Na4La2(CO3)5 and CsNa5Ca5(CO3)8: Two New Carbonates as UV Nonlinear Optical Materials

Soaking the Rare-Earth Carbonates for a Change: An Alternative Approach to Explore Carbonate Nonlinear Optical Crystals

Alkali-metal rare-earth carbonates (ARECs) find great potential in nonlinear optical applications. As the most common method, the hydrothermal reaction is widely used in synthesizing ARECs. The black-box nature of the hydrothermal reaction makes it difficult for understanding the formation processes and therefore may slow down the pace of structural discovery. Here, by simply soaking the rare-earth carbonates in Na2CO3 solutions, we successfully obtain a series of noncentrosymmetric Na3RE(CO3)3·6H2O (RE = Tb 1, Sm 2, Eu 3, Gd 4, Dy 5, Ho 6, and Er 7) compounds without using the high-temperature hydrothermal method. The transformation process, investigated by powder X-ray diffraction and scanning electron microscopy, is governed by the concentration of the soaking solutions. Na3Tb(CO3)3·6H2O, as an example, is studied structurally, and its physical properties are characterized. It exhibits a second harmonic generation effect of 0.5 × KDP and a short UV cutoff edge of 222 nm (5.8 eV). Our study provides insights for exploring new AREC structures, which may further advance the development of carbonate nonlinear optical crystals.

M(NH2SO3)2·xH2O (M = Ca, Pb, x = 0, 1, 4): Effect of Hydrogen Bonding on Structural Transformations and Second Harmonic Generation of Metal Sulfamates

Nonlinear optical (NLO) crystals are very important for laser technology, but the performances of available NLO crystals are still insufficient for increasing demand. Recently, the exploration of new NLO crystals in non-π-conjugated systems with the heteroatomic tetrahedra is attracting a lot of interest. In this work, we systematically explore the metal sulfamates containing [NH₂SO₃] groups and four metal sulfamates, namely, Ca(NH₂SO₃)₂·4H₂O, Ca(NH₂SO₃)₂·H₂O, Pb(NH₂SO₃)₂·H₂O, and Pb(NH₂SO₃)₂ were synthesized by aqueous solution and hydrothermal methods. Notably, these metal sulfamates exhibit different crystal structures and optical properties owing to the diverse arrangement of the functional groups in their structures. In addition, due to hydrogen bond regulation, the centrosymmetric (CS) compound Ca(NH₂SO₃)₂·4H₂O can transform into noncentrosymmetric (NCS) Ca(NH₂SO₃)₂·H₂O, leading to NLO activity. Experimental characterizations and theoretical analysis reveal that these metal sulfamates are ultraviolet transparent and suitable for developing new NLO materials.

A General Principle for DUV NLO Materials: π-Conjugated Confinement Enlarges Band Gap*

Current nonlinear optical materials face a conventional limitation in the trade-off between the band gap and birefringence, especially in the deep UV spectral region. To circumvent this dilemma, we propose a general principle, π-conjugated confinement, to partially decouple the interunit π-conjugated interactions by the separation of non-π-conjugated units. The goal is to further enlarge the band gap to a value larger than that of the singular π-conjugated counterpart and to maintain a suitable density of π-conjugated units to gain a large optical anisotropy. We reveal that π-conjugated confinement is a shared structural feature for all DUV NLO materials known to date, and thus, it provides a novel and essential design criterion for future design synthesis. Guided by this principle, the carbonophosphates are predicted to be a new promising DUV candidate system. Sr₃ Y[PO₄ ][CO₃ ]₃ (1) and Na₃ X[PO₄ ][CO₃ ] (X=Ba, Sr, Ca, Mg, 2-5) exhibit not only greatly enhanced birefringence that is 3-24 times larger than that of singular phosphates but also enhanced band gaps that are 0.2-1.7 eV wider than those of singular carbonates.

Chromous carbonates containing a square-grid layer of {Cr2(CO3)4}n4n- based on a dichromium(II,II) paddlewheel core

Polymeric coordination compounds based on Cr2n+ paddle-wheel building blocks with non-carboxylate O,O-donor ligands chelating and bridging the Cr-Cr centers have been underexplored hitherto. This paper reports the synthesis and crystal structure of a new homo-valent chromium(ii,ii) compound, Na3HCr2(CO3)4·10H2O (1). It has a two-dimensional structure in which the paddlewheel chromium(ii,ii) units of Cr2(CO3)44- are cross-linked through the carbonate groups. The layers are stacked along the [100] direction, and Na ions fill the intra and interlayer spaces with a neighboring layer distance of about 11.3 Å. The investigation of the primary magnetic properties and theoretical studies with density functional theory (DFT) reveal the partial paramagnetic properties of compound 1 arising from the Boltzmann distribution between a ground state σ2π4δ2 with S = 0 and a low-lying excited state σ2π4δδ* with S = 1 for the Cr2(ii,ii) dimer. According to Raman spectra measurements combined with theoretical calculations, the two Raman bands in the small-wavenumber region at 335 and 297 cm-1 were assigned to the vibration of the Cr-O bonds and the band at 371 cm-1 was assigned to the stretching of the Cr-Cr quadruple bond.

