Explorations of New SHG Materials in the Alkali-Metal–Nb5+–Selenite

Article pubs.acs.org/IC

Explorations of New SHG Materials in the Alkali-Metal−Nb5+− Selenite System Xue-Li Cao, Chun-Li Hu, Fang Kong, 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 S Supporting Information *

ABSTRACT: Standard high-temperature solid-state reactions of NaCl, Nb2O5, and SeO2 resulted in two new sodium selenites containing a second-order Jahn−Teller (SOJT) distorted Nb5+ cation, namely, Na2Nb4O7(SeO3)4 (P1̅; 1) and NaNbO(SeO3)2 (Cmc21; 2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)4]2− anionic network composed of 2D [Nb4O11(SeO3)2]6− layers which are further bridged by additional SeO32− anions via corner sharing; the 2D [Nb4O11(SeO3)2]6− layer is formed by unusual quadruple [Nb4O17]14− niobium oxide chains of corner-sharing NbO6 octahedra being further interconnected by selenite anions via Nb−O−Se bridges. The polar compound 2 features a 1D [NbO(SeO3)2]− anionic chain in which two neighboring Nb5+ cations are bridged by one oxo and two selenite anions. The alignments of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of ∼7.8 × KDP (∼360 × α-SiO2), which is the largest among all phases found in metal−Nb5+−Se4+/ metal−Nb5+−Te4+−O systems. Furthermore, the material is also type I phase matchable. The above experimental results are consistent with those based on DFT theoretical calculations. Thermal stabilities and optical properties for both compounds are also reported.

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(e.g., Nb5+, Ta5+, Mo6+, W6+) and cations with stereochemically active lone pair electrons (e.g., I5+, Pb2+, Bi3+, Se4+, Te4+) into the same material is a very effective route to obtain materials with NCS structures and strong frequency-doubled effects.6−11 For example, the introduction of iodate groups into barium niobate resulted in BaNbO(IO3)5 with a strong SHG effect of ∼14 × KDP (∼660 times that of α-SiO2).6a Selenite and tellurite with also a stereoactive lone pair as iodate have been combined with various d0-TM cations, which led to the discovery of many good NLO materials such as BaM2TeO9 (M = Mo, W), displaying very strong SHG responses of 500−600 × α-SiO2.7c However, to date, only a few quaternary metal niobates containing Se4+ or Te4+ cations have been synthesized, including KNb 3 Se 2 O 1 2 , Ba 2 Nb 6 Te 2 O 2 1 , TlNbTeO 6 , Pb4Te6Nb10O41, Pb2Te2Nb4O15, BiNbTe2O8, and InNb(TeO4)2.12,13 Unfortunately, only InNb(TeO4)2 exhibits a NCS structure and shows a moderate SHG response of 100 × α-SiO2.13d Hence, more systematic synthetic explorations are needed in these systems in order to better understand their structure−property relationships and develop new highperformance SHG materials based on metal niobates. Our systematic explorations in the alkali metal−Nb5+−Se4+− O system afforded two new compounds: namely, Na2Nb4O7(SeO3)4 (1) and NaNbO(SeO3)2 (2). The polar NaNbO(SeO3)2 exhibits a strong frequency-doubled effect of 7.8 × KDP (∼360 × α-SiO2), which is the highest among all

INTRODUCTION During the past few decades, metal niobates have been widely investigated as a class of important photoelectric functional materials in modern photoelectron technology, especially in second-harmonic generation (SHG), since the octahedrally coordinated Nb5+ cation with d0 electronic configuration is subject to second-order Jahn−Teller (SOJT) distortion.1,2 Its distortion can be toward a corner (the local C4 direction), a face (the local C3 direction), or an edge (the local C2 direction).3 These distortions can induce the compounds to crystallize in a noncentrosymmetric (NCS) space group with strong SHG effect. To date, a number of metal niobates with strong frequency-doubled effects have been synthesized and their large-size single crystals have been grown for important applications in photonic technologies, such as LiNbO3 (LN) with ilmenite structure, KNbO3 (KN) with perovskite structure, and NaBa2Nb5O15 (BNN) with tungsten-bronze structure, with SHG responses of about 13.6 × KDP, 31 × KDP and 50 × αSiO2, respectively.4,5 The huge differences in the SHG responses for these materials can be attributed to the different types of distortions for the NbO6 octahedra. However, there are still some drawbacks for these niobate materials which limit their crystal growths and their practical applications. For example, LN is prone to phase transitions at different temperatures, LN and KN have a low laser damage threshold, and BNN shows a weak SHG effect. Hence, explorations of new types of niobates with better optical properties are needed. Recent studies show that the combination of two classes of cations such as octahedrally coordinated SOJT d0-TM cations © XXXX American Chemical Society

