Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3): Nonlinear-Optical Materials with Short UV Cutoff Edges Jianghe Feng,† Xiang Xu,† Chun-Li Hu, and Jiang-Gao Mao* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China
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ABSTRACT: Three new acentric Sc-based borates, K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3; space group R32), have been obtained and characterized. They are isostructural, and all exhibit three-dimensional [Sc2(B5O10)3]9− anionic architecture composed of B5O10 clusters and ScO6 octahedra with the alkali- and alkaline-earth-metal cations occupying the cavities and keeping the charge balance. These compounds possess moderate second-harmonic-generation (SHG) responses (∼0.2×β-BaB2O4 at 532 nm and ∼0.4×KH2PO4 at 1064 nm) with phase-matching abilities and importantly display short cutoff edges below 200 nm. Furthermore, the relationship between the crystal structure and the SHG property has also been discussed based on the theoretical calculations.
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INTRODUCTION As an essential class of nonlinear-optical (NLO) materials, metal borates exhibit a great advantage of coherent light generation in the ultraviolet (UV) and deep-UV areas.1−7 In borates, the B atoms usually adopt two types of coordination geometries, a BO4 tetrahedron and a BO3 planar triangle, which can also be polymerized into various fundamental building blocks, resulting in the rich structural diversity of borates.8−10 On the basis of the anionic group theory, the BO3 unit is a superduper NLO-active group for great microscopic NLO coefficients.11 Furthermore, both units are favored for the cutoff edge (λcutoff) moving to a short UV region. During the past decades, numerous alkali- or alkaline-earth-metal borates have been discovered and commercially applied as UV NLO crystals, such as KBe2BO3F2 (KBBF), LiB3O5 (LBO), βBaB 2 O 4 (BBO), CsB 3 O 5 (CBO), and CsLiB 6 O 1 0 (CLBO).4,12−16 To further explore new UV NLO materials, much attention has been paid to the rare-earth borate system.17−33 Because the rare-earth ions with closed-shell electron configurations, i.e., Sc3+, Y3+, La3+, and Lu3+, have no f−f or d−d electronic transitions, the corresponding rare-earth borates are expected to possess high optical transmission in the UV region. As an example, the derivatives of huntite Lu0.66La0.95Sc2.39(BO3)4 and Y0.57La0.72Sc2.71(BO3)4 were both reported to exhibit a short λ cutoff value of approximately 190 nm and possess the large second-harmonic-generation (SHG) coefficients d11 of 1.74 and 1.70 pm/V, respectively.22−24 Besides the mixed rare-earth metal borates, the ternary alkali- or alkaline-earth-metal rareearth borates, i.e., A2O/AeO−RE2O3−B2O3 (A = alkali metal, Ae = alkaline-earth metal, and RE = rare earth), have also been well studied, which produces a series of new structure types © XXXX American Chemical Society
and a few promising UV NLO crystals. For example, La2CaB10O19 with a large SHG coefficient d22 of 1.04 pm/V and an excellent UV transmission extending to 170 nm can produce a high-power picosecond 355 nm laser.27,29,30 Most recently, new UV NLO materials have also been discovered within the quaternary rare-earth borate system with two different alkali cations or both alkali and alkaline-earth cations (A2O−A′2O/AeO−RE2O3−B2O3, where A, A′ = alkali metals, Ae = alkaline-earth metal, and RE = rare earth), such as K7CaY2(B5O10)3 with a SHG effect of 0.9×KH2PO4 (KDP) and a λcutoff value below 190 nm and K6Li3Sc2(B5O10)3 with a SHG effect similar to that of KDP and a λcutoff value of 190 nm.31,33 It is suggested that the introduction of more alkali/ alkaline earth into rare-earth borates can further enrich the structure diversity of borates and facilitate the design of new materials with outstanding UV NLO properties. Hence, we carried out the systematic synthetic studies based on the unexplored quinary A2O−A′2O−AeO−Sc2O3−B2O3 system with the expectation of finding new UV NLO materials. The title noncentrosymmetric compounds, K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3), have been isolated successfully. These compounds all exhibit moderate SHG effects and short λcutoff values below 200 nm, indicating that they are good candidates for UV NLO applications. Herein, we present their singlecrystal structures, thermal behaviors, UV−vis spectra, and SHG properties. Received: December 13, 2018
