Rational Design of the Alkali Metal Sn-Based Mixed Halides with Large Birefringence and Wide Transparent Range.

Birefringent materials are widely used in various advanced optical systems, owing to their vital role in creating and controlling polarized light. Currently, Sn2+-based compounds containing stereochemically active lone-pair (SCALP) cations are extensively investigated and considered as one class of promising birefringent materials. To solve the problem of relatively narrow bandgap of Sn2+-based compounds, alkali metals and multiple halogens are introduced to widen the bandgap during the research. Based on this strategy, four new Sn2+-based halides, A2Sn2F5Cl and ASnFCl2 (A = Rb and Cs), with large birefringence, short ultraviolet (UV) cutoff edge, and wide transparent range are successfully found. The birefringences of A2Sn2F5Cl (A = Rb and Cs) are 0.31 and 0.28 at 532 nm, respectively, which are among the largest in Sn-based halide family. Remarkably, A2Sn2F5Cl possess relatively shorter UV cutoff edge (<300 nm) and broad infrared (IR) transparent range (up to 16.6 µm), so they can become promising candidates as birefringent materials applied in both UV and IR regions. In addition, a comprehensive analysis on crystal structures and structure-property relationship of metal Sn2+-based halides is performed to fully understand this family. Therefore, this work provides insights into designing birefringent materials with balanced optical properties.

Beyond π-π Stacking: Understanding Inversion Symmetry Breaking in Crystalline Racemates.

The design of noncentrosymmetric (NCS) solid state materials, specifically how to break inversion symmetry between enantiomers, has intrigued chemists, physicists, and materials scientists for many years. Because the chemical complexity of molecular racemic building units is so varied, targeting these materials is poorly understood. Previously, three isostructural racemic compounds with a formula of [Cu(H2O)(bpy)2]2[MF6]2·2H2O (bpy = 2,2'=bipyridine; M = Ti, Zr, Hf) were shown to crystallize in the NCS space group Pna21, of polar, achiral crystal class mm2. In this work, we synthesized five new racemic compounds with the formula [Cu(H2O)(dmbpy)2]2[MF6]2·xH2O (dmbpy = 4,4'/5,5'-dimethyl-2,2'-bipyridine; M = Ti, Zr, Hf). Single crystal X-ray diffraction reveals that the five newly synthesized compounds feature equimolar combinations of Δ- and Λ-Cu(dmbpy)2(H2O)2+ complexes that are assembled into packing motifs similar to those found in the reported NCS structure but all crystallize in centrosymmetric (CS) space groups. Seven structural descriptors were created to analyze the intermolecular interactions on the assembly of Cu racemates in the CS and NCS structures. The structural analysis reveals that in the CS structures, the inversion center results from parallel heterochiral π-π stacking interactions between adjacent Cu racemates regardless of cation geometries, hydrogen bonding networks, or interlayer architectures, whereas in the NCS structure, nonparallel heterochiral π-π interactions between the adjacent Cu racemates preclude an inversion center. The parallel heterochiral π-π interactions in the CS structures can be rationalized by the restrained geometries of the methyl-substituted ligands. This work demonstrates that the introduction of nonparallel stacking can suppress the formation of an inversion center for an NCS racemate. A conceptual framework and practical approach linking the absence of inversion symmetry in racemates is presented for all NCS crystal classes.

NaRb6(B4O5(OH)4)3(BO2) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO2.

Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO3] and tetrahedral [BO4] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO2] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb6(B4O5(OH)4)3(BO2), with a linear BO2- unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO2], [BO3], and [BO4] with sp-, sp2-, and sp3-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb6(B4O5(OH)4)3(BO2) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO2- unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric ß-Phase.

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs2I4O11 that was obtained through IO4 polyhedral rearrangements from centrosymmetric (CS) ß-Cs2I4O11. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs2I4O11 crystallizes in the NCS monoclinic space group P21 (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH2PO4) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d34, d21, and d23 SHG tensor components. The synthetic strategy of discovering γ-Cs2I4O11 provides a new way for designing novel NCS SHG materials.

Crystal Structures and Property Measurements of Rare Earth Magnesium Thiosilicates Synthesized via Flux Crystal Growth Utilizing the Boron Chalcogen Mixture (BCM) Method.

