Why Some Noncentrosymmetric Borates Do Not Make Good

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Why Some Noncentrosymmetric Borates Do Not Make Good Nonlinear Optical Materials: A Case Study with K3B5O8(OH)2 Fenghua Ding,†,‡ Matthew L. Nisbet,‡ Weiguo Zhang,§ P. Shiv Halasyamani,§ Liyuan Chai,† and Kenneth R. Poeppelmeier*,‡ †

School of Metallurgy and Environment, Central South University, Changsha 410083, China Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States § Department of Chemistry, University of Houston, Houston, Texas 77204, United States Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/31/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Synthesis of deep-ultraviolet (DUV) transparent materials, especially those with noncentrosymmetric structures, remains a great challenge in the solid-state chemistry community. A new DUV transparent borate, K3B5O8(OH)2, was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd2 (point group mm2), with the following cell parameters: a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. The structure of K3B5O8(OH)2 features a 3D framework composed of [B5O8(OH)2]3− basic building units that are linked by contacts with K+ cations and O−H···O hydrogen bonds. Second harmonic generation (SHG) measurements were performed, and an SHG efficiency of 0.5 × SiO2 was observed. Symmetry dictates that the χ14 component of the macroscopic NLO susceptibility is equal to zero in mm2, which prevents the maximal component of the microscopic NLO susceptibility for [B5O8(OH)2]3− units (χ14) from contributing to the macroscopic NLO susceptibility of the crystal and therefore limits the SHG efficiency. In contrast, large SHG effects can be observed from compounds containing [B5O10] units that crystallize in point groups with nonzero χ14, such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [B5O10]-based compounds and discovering other borate-based DUV materials.

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group, acentric structural units are introduced, such as d0 transition-metal (TM) cations in an octahedral environment,16 d10 cations with large polar displacements,17 and cations with stereochemically active lone pairs.18 However, the incorporation of these units inevitably causes a red-shift of the absorption edge, which prevents materials containing these units from being used in UV or DUV region. Efforts to design DUV borates with large SHG efficiency mostly focus on controlling the arrangement of anionic boron−oxygen moieties in the crystal compound without introducing the cations mentioned above. For example, it is widely accepted that the parallel arrangement of planar BO3 groups contributes a relatively large SHG effect, which is wellreflected in KBBF.19 The last several years have witnessed the discovery of many new members of the KBBF family on the basis of BO3 groups with a parallel arrangement, such as X3Z3Li2Al4B6O20F (X = alkali metal; Z = alkaline-earth metal),20−22 Rb3Al3B3O10F,23 NH4B4O6F,24 Ca2B10O14F6,25 and CsB4O6F.26 Besides KBBF, the first NLO borate crystal described for DUV generation was KB5O8·4H2O (KB5).27 The sixth harmonic of the Nd:YAG laser (1064 nm) has also been

INTRODUCTION Nonlinear optical (NLO) materials have played an important role in laser science and photonic technology.1,2 Coherent light in the ultraviolet (UV) or deep-ultraviolet (DUV) region can be obtained by cascading second harmonic generation (SHG) (i.e., fourth harmonic (1064 nm/4 = 266 nm) or sixth harmonic (1064 nm/6 = 177.3 nm) starting from the common fundamental laser wavelength of 1064 nm (Nd:YAG)).3 The NLO material KBe 2 BO 3 F 2 (KBBF) has been applied successfully to output coherent radiation at 177.3 nm. However, the growth of large KBBF crystals of high optical quality is limited by two factors: synthesis requires the use of the toxic reagent BeO, and crystals of KBBF tend to adopt a layered crystal habit.4−6 Thus, great effort is still needed in synthesizing new DUV NLO compounds. The following two properties are commonly considered to be basic requirements for an NLO material: (1) crystallographic noncentrosymmetry (NCS) and (2) a relatively large SHG efficiency in the required spectroscopic region.7 According to Chen’s theory of anionic groups,8 anions with acentric coordination environments are desired structural components for designing materials with high SHG efficiency, such as p-conjugated planar groups (e.g., BO33−, CO32−, NO3−)9−11 and rigid tetrahedral groups (e.g., PO43−, BO45−, BeO44−).12−15 To encourage crystallization in an NCS space © XXXX American Chemical Society

