PbCdF(SeO3)(NO3): A Nonlinear Optical Material Produced by

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

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PbCdF(SeO3)(NO3): A Nonlinear Optical Material Produced by Synergistic Effect of Four Functional Units Yun-Xiang Ma, Chun-Li Hu, Bing-Xuan Li, 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, P. R. China

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ABSTRACT: A new nonlinear optical material, the first fluoride selenite nitrate PbCdF(SeO3)(NO3), was successfully synthesized by traditional solid state reactions. This compound crystallizes in the polar space group of Pca21, and its structure features a novel 2D layered structure consisting of 1D lead nitrate chains and 2D cadmium selenite layers. PbCdF(SeO3)(NO3) exhibits a phase-matchable SHG efficiency of about 2.6 times that of KDP and a wide band gap of 4.42 eV. Its laser damage threshold was measured to be 135.6 MW/cm2, which is comparative to that of the reported Li7(TeO3)3F with a short ultraviolet cutoff edge. Theoretical calculation confirmed that the synergistic effects of all the four functional units make PbCdF(SeO3)(NO3) a remarkable SHG material.

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tortion when coordinated with oxygen ligands.9 Selenite derivative NLO materials, like selenite-nitrates,10 seleniteborates,11 or selenite-phosphates,12 could be accomplished when the second functional unit such as NO3, BO3, or PO4 group was introduced into the selenite system. Selenite nitrates are a type of interesting compounds constructed by polar basic building units of triangular pyramid and π-conjugate triangle planar simultaneously which are less studied. However, the incorporation of a lone-pair triangular pyramid and πconjugate NO3 triangle can result new structures with excellent SHG properties, such as Pb4(OH)4(BrO3)3(NO3) (Cc, 1 × KDP)13a and Pb2(SeO3)(NO3)2 (Pmn21, 2 × KDP).13b As the countercation, Pb(II) can also be regarded as an SHG functional unit when it coordinates with oxygen or fluorine ligands due to its 6s2 lone pair electrons on the condition that they are stereoactive.13 On the basis of the above description, we focused our research efforts on the lead selenite nitrate system to explore new excellent inorganic NLO materials. Up to now, only four compounds, namely, PbCu3(OH)(NO 3)(SeO3 )3 (H2 O) 0.5 (P1̅),14a Pb2Cu3O2(NO3)2(SeO3)2 (Cmc21),14a Pb2(NO2)(NO 3 )(SeO 3 ) (Pmn2 1 ), 1 4 b and Pb 2 (SeO 3 )(NO 3 ) 2 (Pmn21),13b were reported. It should be noted that three of them crystallized in the polar space groups. Our previous researches in selenites show that the substitution of oxygen atoms with fluorine elements may promote the SHG efficiencies of the replaced compounds and also lead to the

INTRODUCTION The design and synthesis of new structures crystallized in noncentrosymmetric (NCS) space groups have captured the spotlight of chemists and material scientists because NCS compounds can possess some special physical properties which centrosymmetric (CS) compounds cannot, for example, second-order nonlinear optical properties, also called second harmonic generation (SHG).1 However, structures without an inversion center are much more difficult to achieve compared with those symcenter included according to the search result of less than 22% NCS compounds from ICSD (Version 1.9.9). After years of exploration, scientists found that utilization of functional units to construct target structures is an effect strategy to obtain new SHG crystals.2 The most common functional units include: (1) “one side” coordinated lone pair units,3 such as IO3 triangular pyramid in α-LiIO3, (2) distorted polyhedral units containing d0 transition metals,4 such as the NbO6 octahedron in KNbO3, (3) π-conjugate planar units,5 such as the BO3 triangle in β-BaB2O4, and (4) symcenter excluded tetrahedral units,6 such as PO4 tetrahedron in KDP (KH2PO4). Recent research shows that introducing two or more different functional units to one structure may enhance its SHG efficiency if their polarizations can be constructively added.7,8 Many excellent SHG materials were reported based on this method, for example, Rb3VO(O2)2CO3 (21 × KDP),7d BiSeO3F (13.5 × KDP),8a Pb2BO3I (13.5 × KDP),8c and Pb2(BO3)(NO3) (9 × KDP).8d Metal selenites have been considered as an important kind of SHG candidates due to the stereoactive lone pair cation of Se(IV) which will undergo second-order Jahn−Teller dis© XXXX American Chemical Society

Received: July 20, 2018

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DOI: 10.1021/acs.inorgchem.8b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry blue shift of their cutoff edges.15 A larger optical band gap usually indicates larger laser damage threshold (LDT).16 To create new excellent SHG materials and enrich the structure chemistry of metal selenite nitrates, the most electronegative anion of F− and large displacement cation of Cd(II)5a were also introduced into the Pb(II)-SeO3-NO3 system simultaneously, which had not been explored before. Our efforts in this area result in the first fluorinated selenite nitrate compound, PbCdF(SeO3)(NO3), the space group of which is indeed noncentrosymmetric and polar. Our researches show that PbCdF(SeO3)(NO3) exhibits a phase-matchable SHG efficiency of about 2.6 × KDP, a broad optical band gap of 4.42 eV, as well as a large laser damage threshold of 135.6 MW/ cm2. Herein, we report the synthesis, structure, and particularly the optical properties of PbCdF(SeO3)(NO3) in detail.