Nine new rare earth magnesium-containing thiosilicates of the formula RE3Mg0.5SiS7 (Ln = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) were synthesized in an alkali halide flux using the boron chalcogen mixture (BCM) method. Crystals of high quality were produced, and their structures were determined by single-crystal X-ray diffraction. The compounds crystallize in the hexagonal crystal system in the P63 space group. Phase pure powders of the compounds were used for magnetic susceptibility measurements and for second-harmonic generation (SHG) measurements. Magnetic measurements indicate that Ce3Mg0.5SiS7, Sm3Mg0.5SiS7, and Dy3Mg0.5SiS7 exhibit paramagnetic behavior with a negative Weiss temperature over the 2-300 K temperature range. SHG measurements of La3Mg0.5SiS7 demonstrated SHG activity with an efficiency of 0.16 times the standard potassium dihydrogen phosphate (KDP).

Phosphorescence in Mn4+-Doped R+/R2+ Germanates (R+ = Na+ or K+, R2+ = Sr2+).

Single crystals of three new compounds, Na0.36Sr0.82Ge4O9 (1, proposed composition), Na2SrGe6O14 (2), and K2SrGe8O18 (3), were obtained and characterized using single-crystal X-ray diffraction. Their structures contain three-dimensional (3D) anionic frameworks built from GeO4 and GeO6 polyhedra. The presence of octahedral Ge4+ sites makes the new phases suitable for Mn4+ substitution to obtain red-emitting phosphors with a potential application for light conversion. Photoluminescence properties of Mn4+-substituted Na2SrGe6O14 (2) and K2SrGe8O18 (3) samples were studied over a range of temperatures, and red light photoluminescence associated with the electronic transitions of tetravalent manganese was observed. The Na2SrGe6O14 (2) phase was also substituted with Pr3+ on the mixed Na-Sr site similar to the previously studied Na2CaGe6O14:Pr3+. The red emission peak of the Pr3+ activator occurs at a shorter wavelength (610 nm) compared to that of Mn4+ (662-663 nm). Additionally, second harmonic generation (SHG) data were collected for the noncentrosymmetric Na2SrGe6O14 (2) phase, indicating weak SHG activity. Diffuse reflectance spectroscopy and density of states calculations were performed to estimate the band gap values for pristine Na2SrGe6O14 (2) and K2SrGe8O18 (3) phases.

Perovskite-like K3TiOF5 Exhibits (3 + 1)-Dimensional Commensurate Structure Induced by Octahedrally Coordinated Potassium Ions.

Elpasolite- and cryolite-type oxyfluorides can be regarded as superstructures of perovskite and exhibit structural diversity. While maintaining a similar structural topology with the prototype structures, changes in the size, electronegativity, and charge of cation and/or anion inevitably lead to structural evolution. Therefore, the nominal one-to-one relation suggested by a doubled formula of perovskite does not guarantee a simple 2-fold superstructure for many cases. Herein, the commensurately modulated perovskite-like K3TiOF5 was refined at 100 K from single-crystal X-ray diffraction data by using a pseudotetragonal subcell with lattice parameters of a = b = 6.066(2) Å and c = 8.628(2) Å. The length of the modulation vector was refined to 0.3a* + 0.1b* + 0.25c*. In the commensurate supercell of K3TiOF5, the B-site Ti4+ and K+ cations in [TiOF5]3- and [KOF5]6- octahedral units were found to be significantly displaced from the average atomic positions refined in the subcell. The displacements of the K+ cations are ±0.76 Å, and those for the Ti4+ cations are approximately ±0.13 Å. One- and two-dimensional solid-state 19F NMR measurements revealed two tightly clustered groups of resonances in a ratio of ca. 4:1, assigned to equatorial and axial fluorine, respectively, consistent with local [TiOF5]3- units. S/TEM results confirmed the average structure. Electronic structure calculations of the idealized I4mm subcell indicate the instability to a modulated structure arises from soft optical modes that is controlled by the octahedrally coordinated B-site potassium ions in the cryolite-type structure.

Synthesis of Hydrated Ternary Lanthanide-Containing Chlorides Exhibiting X-ray Scintillation and Luminescence.