A
DOI: 10.1021/acs.inorgchem.8b03479 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinements for AK6CaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3) fw temp, K space group a, Å c, Å volume, Å3 Z Dcalc, g/cm−3 F(000) completeness, % μ, mm−1 GOF on F2 Flack factor R1, wR2a [I > 2σ(I)] R1, wR2 (all data)
K6LiCaSc2(B5O10)3
K6NaCaSc2(B5O10)3
K6Li0.7Na0.3CaSc2(B5O10)3
1013.69 293(2) R32 12.6595(7) 15.1804(13) 2106.9(2) 3 2.397 1482 99.5 1.678 1.065 0.01(5) 0.0240, 0.0554 0.0263, 0.0571
1029.74 293(2) R32 12.7621(5) 15.0323(8) 2120.31(16) 3 2.419 1506 99.4 1.684 1.101 −0.04(4) 0.0269, 0.0704 0.0277, 0.0711
1018.45 293(2) R32 12.6904(2) 15.1164(3) 2108.29(6) 3 2.406 1489 99.6 1.682 1.031 0.02(4) 0.0226, 0.0581 0.0233, 0.0588
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑w[(Fo)2 − (Fc)2]2/∑w[(Fo)2]2}1/2.
a
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consistent with those obtained by single-crystal X-ray diffraction (XRD) studies. Large crystal growth was performed by the top-seeded solution method by using B2O3−K2O−LiF (LiF was used to reduce the viscosity) as a flux. The mixture of NaK6CaSc2(B5O10)3, K2CO3, B2O3, and LiF at the ratio of 1:1:2:3 was heated to 880 °C, held for 1 day, and then cooled slowly (1 °C/h) to 840 °C. Here a platinum wire was employed for the seed crystal growth, which was then introduced into the saturation solution (determined by the method described in ref 35) at a 10 rpm rotation rate. Subsequently, the single crystal was grown through cooling at a 0.3 °C/day rate until the considerable crystal size was formed. The as-grown crystal was drawn out from the solution slowly when the crystal growth was finished and then cooled to room temperature at the rate of 10 °C/h. The ICP/ EDS (giving an average K/Li/Na/Ca/Sc/B molar ratio of 20.5:2.4:1.0:3.1:6.4:48.3), single-crystal XRD, and PXRD analyses gave the formula of K6Li0.7Na0.3CaSc2(B5O10)3 (Figure S1c). Single-Crystal Structure Determinations. Bulk single crystals glued onto glass fibers were mounted on a SuperNova (Mo) X-ray source (λ = 0.71073 Å) at 293(2) K for reflection data collection. All data sets were corrected by the multiscan method for polarization, Lorentz factors, and absorption.36 The structures were solved by direct methods, then refined by a full-matrix least-squares fitting on F2 by SHELX-97,37 and checked for missing symmetry elements and possible twinning by using PLATON, but none was found.38 All of the atoms were refined with anisotropic thermal parameters. The refined Flack parameters of 0.01(5), 0.02(4), and −0.04(4) were for Li-, Li0.7Na0.3-, and Na-based compounds, respectively, confirming the correctness of their absolute structures.39,40 Interestingly, the absolute structure of the Na-based compound is opposite to those of the other two compounds. As a result, the crystallographic data and structural refinements are listed inTable 1, and the important bond lengths and angles are summarized in Tables S1 and S2, respectively. Computational Descriptions. Because the title compounds are isostructural, only K6LiCaSc2(B5O10)3 was taken as representative for the calculation. Here, the density functional theory (DFT) method in the CASTEP program was used for the electronic structure and optical property calculations.41,42 The Perdew−Burke−Ernzerhof in generalized gradient approximation was used for the exchange-correlation (XC) function. 43 The norm-conserving pseudopotential was employed to treat the core−electron interactions.44 The following orbital electrons were regarded as valence electrons: Li 2s1, K 3s23p64s1, Ca 3s23p64s2, Sc 3d14s2, B 2s22p1, and O 2s22p4. During the calculation, a k-point sampling of 2 × 2 × 2 and a cutoff energy of 800 eV were used to achieve numerical integration of the Brillouin zone. For the optical property calculations, more than 880 empty bands were employed to ensure convergence of the NLO properties and