Received: September 9, 2015

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DOI: 10.1021/acs.inorgchem.5b02074 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry compounds found in metal−Nb5+−Se4+/metal−Nb5+−Te4+−O systems. Herein, the syntheses, crystal structures, SHG properties, and theoretical calculations are represented.

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and structural refinement details and selected bond lengths are given in Tables 1 and 2, respectively. Other related crystallographic data about the two compounds are supplied as the Supporting Information.

Table 1. Crystal Data and Structural Refinement Details for the Title Compounds

EXPERIMENTAL SECTION

Reagents and Instruments. NaCl (99+%, AR), Nb2O5 (99+%, AR), and SeO2 (99+%, AR) were purchased from the Shanghai Reagent Factory. The X-ray powder diffraction patterns were measured in a successive fashion with a step size of 0.02° in a 2θ ambit of 5−65° on a Rigaku MiniFlex II diffractometer at 293 K (Cu Kα radiation). Elemental analyses were performed on a field emission scanning electron microscope (FESEM, JSM6700F) furnished with an energy dispersive X-ray spectroscope (EDS, Oxford INCA). With pure KBr pellets as a standard, the infrared spectra were collected in the ambit of 4000−400 cm−1 with a resolution of 2 cm−1 at 293 K on a Magna 750 FT-IR spectrometer. The measurements of UV−vis diffuse reflectance spectra were carried out with a PE Lambda 900 UV−vis−NIR spectrophotometer in the range 250−2500 nm at 293 K.14 The curves of differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA) were measured on a NETZCH STA449C instrument under a nitrogen atmosphere with a heating speed of 8 °C/min. The powder samples of NaNbO(SeO3)2 were ground and sieved into a series of distinct particle size ranges of 13−25, 25−44, 44−53, 53−74, 74−105, 105−149, 149−210, and 210−300 μm, which were used for SHG measurements on a Q-switched Nd:YAG laser with a wavelength of 1064 nm.15 KDP and α-SiO2 samples sieved in the same particle size range were used as the references. Syntheses. Both compounds were initially prepared by standard high-temperature solid-state reactions of a mixture of NaCl (46.8 mg, 0.8 mmol), Nb2O5 (53.2 mg, 0.2 mmol), and SeO2 (221.9 mg, 2.0 mmol). In order to guarantee the best reaction activity and uniformity, the mixture was ground thoroughly in an agate mortar and then made into pellets, which were sealed into two evacuated quartz tubes. The first quartz tube was heated at 350 °C for 1 day and then at 800 °C for 5 days and subsequently cooled to 300 °C at 2 °C/h before taking off the furnace. A small quantity of colorless brick-shaped single crystals of compound 1 was obtained after washing excess NaCl and SeO2. The second quartz tube was allowed to react at 750 °C for 5 days and then cooled to 300 °C at 2 °C/h. After washing by distilled water, a mixture of colorless brick-shaped crystals of compound 1 and colorless rodlike crystals of compound 2 was obtained. After structural refinements of both compounds, powder crystalline samples were obtained through the solid-state methods of a NaCl−Nb2O5−SeO2 mixture in molar ratios of 10:1:10 and 4:1:10 at 780 °C (for 1) and 750 °C (for 2) for 5 days. The resultant products were washed with distilled water to remove excess NaCl and SeO2. A colorless crystalline single phase of compound 1 was obtained with a yield up to 87% on the basis of metallic Nb. The product of compound 2 contains a small number of brick-shaped crystals of compound 1 as impurities, which were removed by manual picking, and finally a single-phase sample with a yield of about 66% (based on metallic Nb) was obtained. Their purities were testified by power-XRD studies (Figure S1 in the Supporting Information). EDS elemental analyses on several single crystals revealed average Na:Nb:Se mole ratios of 1:1.98:2.05 and 1:0.90:2.13 for 1 and 2, respectively, being very close to those from single-crystal X-ray structural analyses. Single-Crystal Structure Determination. A colorless brickshaped crystal of Na2Nb4O7(SeO3)4 (0.30 × 0.23 × 0.20 mm3) and rodlike crystal of NaNbO(SeO3)2 (0.36 × 0.11× 0.09 mm3) were selected for structural determinations. Data collections were performed on a Mercury CCD instrument with Mo Kα radiation (λ = 0.71073 Å) at 293 K. The data sets were corrected by the multiscan method for Lorentz and polarization factors as well as for absorption.16a Both structures were established by direct methods and refined by full-matrix least-squares techniques with SHELXL97.16b No higher symmetry element was found from PLATON checking.16c The Flack parameter of 0.048(12) for NaNbO(SeO3)2 verifies the correctness of its absolute structure. Crystallographic data

fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) μ(Mo Kα) (mm−1) GOF on F2 Flack factor R1, wR2 (I > 2σ(I))a R1, wR2 (all data)

Na2Nb4O7(SeO3)4

NaNbO(SeO3)2

1037.46 triclinic P1̅ 3.9394(4) 10.1314(14) 10.8820(14) 106.084(5) 90.080(4) 97.753(7) 413.16(9) 1 4.170 11.668 1.085 N/A 0.0304, 0.0590 0.0369, 0.0621

385.82 orthorhombic Cmc21 10.952(4) 7.676(3) 7.357(3) 90 90 90 618.5(4) 4 4.143 13.771 1.131 0.048(12) 0.0159,0.0356 0.0164,0.0358

R1 = ∑||F o |}/||F c |/∑|F o |; wR2 = {∑w[(F o ) 2 − (F c ) 2 ] 2 / ∑w[(Fo)2]2}1/2.

a

Computational Methods. The electronic structure and SHG property calculations for the noncentrosymmetric NaNbO(SeO3)2 were performed by DFT methods within the CASTEP code.17 During the calculations, the exchange-correlation function of GGA-PBE and norm-conserving pseudopotential were chosen, in which Na-2s2sp63s1 and Nb-4d45s1 as well as the outermost electrons of Se and O were treated as valence electrons.18,19 The k-point sampling and the cutoff energy were set to be 2 × 3 × 3 and 750 eV, respectively. In the optical property calculations, 272 empty bands were used to ensure the accuracy of the calculated refractive indices and the SHG coefficients. When the static SHG coefficients were calculated, Lin’s formula was adopted,20 which has been widely used in solid-state systems.

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RESULTS AND DISCUSSION Syntheses. Two new sodium selenites containing the Nb5+ cation, namely, Na2Nb4O7(SeO3)4 (1) and NaNbO(SeO3)2 (2), have been successfully prepared by standard solid-state reactions at high temperatures. During the preparations of these two compounds, flux is necessary due to the high melting point and low reactivity of the niobium(V) oxide. Hence, excess NaCl and SeO2 were applied as flux. Furthermore, Na2Nb4O7(SeO3)4 can be obtained with different molar ratios of the starting materials and different reaction temperatures. In comparison to Na2Nb4O7(SeO3)4, the growth of Na2Nb4O7(SeO3)4 needs relatively rigorous reaction conditions. Structure of Na2Nb4O7(SeO3)4. Na2Nb4O7(SeO3)4 crystallizes in the space group P1̅ (No. 2). Its structure can be described as a 3D [Nb4O7(SeO3)4]2− anionic network with 1D tunnels of Nb4Se4 eight-membered rings (8-MRs) along the a axis being filled by Na+ cations (Figure 1). The asymmetrical unit contains one Na+, two Nb5+, and two Se4+ ions. Both Nb5+ and Se4+ cations are in asymmetric coordination arrangements attributed to the SOJT effect. Se(1) and Se(2) atoms adopt a ψ-SeO3 trigonal-pyramidal coordination geometry with the lone pair electrons of Se4+ cations occupying the pyramidal site. The Se−O bond lengths are 1.644(4)−1.751(4) Å (Table 2), which B