A series of new ternary lanthanide-based chlorides, Cs2EuCl5(H2O)10, Cs7LnCl10(H2O)8 (Ln = Gd or Ho), Cs10Tb2Cl17(H2O)14(H3O), Cs2DyCl5(H2O)6, Cs8Er3Cl17(H2O)25, and Cs5Ln2Cl11(H2O)17 (Ln = Y, Lu, or Yb), were prepared as single crystals via a facile solution route. The compounds with compositions of Cs7LnCl10(H2O)8 (Ln = Gd or Ho) and Cs5Ln2Cl11(H2O)17 (Ln = Y, Lu, or Yb) crystallize in a monoclinic crystal system in space groups C2 and P21/c, respectively, whereas Cs2EuCl5(H2O)10, Cs10Tb2Cl17(H2O)14(H3O), and Cs8Er3Cl17(H2O)25 crystallize in orthorhombic space groups Pbcm, Pnma, and P212121, respectively. Cs2DyCl5(H2O)6 crystallizes with triclinic symmetry in space group P1̅. All of these compounds exhibit complex three-dimensional structures built of isolated lanthanide polyhedral units that are linked together by extensive hydrogen bonds. Cs2EuCl5(H2O)10 and Cs10Tb2Cl17(H2O)14(H3O) luminesce upon irradiation with 375 nm ultraviolet light, emitting intense orange-red and green color, respectively, and Cs10Tb2Cl17(H2O)14(H3O) scintillates when exposed to X-rays. Radioluminescence (RL) measurement of Cs10Tb2Cl17(H2O)14(H3O) in powder form shows that the RL emission integrated in the range of 300-750 nm was ∼16% of BGO powder.

Interplay between Oxo and Fluoro in Vanadium Oxyfluorides for Centrosymmetric and Non-Centrosymmetric Structure Formation.

Herein, we report the syntheses of two lithium-vanadium oxide-fluoride compounds crystallized from the same reaction mixture through a time variation experiment. A low temperature hydrothermal route employing a viscous paste of V2O5, oxalic acid, LiF, and HF allowed the crystallization of one metastable phase initially, Li2VO0.55(H2O)0.45F5⋅2H2O (I), which on prolonged heating transforms to a chemically similar yet structurally different phase, Li3VOF5 (II). Compound I crystallizes in centrosymmetric space group, I2/a with a = 6.052(3), b = 7.928(4), c = 12.461(6) Å, and ß = 103.99(2)°, while compound II crystallizes in a non-centrosymmetric (NCS) space group, Pna21 with a = 5.1173(2), b = 8.612(3), c = 9.346(3) Å. Synthesis of NCS crystals are highly sought after in solid-state chemistry for their second-harmonic-generation (SHG) response and compound II exhibits SHG activity albeit non-phase-matchable. In this article, we also describe their magnetic properties which helped in unambiguous assignment of mixed valency of V (+4/+5) for Li2VO0.55(H2O)0.45F5⋅2H2O (I) and +4 valency of V for Li3VOF5 (II).

Periodic Tendril Perversion and Helices in the AMoO2F3 (A = K, Rb, NH4, Tl) Family.

Although compounds of the formula AMoO2F3 (A = K, Rb, Cs, NH4, Tl) have been known for decades, crystal structures have only been reported for CsMoO2F3 and NH4MoO2F3. The three compounds (Rb/NH4/Tl)MoO2F3 are isostructural and crystallize in the centrosymmetric space group C2/c (No. 15). The compounds contain the MoO2F3- anionic chain, composed of corner-sharing MoO2F4 octahedra, with Mo6+ coordinated by two cis bridging fluoride anions that are trans to terminal oxide anions. The MoO2F3- chain has a very unusual and complex chain structure; a single chain contains alternating zigzag and helical sections. These helical regions alternate in chirality along the chain, and thus the chains exhibit periodic tendril perversion. To the best of the authors' knowledge, no other materials with a similar chain structure have been reported. On the other hand, KMoO2F3 is noncentrosymmetric and chiral, crystallizing in the enantiomorphic space group P212121 (No. 19). KMoO2F3 also contains the MoO2F3- anionic chain. However, the chain is helical, with only one enantiomer present, resulting in a chiral, noncentrosymmetric structure.

