.