Received: July 13, 2018

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DOI: 10.1021/acs.inorgchem.8b01965 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

powder was obtained after filtering the product and successively washing the solid powder with acetone and ethyl alcohol. Single crystals of the compound K3B5O8(OH)2 were synthesized by a solvothermal method. K2B4O5(OH)4(H2O)2 (1.0 g) and 0.3 mL of ethyl alcohol were sealed with an impulse sealer in Teflon pouches and placed into a 125 mL Parr autoclave with a backfill of 50 mL of ethyl alcohol and 1 g of KOH. The autoclave was quickly heated to 240 °C, held at this temperature for 48 h, and cooled to room temperature at a rate of 0.1 °C/min. The single crystals were recovered in air after vacuum filtration, and the purity of sample was confirmed by powder XRD measurements (Figure S1). Single-Crystal Structure Determination. A colorless and transparent K3B5O8(OH)2 block crystal with dimensions of 0.046 × 0.063 × 0.063 mm was chosen for structure determination. Single crystal XRD data was obtained at 100 K with a Bruker Kappa APEX 2 CCD diffractometer with monochromated Mo Kα radiation (λ = 0.7107 Å). The crystal-to-detector distance was set to 60 mm. The SAINT program was used for data reduction and integration.38 The structure was established by direct methods and refined through full matrix least-squares fitting on F02 using OLEX2.39 All atoms were refined using full matrix least-squares techniques, and final leastsquares refinement was on F02 with data F02 ≥ 2σ(F02). Numerical absorption corrections were carried out using the SADABS program for area detector. The structure was solved with the use of Shel-XS to determine the atomic coordinates of the metallic cations.39 The structure was examined for possible missing symmetry elements with PLATON and no additional symmetry was found.40 The final refined atomic positions and isotropic thermal parameters are summarized in Table 1. Other crystallographic data are reported in the Supporting Information.

achieved in KB5, but the output power is low due to the compound’s weak NLO efficiency (∼0.1 × KDP).28,29 In recent years, several new pentaborates have been reported, some of which possess the same anionic [B5O10] units as KB5, such as K2Al[B5O10]·4H2O,30 (NH4)2Al[B5O10]·4H2O,30 and Rb2Al[B5O10]·4H2O.31 Compared to KB5, these compounds exhibit relatively large SHG efficiencies (2.0 × KDP). This difference in SHG behavior prompted examination of the relationship between the crystal structure and NLO properties among pentaborate compounds. In the current paper, we present a new DUV pentaborate, K3B5O8(OH)2, synthesized by a solvothermal method. Taking this new pentaborate as an example, we discuss the structural diversity of pentaborates and the desired arrangement of [B5O10] groups to achieve a relatively large SHG effect. Moreover, the synthesis, crystal growth, related optical properties, thermal stability, and electronic structure of the compound are reported.

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EXPERIMENTAL SECTION

Materials and Instruments. Reagents. Boric acid (H3BO3, 99.0%), potassium tetraborate tetrahydrate (K2B4O7·4H2O, 99.0%), potassium hydroxide (KOH, 90.0%), calcium oxide (CaO), and ethyl alcohol (C2H6O, Pure 200 proof) were used as received from SigmaAldrich. Deionized water was used during all experiments. Infrared (IR) Spectroscopy. The Fourier transform infrared (FTIR) spectrum in the range from 600 to 4000 cm−1 was recorded on a Bruker 37 Tensor FTIR spectrometer at room temperature. UV−Vis Diffuse Reflectance Spectra. The UV−vis diffuse reflectance spectrum was collected with a UV-3600 Shimadzu UV− vis-NIR spectrophotometer over the spectral range of 200−1500 nm at room temperature. Barium sulfate (BaSO4) was used as a standard sample for the baseline correction. The sample was thoroughly mixed with BaSO4, and this mixture was used for UV−vis measurements. The reflectance spectrum was converted to absorbance using the Kubelka−Munk equation.32,33 Thermal Analysis. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out with a NETZSCHProteus-61 analyzer instrument. Crystalline samples were placed in an aluminum crucible and heated from room temperature to 600 °C at a rate of 10 K/min, then cooled to room temperature at the same rate under flowing helium with a flow rate of 25 mL/min. Second-Order NLO Measurements. SHG measurements of polycrystalline samples were performed with a Nd:YAG laser (λ = 1064 nm) as the incident light source. Samples of the title compound were ground and sieved into seven distinct size ranges for the test. Ground α-SiO2 powder was used as a reference and sieved into the same size ranges. The intensities of the frequency-doubled output emitted from the samples were detected by a photomultiplier tube. Calculation. The electronic structure calculations were performed on the basis of density functional theory (DFT) using the Vienna Abinitio Simulation Package (VASP).34,35 The projector augmented wave (PAW) method36 was used to treat the core and valence electrons using the following electronic configurations: 2s22p4 for O, 2p63s1 for K, 2s22p1 for B, and 1s1 for H. The Brillouin zone is sampled using a 3 × 6 × 4 Γ-centered Monkhorst−Pack k-point mesh. Integrations were performed using Gaussian smearing with a width of 50 meV. The revised Perdew−Burke−Ernzerhof (PBE) functional for solids (PBE-sol)37 were used in our calculation. P r e p a ra t i on o f K 3 B 5 O 8 ( O H ) 2 . I n i t i a l l y , p r e c u r s o r K2B4O5(OH)4(H2O)2 was prepared. A mixture of K2B4O7·4H2O (1.4 g), KOH (0.35 g), and H3BO3 (0.8 g) was loaded into a 15 mL polytetrafluorethylene (PTFE) tube with 8 mL H2O. The tube was then placed in a microwave furnace, heated to 150 °C in 30 min and kept for 2 h, after which the furnace cooled down to room temperature in 1 h. During the heating and holding procedures, the pressure in the microwave ranged from 37 to 46 bar. Colorless