EXPERIMENTAL SECTION
Materials and Methods. LiF (99.9%), M2CO3 (M = Li, Na, K; 99.9%), CaCO3 (99.9%), Sc2O3 (99.9%), and H3BO3 (99.99%) were used as reagents and purchased from Shanghai Titan Scientific Co. Ltd. Powder X-ray diffraction (PXRD) analyses were performed on a MiniFlex II θ−2θ diffractometer equipped with Cu Kα radiation (λ = 1.540598 Å) within the 2θ range of 10−70° with a step size of 0.02°. Inductively coupled plasma (ICP) measurements were tested on an Ultima 2 inductively coupled with plasma OES spectrometer. Microprobe elemental analyses were investigated by using fieldemission scanning electron microscopy (JSM6700F) with energydispersive X-ray spectroscopy (EDS; Oxford INCA). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies were performed on a NETZSCH STA 449F3 unit under a N2 atmosphere at a heating rate of 10 °C/min. The IR spectra were measured on a VERTEX 70 Fourier transform infrared spectrophotometer in the 4000−400 cm−1 range. The samples were mixed in a KBr matrix with a ratio of 1:100 and pressed into sheets for the measurement. The room-temperature optical absorption spectra were performed on a PE Lambda 950 spectrophotometer in the range of 200−2500 nm, and a BaSO4 plate was employed as the standard (100% reflectance). The powder frequency-doubling-effect measurements were tested by adopting a modified method of the Kurtz and Perry method under the incident Q-switched Nd:YAG lasers with wavelengths of 1064 and 532 nm.34 Samples of the title compounds and the references of KDP and BBO with several particle size ranges (53−62, 62−75, 75−90, 90−109, 109−150, 150−210, and 210−270 μm) were employed to evaluate the SHG properties. Crystal Growth. Small single crystals of K6LiCaSc2(B5O10)3 and K6NaCaSc2(B5O10)3 were obtained through spontaneous crystallization with the reagents of A2CO3 (A = Li, Na, K), CaCO3, Sc2O3, and H3BO3 at the stoichiometric molar ratio. A2CO3 and CaCO3 were heated at 200 °C for 1 h before being used. Concretely, the reactions were mixed and ground thoroughly by hand in mortars and then put into platinum crucibles, which were heated to 300 °C and held there for10 h and then to 850 °C for Li-based compounds and to 910 °C for Na-based compounds in 1 day and kept for 7 days and finally decreased to 400 °C in 3 days and finished. Then small single crystals were isolated after the obtained samples were soaked in deionized water for 1 day. The high purities of the crystals were identified by PXRD analyses, as shown in Figure S1a,b. Elemental analyses of EDS/ ICP show an average K/Li/Ca/Sc/B molar ratio of 5.9:1.1:1.0:1.8:14.9 for K6LiCaSc2(B5O10)3 and an average K/Na/ Ca/Sc/B molar ratio of 5.8:1.0:1.1:1.9:15.1 for K6NaCaSc2(B5O10)3, B
DOI: 10.1021/acs.inorgchem.8b03479 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Scheme showing the substructures of the B5O10 cluster (a) and ScO6 octahedron (b), views of the 3D [Sc2(B5O10)3]9− anionic network along the c axis (c) and the 1D Sc−B5O10 chains (d) and of the 3D structure of K6LiCaSc2(B5O10)3 along the c axis (e), and the arrangement of ScO6, LiO6, and CaO6 octahedra along the c axis. SHG coefficients. The other parameters and convergent criteria were set as the default values of the CASTEP code. The second-order NLO properties were evaluated according to the length-gauge formalism within the independent particle approximation.45,46 Chen’s static formula derived by Rashkeev et al. and improved by Chen’s group was adopted.47−49
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identical bond lengths [2.219(2) Å for Li1−O4 and 2.354(2) Å for Ca−O1; Figure S2] to form LiO6 and CaO6 octahedra, which exhibit D3 local symmetry. K1 and K2 are 8- and 10coordinated by O atoms (Figure S2), respectively, and the K− O bond lengths cover the 2.635(2)−3.292(2) Å region. The bond valence sums are calculated to be 0.78, 1.10/1.12, 2.11, 2.99, and 3.02/3.08/3.01 for Li1, K1,2, Ca1, Sc1, and B1,2,3, respectively, being close to their corresponding expected oxidation states. One B2O4 tetrahedron and a pair of B1O3 and B3O3 units constitute a three-membered ring by