DOI: 10.1021/acs.inorgchem.5b02074 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Bond Distances (Å) for the Title Compoundsa Na(1)−O(3) Na(1)−O(6)#1 Na(1)−O(6)#2 Na(1)−O(6) Na(1)−O(2) Na(1)−O(2)#2 Na(1)−O(4) Nb(1)−O(7)#3 Nb(1)−O(8)

2.360(4) 2.393(4) 2.411(4) 2.493(5) 2.692(4) 2.807(4) 3.013(5) 1.783(3) 1.881(4)

Na(1)−O(3)#1 Na(1)−O(3)#2 Na(1)−O(3) Na(1)−O(3)#3 Na(1)−O(2) Na(1)−O(2)#3

2.396(5) 2.396(5) 2.447(5) 2.447(5) 2.826(5) 2.826(5)

Na2Nb4O7(SeO3)4 Nb(1)−O(5)#4 Nb(1)−O(1)#5 Nb(1)−O(2)#6 Nb(1)−O(7) Nb(2)−O(10) Nb(2)−O(9) Nb(2)−O(8) Nb(2)−O(4)

1.956(4) 2.034(4) 2.119(3) 2.163(3) 1.787(3) 1.909(5) 1.916(4) 2.046(4)

Nb(2)−O(3) Nb(2)−O(10)#3 Se(1)−O(2) Se(1)−O(3) Se(1)−O(1) Se(2)−O(6) Se(2)−O(4) Se(2)−O(5)

2.117(3) 2.167(3) 1.692(3) 1.693(4) 1.702(4) 1.644(4) 1.703(4) 1.751(4)

NaNbO(SeO3)2 Na(1)−O(1)#1 Na(1)−O(1)#2 Nb(1)−O(4)#4 Nb(1)−O(1)#3 Nb(1)−O(1) Nb(1)−O(2)#5

3.028(6) 3.028(6) 1.795(5) 1.977(3) 1.977(3) 1.986(3)

Nb(1)−O(2)#4 Nb(1)−O(4) Se(1)−O(3) Se(1)−O(1) Se(1)−O(2)

1.986(3) 2.130(5) 1.636(3) 1.738(4) 1.746(4)

a Symmetry transformations used to generate equivalent atoms are as follows. For Na2Nb4O7(SeO3): (#1) −x + 1, −y, −z; (#2) x − 1, y, z; (#3) x + 1, y, z; (#4) −x + 1, −y + 1, −z; (#5) −x + 1, −y + 1, −z + 1; (#6) x, y + 1, z. For NaNbO(SeO3)2: (#1) −x, −y, z − 1/2; (#2) x, −y, z − 1/2; (#3) −x, y, z; (#4) −x, −y + 1, z + 1/2; (#5) x, −y + 1, z + 1/2.