Table 1. Crystal Data and Structure Refinement for K3B5O8(OH)2 empirical formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) volume (Å3) Z ρcalc (g/cm3) μ (mm−1) F(000) crystal size (mm3) radiation 2θ range for data collection (deg) index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)]a final R indexes [all data] largest diff. peak/hole (e·Å−3) flack parameter

K3B5O10H2 333.43 99.87 orthorhombic Fdd2 13.736(9) 19.317(12) 7.606(5) 2018.4(2) 8 2.1938 1.391 1318.0 0.063 × 0.063 × 0.046 Mo Kα (λ = 0.71073) 6.48−59.54 −18 ≤ h ≤ 19, −26 ≤ k ≤ 26, −10 ≤ l ≤ 10 7095 1430 [Rint = 0.0256, Rsigma = 0.0185] 1430/1/88 1.022 R1 = 0.0137, wR2 = 0.0373 R1 = 0.0138, wR2 = 0.0373 0.25/−0.16 0.05(2)

R1 = Σ||Fo| − |Fc||/Σ|F0| and wR2 = [Σw(Fo2 − Fc2)2/ΣwFo4]1/2 for Fo2 > 2σ(Fo2). a

B

DOI: 10.1021/acs.inorgchem.8b01965 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) ORTEP view of the asymmetric unit of K3B5O8(OH)2. (b) Basic building unit of K3B5O8(OH)2.

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RESULTS AND DISCUSSION Structure and Description. The title compound K3B5O8(OH)2 crystallizes in the NCS, polar space group, Fdd2 (no. 43), with orthorhombic cell parameters of a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. Two unique K atoms, three unique B atoms, five unique O atoms, and one H atom are present in the asymmetric unit (Figure 1). For all of the structural figures, the program Crystalmaker41 was used. Atomic coordinates (×104) with equivalent isotropic displacement parameters (×103 Å2 ), anisotropic displacement parameters, selected bond lengths (Å) and angles (deg) for K3B5O8(OH)2 are listed in Tables S1−S4, respectively. The title compound exhibits a 3D framework structure formed from [B5O8(OH)2]3− building units (Figure 1). This basic building unit contains four triangular BO3 groups and one BO4 tetrahedra, where two rings share a common BO4 tetrahedron. According to Burns42 descriptions, this unit can be described as 4Δ1T:⟨2Δ1T⟩−⟨2Δ1T⟩. The [B5O8(OH)2]3− units are connected by O−H···O hydrogen bonds to form a pseudoframework, in which the K atoms occupy cavities and balance the valence (Figure 2). The dangling oxygen atoms in

The noncentrosymmetry of the structure originates from the unique 2-fold axis symmetry along the c-direction. The [B5O8(OH)2]3− units present four free vertices that allow O−H···O connections to form between adjacent [B5O8(OH)2]3− units. Glide planes along the a- and bdirections generate two opposite configurations, namely, lefthanded and right-handed [B5O8(OH)2] units. The left- and right-handed units then alternate in a parallel pattern and connect with each other through O−H···O bonds (Figure 3). In this way, only glide planes relate the right- and left-handed units, and the overall structure is NCS.

Figure 3. (a) Right- and left-handed units are related by glide planes, rather than inversion, to form the NCS (Fdd2) structure. (b) View along the x-axis of a single layer of left-handed units.