BaF2TeF2(OH)2: A UV Nonlinear Optical Fluorotellurite Material Designed by Band-Gap Engineering.

Balancing the wide band gap, large second harmonic generation (SHG) response, and moderate birefringence are significant but addressable challenges for designing nonlinear optical (NLO) materials. Based on the band-gap engineering in perovskite solar cell materials, we have successfully synthesized a new fluorotellurite, BaF2TeF2(OH)2, which exhibits a pseudo-Aurivillius structure and crystallizes in a noncentrosymmetric and polar space group Pmn21. The physical property measurements show that this material can effectively balance the requirements among the short UV absorption edge (∼205 nm), large SHG response (∼3 × KDP) and moderate birefringence (∼0.078@350-700 nm) and is a promising ultraviolet NLO crystal.

Effect of Pb Substitution in Sr2-xPbxGeSe4 on Crystal Structures and Nonlinear Optical Properties Predicted by DFT Calculations.

We investigated the Sr2-xPbxGeSe4 series from 0 ≤ x ≤ 2 to study the impact of Pb on structure and properties. While the noncentrosymmetric (NCS) compounds γ-Sr2GeSe4 and α-Pb2GeSe4 have already been reported previously, the substitution variants Sr1.31Pb0.69GeSe4 (space group Ama2, a = 10.31220(1) Å, b = 10.39320(1) Å, c = 7.42140(1) Å) and Sr0.19Pb1.81GeSe4 (I4̅3d, a = 14.6177(3) Å) are introduced here for the first time. The experimentally determined optical band gaps decrease as predicted with increasing Pb content from γ-Sr2GeSe4 to Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 from 2.00, to 1.65, 1.45 and 1.42 eV, respectively. The nonlinear optical (NLO) properties of the orthorhombic compounds γ-Sr2GeSe4 and Sr1.3Pb0.7GeSe4 (approximated with the supercell "Sr3PbGe2Se8") were studied both theoretically, using first-principle calculations, and experimentally. The calculations found the effective nonlinear susceptibility, deff, of γ-Sr2GeSe4 and "Sr3PbGe2Se8" at the static limit to be 10.8 and 8.8 pm V-1, respectively. The experimental deff values of γ-Sr2GeSe4, Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 were 2.6, 2.3, 0.68, and 0.79 pm V-1, respectively.

Polymorphism and Molten Nitrate Salt-Assisted Single Crystal to Single Crystal Ion Exchange in the Cesium Ferrogermanate Zeotype: CsFeGeO4.

Two polymorphs of a new cesium ferrogermanate zeotype, CsFeGeO4, were synthesized using the molten CsCl-CsF flux growth approach at 900 °C. The orthorhombic polymorph, referred to as (1), crystallizes in the centrosymmetric nonpolar Pbcm space group. The compound exhibits a three-dimensional porous framework structure composed of disordered (Fe/Ge)O4 corner-sharing tetrahedra that generate large eight-sided channels running down the b-axis. These channels are occupied by Cs ions that provide charge balance to the anionic framework. Minor modifications in the reaction conditions lead to the synthesis of a monoclinic polymorph of CsFeGeO4, referred to as (2), crystallizing in the noncentrosymmetric polar space group P21 and exhibiting an identical framework structure to (1), albeit featuring ordered FeO4 and GeO4 tetrahedra. Solid state synthesis of CsFeGeO4 produces a polycrystalline mixture of (1) and (2), referred to as (6). Polarization-electric field (P-E) measurements of (6) indicate that the material is not ferroelectric. Powder second harmonic generation (SHG) measurements of (2) and (6) revealed them to be SHG active with intensities of 1.5 and 0.2 times that of α-SiO2, respectively. The temperature dependent magnetic susceptibility of (2) exhibits a downturn at T = 2.6 K, indicative of antiferromagnetic ordering. First-principles calculations in the form of density functional theory showed that (1) and (2) differ in stability by only 1.3 meV/atom, with (2) being the thermodynamically stabilized phase. Additional calculations for (1), using molten nitrate as reference, predicted the formation of energetically favorable phases, KFeGeO4 (3) and RbFeGeO4 (4). They were subsequently prepared via a molten nitrate salt bath treatment of (1) to replace Cs with K and Rb, affording (3) and (4) as single-crystal to single-crystal ion exchange products. Structure determination and property measurements for a pyroxene phase, CsFeGe2O6, referred to as (5), are also reported. This compound crystallized as a side product in the flux synthesis of CsFeGeO4.