quadruple [Nb4O17]14− niobium oxide chain (Figure 1a). These quadruple niobium oxide chains are bridged by Se(1)O3 groups into a 2D [Nb4O11(SeO3)2]6− anionic layer in the ab plane (Figure 1b), and neighboring 2D layers are further interconnected by bidentate bridging Se(2)O3 groups into a 3D [Nb4O7(SeO3)4]2− network (Figure 1c). Within the 3D network, there are two types of 1D tunnels along the a axis based on Nb4Se2 rings and Nb4Se4 rings, respectively. The Na+ cations are located at the 8-MR channels. Each Na+ ion is seven-coordinated by two bidentate chelating and three unidentate SeO3 groups with Na−O bond lengths falling in the range 2.360(4)−3.013(5) Å (Table 2). Bond valence calculations on Na2Nb4O7(SeO3)4 gave values of 0.96, 5.16− 5.18, and 4.09−4.10 for Na, Nb, and Se atoms, indicating that they are in oxidation states of 1+, 5+, and 4+, respectively.22 Topological analyses were carried out using the TOPOS 4.0 program.23 The 3D [Nb4O7(SeO3)4]2− anionic structure in Na2Nb4O7(SeO3)4 is composed of SeO3 and NbO6 units. Each Se(1)O3 is connected to three NbO6 octahedra, and each Se(2)O3 is connected to two NbO6 octahedra. Each Nb(1)O6 octahedron is connected to three SeO3 units and three other NbO6 units, and each Nb(2)O6 octahedron is linked to four other NbO6 units and two SeO3 units. From a topological viewpoint, the Se(1)O3, Se(2)O3, Nb(1)O6, and Nb(2)O6 units serve as three-, two-, six-, and six-connected nodes, respectively. The two-connected node could be simplified to a straight line. The 3D anionic network of Na2Nb4O7(SeO3)4 can be simplified as a new (3,6,6)-connected net with a new Schläfli symbol of {42;6}{47;68}{49;66} (Figure S2c in the Supporting Information). It is interesting to note that the structure of KNb3O6(TeO3)2 reported previously by our group displays a 3D [Nb3O6(TeO3)2]− anionic network.12c Different from the case for Na 2 Nb 4 O 7 (SeO 3 ) 4 , the NbO 6 octahedra in KNb3O6(TeO3)2 are corner sharing to form a corrugated niobium(V) oxide layer parallel to the ac plane. Structure of NaNbO(SeO3)2. NaNbO(SeO3)2 crystallizes in the polar space group Cmc21 (No. 36). Its structure can be described as 1D [NbO(SeO3)2]− anionic chains separated by Na+ cations (Figure 2). The 1D [NbO(SeO3)2]− anionic chain

Figure 1. (a) 1D [Nb4O17]14− anionic chain along the a axis, (b) view of the 2D [Nb4O11(SeO3)2]6− anionic layer down the a axis, and (c) view of the 3D structure of NaNb4O7(SeO3)4 in the bc plane. Na, Nb, Se, and O atoms are drawn as green, cyan, yellow, and red circles, respectively, and NbO6 octahedra are shaded in cyan.

are close to those reported in other metal selenites.7,12 Both Nb(1) and Nb(2) atoms are octahedrally coordinated. Nb(1) is bonded to three oxo anions and three oxygen atoms from three selenite anions (Figure S2a in the Supporting Information). The Nb−O bonds are two short (1.783(3)−1.881(4) Å), two normal (1.956(4)−2.034(4) Å), and two long (2.119(3)− 2.163(3) Å); hence, the distortion of Nb(1)O6 octahedron is toward an edge of the octahedron (local C2 direction). The magnitude of the out-of-center distortion (Δd) was calculated to be 0.71.21 Nb(2) is surrounded by four oxo anions and two oxygen atoms from two selenite anions (Figure S2b). The Nb− O bonds are one short (1.787(3) Å), four normal (1.909(5)− 2.117(3) Å), and one long (2.167(3) Å); hence, the Nb(2)O6 octahedron is distorted toward a corner. The out-of-center distortion (Δd) was calculated to be 0.73,21 which is close to that in KNb3O6(TeO3)2.12c Neighboring Nb(1)O6 or Nb(2)O6 octahedra are cornersharing (O(7) or O(10)) into 1D chains along the a axis. A pair of Nb(2)O6 octahedral chains form a double chain by interchain corner sharing (O(9)), and the double chain further connects with two Nb(1)O6 octahedral chains from both sides via interchain corner sharing (O(8)), leading to a novel C

DOI: 10.1021/acs.inorgchem.5b02074 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