Description and Discussion of the Structural Diversity of Pentaborates. Pentaborates are a structurally diverse family, with many distinct kinds of fundamental building blocks (FBBs) found in existing pentaborates owing to three principal factors. First, [B5On] (n = 10−12, 14) clusters can include four FBBs with the arrangements of the BO3 triangles and BO4 tetrahedra: B5O10, (5:4Δ+T); B5O11, (5:3Δ+2T); B5O12, (5:2Δ+3T); B5O14, (5:5T) according to the classification by Heller, Christ, and Clark.43,44 Among them, (5:2Δ+3T) FBB can be further classified into two types: 2Δ3T: ⟨2ΔT⟩−⟨3T⟩ (e.g., M2B5O9(OH) (M = Sr, Ba))45 and 2Δ3T:⟨Δ2T⟩−⟨Δ2T⟩ (e.g., Pb2B5O9I).46,47 Moreover, the oxygen atoms of [B5On] may be connected by hydrogen atoms to form [B5On(OH)m] or [B5OnFm] clusters. Especially, the addition of fluorine will lead to the blueshift of the transmission spectrum (i.e., a larger bandgap) which is desirable for NLO compounds.48 Each of these units can be combined with others of its class to form different arrangements, including pairs, chains, sheets, or networks.44

Figure 2. (a) Ball-and-stick and (b) polyhedral representation and of the structure of K3B5O8(OH)2 (K−O bonds are removed for clarity).

each basic building units were removed through O−H linkages, which increase transparency in the DUV region by raising the energy gap of the anionic group.12 In the structure of K3B5O8(OH)2, the B−O bond lengths in the BO4 units vary from 1.341 to 1.507 Å. Each K atom is coordinated by eight oxygen atoms with the K−O bond lengths ranging from 2.610 to 3.291 Å (Figure S2). The results of bond valence calculations (K, 1.00−1.20; B, 2.97−3.01; O, 1.95−2.19; H, 1.05) indicate that the K, B, O, and H atoms are in oxidation states of +1, +3, −2, and +1, respectively. C

DOI: 10.1021/acs.inorgchem.8b01965 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Possible structural combinations of pentaborates. Anionic chains, sheets, and frameworks made up of B5On, [B5On(H/F)m] or [MB5O13] (M = Zn, Al, Ga) units were observed in existing pentaborates. Fillers to balance charge and fill cavity could be inorganic cations, organic amines metal complexes, H2O molecules, or some small anions.

Second, [B5On] or [B5On(OH)m] clusters/frameworks can flexibly accommodate various shapes, sizes, and charges of protonated organic amines,31 inorganic cations,45,49−52 metal complexes,53−55 and even OH− ions,46,56−58 halogen anions,59 and H2O molecules. Hydro/solvothermal methods have been extensively explored, leading to the discovery of several new oxo-boron clusters and open-framework metal borates.60 Among them, there are several NCS alkaline-earth metal borates, for example, Ca2[B5O9]·(OH)·H2O,46 Ca 2 [B 5 O 8 (OH)] 2 [B(OH) 3 ]·H 2 O, 61 and Sr 2 B 5 O 9 (OH)· H2O,62 that possess short cutoff edges lower than 200 nm. Third, [B5On] or [B5On(OH)m] clusters can connect with other species (such as AlO4, GaO4, ZnO4) to form additional derivative open-frameworks.30,31,63 Attempting to introduce heteroatoms into borate backbones has resulted in new pentaborates that integrate zeolitic porosity with extraordinary optical properties. For example, the aluminoborate Rb2AlB5O10·4H2O exhibits a high NLO efficiency (∼2 × KDP) with a wide band gap (5.6 eV).31 The possible structural combination of pentaborates with diversity are shown in Figure 4, and the representative pentaborates are listed in Table 2. Nonlinear Optical Property. K3B5O8(OH)2 crystallizes in the NCS space group Fdd2 (point group mm2), which can exhibit SHG behavior. The SHG intensity is about 0.5 times that of SiO2 at the same particle size range (Figure S3). This

Table 2. Summary of UV and DUV NLO Pentaborates compound

space group

FBBs

UV cutoff edge/ band gap

Ca2[B5O9]·(OH)·H2O Ba2[B5O9]Cl·0.5H2O Na[B5O7(OH)2](H2O) Na2B5O8(OH)·2H2O K2B5O8(OH)·2H2O Li3B5O8(OH)2 K2Al[B5O10]·4H2O (NH4)2Al[B5O10]·4H2O Rb2AlB5O10·4H2O K2[Ga(B5O10)]·4H2O K3B5O8(OH)2

Cc Pnn2 Pca21 Pna21 Pna21 Pnc2 C2221 C2221 C2221 C2221 Fdd2

2Δ+3T 2Δ+3T 2Δ+3T 3Δ+2T 3Δ+2T 3Δ+2T 4Δ+T 4Δ+T 4Δ+T 4Δ+T 4Δ+T