NaRb3B6O9(OH)3(HCO3): A Borate-Bicarbonate Nonlinear Optical Material.

A noncentrosymmetric mixed alkali metal borate-bicarbonate, NaRb3B6O9(OH)3(HCO3), was synthesized and characterized. The compound crystallizes in the monoclinic space group P21 (No. 4) with a = 8.988(3) Å, b = 8.889(2) Å, c = 10.068(4) Å, and ß = 114.6(4)°. The structure features a combination of chains of boron-oxygen [B6O9 (OH)3]3- groups and isolated HCO3- groups, with charge compensation provided by Rb+ and Na+ cations. It exhibits a second harmonic generation response of about 0.5 × KDP. The UV-vis-NIR absorption spectrum indicated a transparency of about 40% at 200 nm. The IR spectrum confirms the coordination environments of anionic groups, and thermogravimetric measurements indicate the material is thermally stable up to approximately 320 °C. Additionally, first-principles calculations were performed in order to gain insight into the role of boron-oxygen and HCO3- groups with respect to the band structure and NLO properties.

Designing Silicates as Deep-UV Nonlinear Optical (NLO) Materials using Edge-Sharing Tetrahedra.

Discovering new deep-ultraviolet (DUV) nonlinear optical (NLO) materials is currently a great challenge. The reported DUV NLO materials are almost exclusively borates or phosphates. Silicates-the largest constituent of the earth's crust-are excluded owing to their weak second harmonic generation (SHG) response. We report a silicate, Li2 BaSiO4 , with edge-sharing LiO4 -SiO4 tetrahedra that achieves the balance between a short UV absorption edge, below 190 nm, and a large SHG response, 2.8×KDP. The SHG intensity is the largest for silicates without second-order Jahn-Teller cations, and exceeds that of non-isomorphic Li2 SrSiO4 by more than an order of magnitude. As such Li2 BaSiO4 may be seen as a promising DUV-UV NLO material. This research indicates that edge-sharing tetrahedra is a new design parameter for discovering new DUV NLO materials.

New Members of SHG Active Dugganite Family, A3BC3D2O14 (A = Ba, Pb; B = Te, Sb; C = Al, Ga, Fe, Zn; D = Si, Ge, P, V): Synthesis, Structure, and Materials Properties.

A family of compounds, A3BC3D2O14 (A = Ba, Pb; B = Te, Sb; C = Al, Ga, Fe, Zn; D = Si, Ge, P, V), with the Dugganite structure was prepared employing traditional solid-state chemistry methods. PXRD and Rietveld refinement studies indicate that the compounds are stabilized in P321 space group (no. 150). The compounds are found to be SHG active with values ranging from 1.9 to 15.0 × KDP. The compounds exhibit high dielectric constants and low loss in our studies. The noncentrosymmetry related properties of the new Dugganites were understood by band structure calculations. We also explored the present Dugganite-structured oxides for the development of new inorganic colored materials by substituting Co2+, Ni2+, Cu2+, and Fe3+ in place of Zn2+. Thus, substitution of Co2+ and Fe3+ together tunes the blue color of the cobalt compound to blue-green color arising from metal-to-metal charge transfer (MMCT) of Fe3+ and Co2+ ions. The tetrahedrally coordinated Ni2+ in the Dugganite imparts a magenta color.

BaCuSiTe3: A Noncentrosymmetric Semiconductor with CuTe4 Tetrahedra and Ethane-like Si2Te6 Units.

BaCuSiTe3 was prepared from the elements in a solid-state reaction at 973 K, followed by slow cooling to room temperature. This telluride adopts a new, hitherto unknown structure type, crystallizing in the noncentrosymmetric space group Pc with a = 7.5824(1) Å, b = 8.8440(1) Å, c = 13.1289(2) Å, ß = 122.022(1)°, and V = 746.45(2) Å3 (Z = 4). The structure consists of a complex network of two-dimensionally connected CuTe4 tetrahedra and ethane-like Si2Te6 units with a Si-Si bond. This semiconducting material has an optical band gap of 1.65 eV and a low thermal conductivity of 0.50 W m-1 K-1 at 300 K. Calculations of its optical properties revealed a moderate birefringence of 0.23 and a second-order harmonic generation response of deff = 3.4 pm V-1 in the static limit.