°C in its DSC curve. The experimental weight loss of 57.9% is close to the calculated value of 57.5%, corresponding to the release of two molecules of SeO2. The final residual is expected to be NaNbO3.1e Optical Properties. The IR spectrum measurements of Na2Nb4O7(SeO3)4 and NaNbO(SeO3)2 have been performed in the wavelength range of 4000−400 cm−1 at 293 K (Figure S4 and Table S1 in the Supporting Information). There is no absorption peak observed in the range 4000−1000 cm−1. The peaks attributed to the Se−O, Nb−O, and Nb−O−Se vibrations appear at 400−1000 cm−1. The absorption peaks at 880−960 and 530 cm−1 belong to the characteristic stretching of the Nb−O bonds. The IR peaks at 400−500 and 690−800 cm−1 are characteristic stretches of Se−O bonds. The absorption peaks associated with the Nb−O−Se vibrations were observed at 550−660 cm−1.12,13 UV−vis absorption spectra analyses for Na2Nb4O7(SeO3)4 and NaNbO(SeO3)2 indicate that they are transparent in the ranges of 0.425−2.5 and 0.386−2.5 μm, respectively (Figure S5 in the Supporting Information). The diffuse reflectance spectra of Na2Nb4O7(SeO3)4 and NaNbO(SeO3)2 revealed band gaps of about 3.56 and 3.65 eV (Figure S6 in the Supporting Information), respectively, indicating that both compounds are wide band gap semiconductors. Second-Harmonic Generation (SHG) Properties. It is imperative to measure the SHG property of NaNbO(SeO3)2 due to its polar structure. The curves of SHG measurements on NaNbO(SeO3)2 and KDP as well as α-SiO2 powders in the range 150−210 μm indicate that NaNbO(SeO3)2 shows a strong SHG effect of ∼7.8 × KDP (∼360 times that of α-SiO2), and it is type I phase matching (Figure 3). It is worth noting

Figure 2. (a) View of the structure of NaNbO(SeO3)2 in the ab plane, (b) a 1D [NbO(SeO3)2]− chain along the c axis, and (c) view of the structure of the 1D [NbO(SeO3)2]− chain in the ab plane. Na, Nb, Se, and O atoms are drawn as green, cyan, yellow, and red circles, respectively, and NbO6 octahedra are shaded in cyan.

down the c axis is composed of a 1D chain of corner-sharing NbO6 octahedra in which a pair of neighboring Nb atoms are further bridged by a pair of selenite anions (Figure 2b). Its asymmetrical unit includes one Na+, one Nb5+, and one Se4+ ion. Both Nb5+ and Se4+ ions are in asymmetric coordination arrangements due to SOJT effects. The Nb5+ cation connects with two oxo anions and four selenite groups in a unidentate fashion (Figure 2b). The Nb−O bonds are one short (1.795(5) Å), four normal (1.977(3)−1.986(3) Å), and one long (2.130(5) Å); hence, the NbO6 octahedron is distorted toward a corner (local C4 direction). The out-of-center distortion (Δd) is calculated to be 0.34,21 which is much smaller than those of NbO6 octahedra in KNb3O6(TeO3)212c and Na2Nb4O7(SeO3)4. The Se4+ cation adopts a ψ-SeO3 trigonal-pyramidal environment with the lone pair electrons occupying the pyramidal site. The Se−O bond lengths range from 1.636(3) to 1.746(4) Å, which are close to those in Na2Nb4O7(SeO3)4. Neighboring NbO6 octahedra are interlinked by corner sharing (O(4)) into a 1D chain along the c axis. Within the chain, each pair of neighboring Nb atoms are further bridged by a pair of selenite anions, resulting in the formation of a 1D [NbO(SeO3)2]− anionic chain (Figure 2b). The above chains are separated by the Na+ countercations (Figure 2a). The Na+ ion is eightcoordinated by four bidentate chelating SeO3 groups with Na− O bond distances in the range of 2.396(5)−3.028(6) Å (Table 2). The total bond valences for Na+, Nb5+, and Se4+ cations have been calculated to be 0.94, 5.23, and 4.01, indicating their oxidation states of 1+, 5+, and 4+, respectively.22 It is worth noting that Pb2NbO2(SeO3)2Cl features a different 1D Nb−Se−O chain.7a The [NbO2(SeO3)2]3− in Pb2NbO2(SeO3)2Cl is composed by corner-sharing NbO6 octahedra which are decorated by SeO3 groups in a bidentate chelating or unidentate fashion. Unlike the bidentate chelating selenite anion in NaNbO(SeO3)2, which connects two consecutive Nb atoms, that in Pb2NbO2(SeO3)2Cl forms a bidentate chelate with two interval Nb atoms; hence the Nb− selenite chain in Pb2NbO2(SeO3)2Cl is zigzag rather than linear.7a TGA. The TGA and DSC curves for Na2Nb4O7(SeO3)4 and NaNbO(SeO3)2 indicate that they are stable up to 417 and 303 °C, respectively (Figure S3 in the Supporting Information). The TGA curve of Na2Nb4O7(SeO3)4 reveals one step of weight loss from 417 to 677 °C, and its DSC curve shows one endothermic peak at 609 °C, during which four SeO2 molecules are released. The experimental weight loss of 41.2% matches well with the calculated value of 42.8%. The final residual is expected to be Na2Nb4O11.1f The TGA curve for NaNbO(SeO3)2 reveals one step of weight loss at 303−841 °C with one endothermic peak at 524