High-pressure synthesis and crystal structure of non-centrosymmetric K2Ca3(CO3)4

A new non-centrosymmetric K2Ca3(CO3)4 crystal (P212121, a = 7.39134(18) Å, b = 8.8153(2) Å, c = 16.4803(4) Å) was synthesized in a multi-anvil press at a pressure of 3 GPa and temperature of 975 °C.

Ba3 (BO3 )(CO3 )F: The First Borate Carbonate Fluoride Synthesized by the High-Temperature Solution Method

In contrast to the well-investigated halogen-containing borates and carbonates, very few halogen-containing borate carbonate compounds have been reported. Specifically, no example of borate carbonate fluoride has been synthesized successfully until now. Herein, the planar π-conjugated units [BO₃ ]^3- and [CO₃ ]^2- and the F^- ions are introduced simultaneously into one crystal structure resulting in the first borate carbonate fluoride, Ba₃ (BO₃ )(CO₃ )F, by the high-temperature solution method in the atmosphere. Its structure features a hexagonal channel formed by the [BO₃ ]^3- and [CO₃ ]^2- units with the [F₃ Ba₈ ]¹³⁺ trimers filled in the channel. Various characterizations including single crystal- and powder-XRD, EDX, IR, UV-vis-NIR, and TG-DSC, together with the first principles calculation have been carried out to verify the structure and fully understand the structure-property relationships.

From centrosymmetric to noncentrosymmetric: intriguing structure evolution in d10-transition metal iodate fluorides

Two new d10-transition metal iodate fluorides, centrosymmetric ZnIO3F and noncentrosymmetric CdIO3F, were successfully synthesized by intriguing structure evolution. Both compounds have higher thermal stability and CdIO3F has a large SHG response and wide band gap. Our study may afford a viable route to design materials with excellent NLO functions.

Y8O(OH)15(CO3)3Cl: an excellent short-wave UV nonlinear optical material exhibiting an infrequent three-dimensional inorganic cationic framework

An excellent short-wave UV nonlinear optical material, Y₈O(OH)₁₅(CO₃)₃Cl, featured an infrequent three-dimensional inorganic cationic framework.

Noncentrosymmetric (NCS) solid solutions: elucidating the structure-nonlinear optical (NLO) property relationship and beyond

A systematic approach toward the discovering of novel functional noncentrosymmetric (NCS) materials revealing technologically useful applications is an ongoing challenge. This Frontiers article investigates a series of NCS solid solutions with respect to their crystal structure and second-harmonic generation (SHG) response. The solid solutions include NCS polar aluminoborates, Al_5-xGa_xBO₉ (0.0 ≤ x ≤ 0.5), rare earth element-doped bismuth tellurites, Bi_2-xRE_xTeO₅ (RE = Y, Ce, and Eu; 0.0 ≤ x ≤ 0.2), layered perovskites, Bi_4-xLa_xTi₃O₁₂ (0.0 ≤ x ≤ 0.75: Aurivillius phases) and CsBi_1-xEu_xNb₂O₇ (0.0 ≤ x ≤ 0.2: Dion-Jacobson phases), calcium bismuth oxides, Ca₄Bi_6-xLn_xO₁₃ (Ln = La and Eu; x = 0, 0.06 and 0.12), and sodium lanthanide iodates, NaLa_1-xLn_x(IO₃)₄ (Ln = Sm and Eu; 0 ≤ x ≤ 1). The origin of SHG for all the NCS solid solutions is discussed and the detailed structure-nonlinear optical (NLO) property relationships are elucidated. In addition, photoluminescence (PL) properties and subsequent energy transfer mechanisms for NCS solid solutions are provided.

Na4Ca(CO3)3: a novel carbonate analog of borate optical materials

Carbonate optical materials currently attract a lot of interest as an unexplored alternative to borate ones.

Role of spin-orbit interaction on the nonlinear optical response of CsPbCO3F using DFT

We explore the effect of spin-orbit interaction (SOI) on the electronic and optical properties of CsPbCO₃F using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCO₃F is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d₃₆ = 0.39 pm V^-1) that of KH₂PO₄ (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran-Blaha modified Becke-Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H-A direction (indirect) transforms to the H-H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(-2ω;ω,ω) dispersion of a CsPbCO₃F single crystal. We found that d₁₁ = d₁₂ = 4.35 pm V^-1 at 1064 nm, which is 11.2 times higher than d₃₆ of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCO₃F is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCO₃F is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region.

Solvent-controlled syntheses of mixed-alkali-metal borates exhibiting UV nonlinear optical properties

A series of mixed-alkali-metal borates have been solvothermally synthesized by using various solvents.

Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures

Solid-state materials with extended structures have revealed many interesting structure-related characteristics. Among many, materials crystallizing in noncentrosymmetric (NCS) space groups have attracted massive attention attributable to a variety of superb functional properties such as ferroelectricity, pyroelectricity, piezoelectricity, and nonlinear optical (NLO) properties. In fact, the characteristics are pivotal to many industrial applications such as laser systems, optical communications, photolithography, energy harvesting, detectors, and memories. Thus, for the past several decades, a great deal of synthetic effort has been vigorously made to realize these technologically important properties by improving the occurrence of macroscopic NCS space groups. A bright approach to increase the incidence of NCS structures was combining local asymmetric units during the initial synthesis process. Although a significant improvement has been achieved in obtaining new NCS materials using this strategy, the majority of solid-state materials still crystallize in centrosymmetric (CS) structures as the locally unsymmetrical units are easily lined up in an antiparallel manner. Therefore, discovering an effective method to control the framework structure and the macroscopic symmetry is an imminent ongoing challenge. In order to more effectively control the overall symmetry of solid-state compounds, it is critical to understand how the backbone and the subsequent centricity are affected during the crystallization. In this Account, several factors influencing the framework structure and centricity of solid-state materials are described in order to more systematically discover novel NCS materials. Recent studies on crystalline solid-state materials suggest three factors affecting the local coordination environment as well as the overall symmetry of the framework structure: (1) size variations of the various template cations, (2) a variable backbone arrangement occurring from the hydrogen-bonding interactions, and (3) the presence of framework flexibility. With regard to the first factor, the impact of size of the various metal cations and coordination numbers on the alignment of other adjacent polyhedra, linkers, and lone pairs determining the framework geometries of mixed metal oxides is analyzed. The second factor considers the regulation of crystallographic centricity determined by the availability of hydrogen-bonding interactions between anionic frameworks containing local asymmetric polyhedra and organic cations. Finally, the third factor explores the framework architecture and the space group symmetry influenced by the flexibility of polyhedra revealing variable coordination numbers. The centricity and framework of new solid-state materials might be controlled by using a variety of synthetically controllable asymmetric units such as organic structure-directing cations and linkers with different sizes and functional groups.

A Polar Titanium-Organic Chain with a Very Large Second-Harmonic-Generation Response

A noncentrosymmetric (NCS) titanium-organic compound, [H₂N(CH₃)₂]TiO{[NC₅H₃(CO₂)₂][NC₅H₄(CO₂)]} (CAUMOF-18), has been synthesized by a solvothermal reaction. The aligned unidimensional polar chain structure of CAUMOF-18 consisting of corner-shared distorted TiO₅N₂ pentagonal bipyramids is attributed to strong hydrogen-bonding and π-π interactions. CAUMOF-18 reveals a very strong second-harmonic-generation efficiency of 400 times that of α-SiO₂ and is phase-matchable (type I). Water-molecule-driven reversible centricity conversion and topotactic transformation to TiO₂ microrods for CAUMOF-18 are also presented.

Connecting effect on the first hyperpolarizability of armchair carbon-boron-nitride heteronanotubes: pattern versus proportion

Carbon-boron-nitride heteronanotubes (BNCNT) have attracted a lot of attention because of their adjustable properties and potential applications in many fields. In this work, a series of CA, PA and HA armchair BNCNT models were designed to explore their nonlinear optical (NLO) properties and provide physical insight into the structure-property relationships; CA, PA and HA represent the models that are obtained by doping the carbon segment into pristine boron nitride nanotube (BNNT) fragments circularly around the tube axis, parallel to the tube axis and helically to the tube axis, respectively. Results show that the first hyperpolarizability (β0) of an armchair BNCNT model is dramatically dependent on the connecting patterns of carbon with the boron nitride fragment. Significantly, the β0 value of PA-6 is 2.00 × 10(4) au, which is almost two orders of magnitude larger than those (6.07 × 10(2) and 1.55 × 10(2) au) of HA-6 and CA-6. In addition, the β0 values of PA and CA models increase with the increase in carbon proportion, whereas those of HA models show a different tendency. Further investigations on transition properties show that the curved charge transfer from N-connecting carbon atoms to B-connecting carbon atoms of PA models is essentially the origin of the big difference among these models. This new knowledge about armchair BNCNTs may provide important information for the design and preparation of advanced NLO nano-materials.

Structure modulation, band structure, density of states and luminescent properties of columbite-type ZnNb2O6

This study reports the four-dimensional commensurately modulated structure of ZnNb₂O₆ using superspace formalism for aperiodic structures considering the modulation vector, q = 1/3 b*.

Synthesis, characterization and theoretical studies of nonlinear optical crystal Sr2B5O9(OH)·H2O

Sr₂B₅O₉(OH)·H₂O has a large SHG effect, a wide transparency window, moderate birefringence, high stability and good growth habit. Theoretical analyses verify the dominating contributions of the borate groups and the distinct contributions of the alkaline-earth cations and OH groups to the SHG effect.