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the date reduction and absorption corrections based on a multi-scan method were used.19a The structure was solved by the direct method and refined by full-matrix least-squares fitting on F2 by SHELX-9719b and also checked by PLATON19c for possible missing symmetry elements. Crystallographic data and structural refinements for the crystal are summarized in Table 1. Important bond distances are listed in Table 2. More details on the crystal structure studies are given as Supporting Information.

Table 1. Crystal Data and Structural Refinements for PbCdF(SeO3)(NO3) formula fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) μ(MoKα) (mm−1) GOF on F2 Flack factor R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)

EXPERIMENTAL SECTION

Materials and Instruments. All the reagents were obtained from commercial sources and employed without further refinement: Pb(NO3)2 (Aladdin, 99.5%), PbF2 (Aladdin, 99.5%), NaF (Shanghai Reagent Factory, 99.0%), CdO (Tianjin Reagent Factory, 99.9%), and SeO2 (Aldrich, 99.9%). Powder X-ray diffraction (PXRD) pattern was collected on a Rigaku MiniFlex II diffractometer using Cu−Kα radiation in the angular range of 2θ = 5−65° with a step size of 0.02°. Microprobe elemental analyses and the elemental distribution maps were measured on a field-emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy-dispersive X-ray spectroscope (EDS, Oxford INCA). IR spectrum was carried out on a Magna 750 FT-IR spectrometer using KBr as the diluent in 4000− 400 cm−1 with a resolution of 2 cm−1 at room temperature. The UV− vis−NIR diffuse reflectance spectrum was measured with a PE Lambda 900 UV−vis−NIR spectrophotometer at 200−2500 nm at room temperature. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449C instrument with a heating rate of 15 °C/min under a nitrogen atmosphere from 30 to 1000 °C. The powder frequency-doubling effect was studied by the method reported before.17 The fundamental wavelength is 1064 nm generated by a Q-switched Nd:YAG laser. Sieved crystals of KDP were used as the references. Powder LDTs of PbCdF(SeO3)(NO3) and Pb2(SeO3)(NO3)213b in the particle size of 70−100 mesh were measured with the reported single-pulse measurement method.18 The average input lasers’ energy density of single pulses were measured to be 72.0 mJ for PbCdF(SeO3)(NO3) and 52.5 mJ for Pb2(SeO3)(NO3)2, when the samples were damaged by the laser with a flat-top laser beam distribution (λ = 1064 nm and τ = 10 ns in a 1 Hz repetition). The area of the laser spot focused on the sample is 5.31 mm2 for PbCdF(SeO3)(NO3) and 6.16 mm2 for Pb2(SeO3)(NO3)2, respectively. Preparations of PbCdF(SeO3)(NO3). The single crystals of PbCdF(SeO3)(NO3) were synthesized by solid state reaction of Pb(NO3)2, CdO, NaF, and SeO2 in a vacuum quartz tube. The constituent parts are as follows: Pb(NO3)2 (0.331 g, 1.0 mmol), CdO (0.128 g, 1.0 mmol), NaF (0.042 g, 1.0 mmol), and SeO2 (0.111 g, 1.0 mmol). The mixture was grinded carefully. The quartz tub was heated at 280 °C for 48 h and then cooled to 30 °C at a rate of 5 °C h−1. Colorless slab-shaped crystals of PbCdF(SeO3)(NO3) were achieved in ca. 48% yield (based on Pb). Some impurities were dissolved and filtered by deionized water. Pure phases of the compound were synthesized by reacting stoichiometric amounts of CdO, PbF2, Pb(NO3)2, and SeO2 in the same way. The purity was checked by PXRD, which is coincidence with its calculated pattern (Figure S1). As shown in the elemental distribution map, elements of Cd, Pb, Se, F, and O are evenly dispersed in the crystal of PbCdF(SeO3)(NO3) (Figure S2). Crystal Structure Determination. Data collection was achieved on an Agilent Technologies SuperNova Dual Wavelength CCD diffractometer with a graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å) at 293 K. The program of CrysAlisPro was applied for