Figure 3. Oscilloscope traces of the SHG signals for the powders (149−210 μm) for KDP and NaNbO(SeO3)2 and phase-matching curve for NaNbO(SeO3)2 on a 1064 nm Q-switched Nd:YAG laser. The curve drawn is to guide the eye and is not a fit to the data.

that InNb(TeO4)2 reported previously shows a SHG response of ∼100 times that of α-SiO2, which is not phase matchable.13d From the above results we think that NaNbO(SeO3)2 is a very promising SHG material. To better understand the relationship between the SHG response and the structure of the material, the local dipole moments for the SeO3 groups and NbO6 octahedra as well as the net dipole moments within a unit cell for NaNbO(SeO3)2 were calculated (Table S2 in the Supporting Information).24,25 The calculated dipole moments for the asymmetric building units NbO6 and SeO3 polyhedra are 2.06 and 9.13 D, respectively, which are in agreement with the previously reported values.13 The x, y, and z components of the dipole moments for four Nb(1)O6 octahedra in a unit cell are 0, 2 × (±1.56 D), and 4 × (−1.35 D), respectively. Hence, the x and y components from four Nb(1)O6 octahedra cancel out D

DOI: 10.1021/acs.inorgchem.5b02074 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry completely and only the z components constructively add to a value of −5.40 D. The dipole moments from SeO3 groups almost cancel out completely due to the opposite orientations of the lone pair electrons on the selenite groups. Therefore, along the polar z axis, the dipole moments from the distorted NbO6 octahedra constructively add up to be a value of 5.46 D (Figure 4). It is also observed that all NbO6 octahedra in the

2s and Se 4s states with small amounts of Se 4p states. In the range of −7.5 to 0 eV and 2.5−10 eV, obvious overlap among Se 4p, Nb 4d, and O 2p states can be found, implying the strong bonding interactions among them. Na has few states in the vicinity of the Fermi level, except for the Na 3s3p peaks around 8−10 eV. It is worth noting that O 2p nonbonding states and the unoccupied Nb 4d states dominate the two sides of the band gap, respectively. Therefore, the band gap of NaNbO(SeO3)2 is determined by Nb and O. NaNbO(SeO3)2 belongs to the mm2 point group, and with the restriction of Kleinman symmetry, there are three independent SHG tensors left. The calculated SHG coefficients in the static limit are 1.01 × 10−9, 1.20 × 10−9, and 7.10 × 10−9 esu for d31, d32, and d33, respectively. The highest SHG tensor d33 is very consistent with the experimentally measured tensor (∼7 × KDP). In addition, we also calculated the refractive indices of the compound. As shown in Figure S8 in the Supporting Information, the frequency-dependent refractive indices curves display high anisotropy and follow an order of nz > ny > nx, and the calculated birefringence Δn is as large as 0.276 in the static limit (0.287 at 1064 nm), which is very helpful for achieving SHG phase matching. Furthermore, to intuitively show the dominant orbitals giving the major contribution to the SHG process, an SHG-density analysis was implemented. On the basis of the “band-resolved” method, each band/orbital contribution in VB and CB to a considered SHG coefficient can be identified.26,27 Summing the SHG-weighted orbital densities in VB and CB, can obtain the SHG density. To explore the SHG origin of NaNbO(SeO3)2, we calculated the SHG density of d33, which can intuitively give the orbitals contributing to d33. As shown in Figure 6, the SHG density in VB lies in O atoms, especially the nonbonding O 2p orbitals. The largest contribution among all O atoms is at O(4), which link to two Nb atoms in the structure. In CB, the SHG effect is greatly strengthened by Nb 4d orbitals, but to some extent, it is weakened by the unoccupied Se 4p as well as slightly by the O 2p orbitals. Summing the SHG density in the constituent groups/ions over VB and CB can give the contribution percentages of the groups/ions. In NaNbO(SeO3)2, the contribution percentages to d33 are calculated to be 59.5%, 40.1%, and 0.07%, respectively, for NbO6 groups, SeO3 groups, and Na+ ions. The results imply that the anionic groups of NbO6 and SeO3 give most of the contributions to the SHG effect of NaNbO(SeO3)2, but the contribution from Na is negligible.