corner-sharing, and two such rings share a B2O4 tetrahedron to form a B5O10 group, which is the typical pentaborate fundamental building block with notation 5:[4Δ+T] introduced by Christ and Clark (Figure 1a).8 It is notable that the two triangular BO3 planes within one ring are both nearly located in the same plane (B1− B2−B3), and two such planes form a B5O10 group with the dihedral angle of 82.7°. Each B5O10 cluster is linked to four ScO6 octahedra, while each ScO6 octahedron is surrounded by six B5O10 clusters. Such an alternative connection of the ScO6 and B5O10 groups leads to the formation of a 3D anionic [Sc2(B5O10)3]9− network (Figure 1c) with large triangleshaped 1D channels along the c axis. It is notable that the [Sc2(B5O10)3]9− network contains the 1D Sc−B5O10 chains parallel to the c axis (Figure 1e), which exhibit a triangular projection along the c axis with ScO6 octahedra occupying the vertexes. K+ cations lie on the above tunnels, while Li+ and Ca2+ occupy the cavities between Sc3+ cations and are arranged together with Sc3+ in the line parallel to the c axis (Figure 1e,f). In addition, K6LiCaSc2(B5O10)3 can be viewed to be derived from K7MIIRE2(B5O10)3 (MII = Ca, Sr, Ba, Zn, Cd, Pb; RE = Y, Gd, Lu) through the chemical substitution of K+ (6coordinated) by Li+, MII by Ca2+ as well as RE3+ by Sc3+ (Figure S3a,b).31,32 However, there is an obvious difference between the structures of K 6 Li 3 Sc(B 5 O 1 0 ) 3 and K6LiCaSc2(B5O10)3; i.e., the Ca2+ sites for K6LiCaSc2(B5O10)3 are occupied by two extremely disordered Li+ ions for K6Li3Sc(B5O10)3 (Figure S3b,c). Thermal Stability. The TGA curves display no obvious weight losses below 900 °C for all three compounds, and the DSC curves present merely one endothermic peak at 885, 927,
RESULTS AND DISCUSSION
Crystal Structure. Isostructural K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3) crystallizes in the chiral trigonal space group of R32 (No. 155), and they can be classified into the K7MIIRE2(B5O10)3 (MII = Ca, Sr, Ba, Zn, Cd, Pb; RE = Sc, Y, Gd, Lu) family. Structures of the title compounds exhibit a three-dimensional (3D) B−O−Sc anionic framework consisting of B5O10 clusters and ScO6 octahedra in a strict alternating fashion, forming one-dimensional (1D) tunnels and cavities that are occupied by the alkali- and alkaline-earth-metal ions (Figure 1). Here, the K6LiCaSc2(B5O10)3 structure is described in great detail as a representative. The crystallographically independent unit of K6LiCaSc2(B5O10)3 contains one Li, two K, one Ca, one Sc, three B, and five O atoms. The Li1 and Ca1 atoms are both located at the 3-fold rotation axis as well as on the 2-fold rotation axis, the Sc1 atom is only lying on the 3-fold rotation axis, and the B2, K1, and K2 atoms are only located at the 2-fold rotation axis; meanwhile, all other atoms are occupied in the general positions. The B2 atom is coordinated by two O2 and two O4 atoms in a marginally distorted tetrahedral geometry with nearly equivalent B−O bond lengths [1.470(3) and 1.465(3) Å] and small dispersive O−B2−O bond angles [106.5(3)−112.4(1)°], whereas the B1 and B3 atoms are both 3-coordinated by O atoms in the triangular geometry with B−O distances and O−B−O bond angles spanning the extents of 1.325(3)−1.412(3) Å and 117.3(2)− 124.7(2)°, respectively (Figure 1a). The Sc1 atom is octahedrally coordinated by three O1 and three O5 atoms, and the relevant bond distances are 2.052(2) Å for Sc1−O5 and 2.173(2) Å for Sc1−O1 (Figure 1b), exhibiting C3 local symmetry with the 3-fold axis across the octahedron’s opposite two faces. The cis O−Sc−O angles fall in the 78.91(7)− 98.43(7)° range and the trans O−Sc−O angle is 166.6(1)°. Both Li1 and Ca1 are octahedrally coordinated with six C
DOI: 10.1021/acs.inorgchem.8b03479 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry and 930 °C for the Li, Na, and Li0.7Na0.3 compounds, respectively (Figure 2). Such thermal features reveal that all of
Figure 3. UV−vis−NIR transmission spectrum of K6Li0.7Na0.3CaSc2(B5O10)3 (the inset is the image of single crystals). Figure 2. TGA and DSC curves for K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3).