PbCdF(SeO3)(NO3) 527.56 orthorhombic Pca21 11.1206(10) 10.3664(10) 5.3950(5) 90 90 90 621.94(10) 4 5.634 36.310 1.090 0.219(11) 0.0326, 0.0764 0.0362, 0.0822

R1 = ∑||F o| − |Fc||/∑|F o|, wR 2 = {∑w[(F o)2 − (Fc)2]2/ ∑w[(Fo)2]2}1/2.

a

Table 2. Selected Bond Distances (Å) for PbCdF(SeO3)(NO3)a Pb(1)−F(1)#1 Pb(1)−F(1)#3 Pb(1)−O(6)#5 Pb(1)−O(5)#5 Cd(1)−O(1)#6 Cd(1)−O(1)#7 Cd(1)−O(3) Cd(1)−F(1)#9 Se(1)−O(1) N(1)−O(4) N(1)−O(6)

2.362(9) 2.505(10) 2.781(13) 2.912(11) 2.257(9) 2.307(9) 2.350(11) 2.691(8) 1.704(9) 1.228(16) 1.269(17)

Pb(1)−O(3)#2 Pb(1)−O(6)#4 Pb(1)−O(2)#3 Pb(1)−O(5) Cd(1)−F(1) Cd(1)−O(2)#8 Cd(1)−O(2) Se(1)−O(2) Se(1)−O(3) N(1)−O(5)

2.380(10) 2.702(9) 2.790(10) 2.930(10) 2.300(8) 2.313(10) 2.477(10) 1.701(10) 1.715(10) 1.233(16)

a Symmetry transformations used to generate equivalent atoms: #1 x − 1, y, z; #2 −x + 5/2, y, z − 1/2; #3 −x + 5/2, y, z + 1/2; #4 x − 1/ 2, −y, z; #5 −x + 2, −y, z − 1/2; #6 x + 1/2, −y + 1, z; #7 −x + 3, −y + 1, z − 1/2; #8 −x + 3, −y + 1, z + 1/2; #9 −x + 7/2, y, z + 1/2.

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RESULTS AND DISCUSSION The first fluorinated selenite nitrate, namely, PbCdF(SeO3)(NO3), was successfully obtained by solid state reaction. The colorless crystals of PbCdF(SeO3)(NO3) are stable in air (Figure 1). It crystallizes in orthorhombic space group Pca21, which is NCS and polar. Its structure features an undulated 2D layer constructed by 1D lead nitrate chains and 2D cadmium selenites layers (Figure 2). The asymmetric unit of CdPbF(SeO3)(NO3) includes one Pb, one Cd, one Se, one N, one F, and six O atoms, totaling 11 unique atoms in general sites. Pb(1) is six-coordinated with four O atoms and two F atoms in B

DOI: 10.1021/acs.inorgchem.8b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

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atoms (Figure S6), with SeO3 units covering on the two sides of cadmium octahedral layers (Figure 2c). The 1D lead nitrate chains and 2D cadmium selenite layer are further linked together to a thick undulated layer via fluorine and oxygen atoms (Figure 2a). The SeO3 triangle pyramid is a hexadentate ligand, chelating bidentately with 1 Cd (O2 and O3) atom and also bridging with 3 Cd and 2 Pb atoms (Figure 3a). The NO3

Figure 1. As-grown small crystals of PbCdF(SeO3)(NO3).

a pentagonal bipyramid with the lone pair located at the axial site.20 The bond distances of Pb−O and Pb−F are falling in the range of 2.380(10)−2.790(10) and 2.362(9)−2.505(10) Å, respectively. Cd(1) is in a CdO5F octahedral geometry with Cd−O lengths of 2.257(9)−2.477(10) Å and the Cd−F bond of 2.300(8) Å. Se(1) is connected with three oxygen in a ψSeO3 quadrihedral coordination environment with the Se−O distances ranging from 1.701(10) to 1.715(10) Å. N(1) is coordinated with three oxygen atoms in a NO3 π-conjugated planar triangle. The N−O bond lengths are ranging from 1.228(16) to 1.269(17) Å. Bond-valence-sum (BVS) calculation results of Pb(1), Cd(1), N(1), and Se(1) are 1.69, 1.83, 5.00, and 3.98, respectively, indicating that the valence states of Pb, Cd, N, and Se atoms are +2, +2, +5, and +4, respectively.21 The deviations of Pb(1) and Cd(1) from the ideal oxidations can be attributed to their weak coordination bonds. If the Pb··· O and Cd···F distances in the range of 2.8−3.0 and 2.5−2.8 Å are considered, the BVSs of the Pb(1) and Cd(1) atoms are 1.93 and 1.95, respectively. The PbO4F2 pentagonal bipyramids are edge-shared (F1··· O6) into a zigzag 1D chain (Figure S5), with NO3 triangles dangling on the one side of the lead polyhedral chain via O(6) atoms (Figure 2b). The CdO5F octahedrons are connected into a 4-connected 2D layer via corner-sharing of oxygen