Molten Alkali Halide Flux Growth of an Extensive Family of Noncentrosymmetric Rare Earth Sulfides: Structure and Magnetic and Optical (SHG) Properties.

Twenty new alkali rare earth thiosilicates and thiogermanates with the general formula ALnTS4 (A = alkali metal, Ln = lanthanide, and T = Si, Ge) were grown as X-ray diffraction-quality single crystals from molten alkali chloride fluxes. These include KNdSiS4, KPrSiS4, RbLnSiS4 (Ln = Ce, Pr, Nd, Gd, Tb, Dy, and Ho), RbLaGeS4, CsLnSiS4 (Ln = La, Pr, and Nd), and CsLnGeS4 (La, Ce, Pr, Nd, Eu, Gd, and Tb). Herein, we discuss the use of a molten chloride flux growth approach for the preparation of the title compounds and their structure determination via single-crystal X-ray diffraction. In addition, we comment on the magnetic properties of RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4, which were found to be paramagnetic for T = 2-300 K and exhibited negative Weiss temperatures with no obvious antiferromagnetic transition down to 2 K. The optical properties of CsLaGeS4 and CsNdTS4(T = Si, Ge) were measured by UV-vis spectroscopy. Second harmonic generation measurements performed on CsLaGeS4 confirmed the crystallization of the compound in the noncentrosymmetric orthorhombic space group, P212121; CsLaGeS4 was found to be SHG-active with nearly half the intensity of α-SiO2 upon irradiation with a Nd:YAG 1064 nm laser, and a semiconductor exhibiting a band gap of 3.60 eV based on UV-vis diffuse reflectance measurements.

Understanding the Behavior of the Above-Room-Temperature Molecular Ferroelectric 5,6-Dichloro-2-methylbenzimidazole Using Symmetry Adapted Distortion Mode Analysis.

The exploitable properties of many functional materials are intimately linked with symmetry-changing phase transitions. These include properties such as ferroelectricity, second harmonic generation, conductivity, magnetism and many others. We describe a new symmetry-inspired method for systematic and exhaustive evaluation of the symmetry changes possible in molecular systems using molecular distortion modes, and how different models can be automatically tested against diffraction data. The method produces a quantitative structural landscape from which the most appropriate structural description of a child phase can be chosen. It can be applied to any molecular or molecular-fragment containing material where a (semi) rigid molecule description is appropriate. We exemplify the method on 5,6-dichloro-2-methylbenzimidazole (DC-MBI), an important molecular ferroelectric. We show that DC-MBI undergoes an unusual symmetry-lowering transition on warming from orthorhombic Pca21 ( T ≲ 400 K) to monoclinic Pc. Contrary to expectations, the high temperature phase of DC-MBI remains polar.

Ba4B8TeO19: A UV Nonlinear Optical Material.

We report a new noncentrosymmetric barium tellurium borate, Ba4B8TeO19 that has potential ultraviolet (UV) nonlinear optical (NLO) applications. Ba4B8TeO19 was synthesized by a flux method and crystallizes in the noncentrosymmetric space group Cc. The material exhibits a framework structure of [B8O17]∞ double layers connected to distorted TeO6 octahedra. Second harmonic generation (SHG) measurements at 1064 and 532 nm on polycrystalline Ba4B8TeO19 indicate that the title compound is phase-matchable (type I) with a moderate SHG response (1 × KH2PO4 at 1064 nm and 0.2 × ß-BaB2O4 at 532 nm). In addition, a short absorption edge (210 nm) was measured. Using density functional theory calculations, we show that the SHG response originates from contributions from O 2p and Te 5s states at the valence and conduction band edges. Finally, by computing the linear optical properties, we find that this compound displays a moderate birefringence of 0.055 at 1064 nm and 0.059 at 532 nm, necessary conditions for phase-matching in UV NLO materials.