To estimate the SHG properties of these title compounds in the UV region, SHG measurements under a 532 nm laser radiation have also been performed. As shown in Figure 4b, all title compounds display SHG effects of ∼0.2 times that of BBO with phase-matching abilities. Hence, the reporting Li, Na, and Li0.7Na0.3 compounds could be used as UV NLO crystals. On the basis of previous studies of the Pan group,32,33 the B5O10 and REO6 groups are the dominant contributors to the SHG effects of K7MIIRE2(B5O10)3 (MII = Zn, Cd, Pb; RE = Sc, Y, Gd, Lu). Because of the same arrangement of the B5O10 groups, the difference in the SHG responses is deduced from the varying degrees of distortion for REO6 octahedra,52 namely, larger REO6 distortion induces stronger SHG effects. For the title compounds, the angle distortions of ScO6 [calculated by the formula Δφ = ∑(180° − θ); here θ is the O−RE−O angle] are weaker because of the effect of the smaller ionic radii of Li+ and Na+ (Table S3), and this would give rise to their relatively weak SHG responses. Theoretical Studies. To disclose the SHG origins theoretically, first-principle calculations of K6LiCaSc2(B5O10)3 as a representative were performed based on the DFT. The calculated band structure is presented in Figure S7. K6LiCaSc2(B5O10)3 possesses an indirect band gap of 4.692 eV because the valence band (VB) maximum (0.00 eV) is located between the G and K points and the conduction band (CB) minimum (4.692 eV) is at the G point. Because of the discontinuity of the XC energy, the energy band gap was underestimated. Because the experimental Eg is 6.52 eV, a scissor (1.828 eV) is used when the optical property is calculated. The density of states (DOS) are calculated (Figure 5) and reveal that the highest VB is mainly from the O 2p nonbonding states; meanwhile, the lowest CB comes from the Li 2s orbital. Hence, the Eg is predominated by Li and O atoms. To profoundly understand the origin of the SHG property, the SHG coefficients dij were calculated. Because of the Kleinman symmetry and space group, K6LiCaSc2(B5O10)3 has one independent second-generation tensor of d22, which is calculated to be 0. 64 pm/V and close to that (0.72 pm/V) of K7CdLu2B15O30. The calculated result (∼1.6×KDP) based on a large single crystal is larger than the experimental value (0.4×KDP) measured by using powder samples. Moreover, the SHG-contributed energy levels can be found in the spectral
the title compounds can be stable up to approximately 900 °C and thermal decomposition and volatilization occur upon further heating. In addition, the PXRD patterns of the residues for the three compounds only exhibit the diffraction peaks of ScBO3, which further confirmed that the decomposition reactions occurred (Figure S4). Vibrational Spectra. The IR absorption spectra for K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3) are quite similar (Figure S5), and all show the characteristic absorption bands of the BO3 and BO4 groups.32,33,50,51 The absorption peaks at 1200−1400 and 900−1100 cm−1 typically belong to the asymmetric stretching of the BO3 and BO4 units, respectively. The B−O bending vibrations of BOn (n = 3, 4) appear at 785 and 728 cm−1. The remaining peaks below 700 cm−1 would stem from the intermixed vibrations of the ScO6 and borate units, which are very difficult to concretely identify. UV−Vis−Near-IR (NIR) Spectra. UV−vis−NIR absorption spectra for the three title compounds all show little absorption ranging from 200 to 2500 nm, indicating that their short-wavelength transmission cutoffs extend to the deep-UV range (