Figure 4. (a) Ball-and-stick representations within a unit cell and (b) the [NbO(SeO3)2]− chain in the structure of NaNbO(SeO3)2. The green arrows indicate the approximate directions of the dipole moments of the NbO6 octahedra in the unit cell. The conelike isosurfaces near top of SeO3 polyhedra are consistent with a stereoactive lone pair on the Se4+ cations. The Na+ cations have been removed for clarity.

Nb−oxo−selenite chain are distorted along the same direction (c axis); hence, their polarizations are constructively added to produce a large macroscopic polarization along the polar axis, which resulted in a large SHG response of about 7.8 × KDP for the compound. Theoretical Calculations. To understand deeply the origin of the strong SHG effect of NaNbO(SeO3)2, first-principles calculations on the compound were performed. The band structure and the partial density of states (PDOS) are displayed in Figure S7 in the Supporting Information and Figure 5,

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CONCLUSIONS In conclusion, we have successfully synthesized two new sodium selenites containing the Nb5+ cation: namely, Na2Nb4O7(SeO3)4 (1) and NaNbO(SeO3)2 (2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)4]2− anionic network composed of 2D [Nb4O11(SeO3)2]6− layers which are further bridged by additional SeO32− anions via corner sharing, and the 2D [Nb4O11(SeO3)2]6− layer is formed by unusual quadruple [Nb4O17]14− niobium oxide chains of corner-sharing NbO6 octahedra being interconnected by selenite anions via Nb−O− Se bridges. The polar compound 2 features a 1D [NbO(SeO3)2]− anionic chain in which two neighboring Nb5+ cations are bridged by one oxo and two selenite anions. The alignment of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of ∼7.8 × KDP (∼360 × α-SiO2), which is the largest among all phases found in metal−Nb5+−Se4+/ metal−Nb5+−Te4+−O systems. Furthermore, the material is

Figure 5. Partial density of states of NaNbO(SeO3)2.

respectively. From Figure S7, the compound exhibits an indirect band gap of 2.807 eV (from G to S point). In comparison to the measured value (3.65 eV), the calculated gap is underestimated, which is due to the well-known limitation of the exchange-correlation function in the DFT method. From the PDOS in Figure 5, some bonding characteristics in the compound can be obtained. Below −9.5 eV, there are mainly O E

DOI: 10.1021/acs.inorgchem.5b02074 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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also type I phase matchable. The material is also of high thermal stability and wide optical transparent range; hence, it is a very promising new SHG crystal. Our work demonstrates that new metal niobate based SHG materials can be developed by introducing selenite or tellurite anions with lone pair electrons. Our future efforts will be devoted to the explorations of new SHG materials in other related systems such as zinc(II) and cadmium(II) niobate selenites.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02074. X-ray crystallographic data (CIF) XRD powder patterns, IR spectra, UV spectra, optical diffuse reflectance spectra, and TGA and DSC diagrams (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J.-G.M.: fax, (+86)591-63173121; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (Grants 21231006, 21203197, and 91222108).

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DOI: 10.1021/acs.inorgchem.5b02074 Inorg. Chem. XXXX, XXX, XXX−XXX