Figure 3. Coordination environments of SeO3 (a) and NO3 (b) groups.

planar triangle is bidentate, bridging with 2 Pb atoms (Figure 3b). If the secondary Pb···O bonds of 2.912(1) and 2.930(1) Å were considered, the NO3 group can also be viewed as a tridentate ligand, chelating bidentately with 1 Pb atom apart from bridging with 2 Pb atoms (Figure 3b). The weak secondary Pb···O bonds strengthened the above 2D thick undulated layers to a pseudo-3D framework with 1D eightmembered ring tunnels running down the c-axis (Figure 4). Besides the title compound of PbCdF(NO3)(SeO3), there are four well characterized lead selenite nitrate compounds in the literature.13b,14 The structure of isomorphic Pb2(SeO3)(NO3)213b and Pb2(SeO3)(NO2)(NO3)14b can be described as a 3D framework composed of sandwich-like lead nitrate layers bridging by selenite units. Pb2Cu3O2(NO3)2(SeO3)2 features a honeycomb-like 2D layered structure with isolated NO3 groups filling between the interlayers, while PbCu3(OH)(NO3)(SeO3)3(H2O)0.5 displays a complicated 3D framework with the free nitrate groups located in the interspaces of the

Figure 2. (a) The 2D undulated layered structure of PbCdF(SeO3)(NO3); (b) the 1D chain of lead nitrate; (c) the 2D layer of cadmium selenite. C

DOI: 10.1021/acs.inorgchem.8b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

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first step started from 385 °C and ended at 530 °C, corresponding to the release of nitrate. The observed weight loss of 8.4% is very close to the calculated value of 8.7%. The second step occurred in the temperature range of 580−960 °C, which can be ascribed to the vaporization of 0.5 F2 and 1 SeO2 molecules. The observed weight loss of 25.5% is coincidence with the calculated value of 24.6%. The final residual of the sample was confirmed to be amorphous phase, and EDS studies show that only Cd, Pb, and oxygen atoms were left which is consistent with the results from the above analysis. SHG and LDT Measurements. As we described above, PbCdF(NO3)(SeO3) is crystallized in noncentrosymmetric and polar space group Pca21 and its fundamental building blocks are SHG-active functional units; therefore, it should exhibit excellent SHG properties. On the basis of the result from the optical diffuse reflectance spectrum of PbCdF(NO3)(SeO3), no obvious absorption peaks were found in the visible to near-infrared frequency band, so the Q-switched Nd:YAG 1064 nm laser was chosen as the fundamental radiation. Powder SHG measurements indicate that the sieved sample (70−100 mesh) of PbCdF(NO3)(SeO3) displays a strong frequency doubling efficiency about 2.6 times of that of commercial KDP, which is also larger than that of the reported Pb2(SeO3)(NO3)2 (2.0 × KDP)13b (Figure 6a). Figure 6b shows that the SHG intensities of PbCdF(NO3)(SeO3) are increased with the particle sizes at first and then keep constant when the particle size is bigger than 150 μm, which indicates that PbCdF(NO3)(SeO3) is phase-matchable. According to the result of the UV−vis−NIR spectrum, PbCdF(NO3)(SeO3) displays a large optical band gap of 4.42 eV, which is 0.66 eV bigger than that of the reported Pb2(SeO3)(NO3)2 (3.76 eV).13b As we stated in the Introduction, a larger optical band gap often means larger LDT value and therefore broader application prospects. So we evaluated the LDT value of PbCdF(NO3)(SeO3) preliminarily by the practical method reported before.18 A Q-switched pulse laser with flat-top laser beam distribution was used to measure the powder LDT of PbCdF(NO3)(SeO3). The LDT value of the sieved crystals of CdPbF(NO3)(SeO3) was measured to be 135.6 MW/cm2, which is similar to that of Li7(TeO3)3F (132.8 MW/cm2).16a The powder LDT value of Pb2(SeO3)(NO3)213b was also examined for comparison, and the value of 85.3 MW/ cm2 is much weaker than that of PbCdF(NO3)(SeO3), which is consistent with the results from the optical diffuse reflectance spectrum. Theoretical Studies. The phase-matchable SHG material of PbCdF(SeO3)(NO3) is composed of four different functional units: SeO3 quadrihedron, NO3 triangle, PbO4F2 pentagonal bipyramid, and CdO5F octahedron. It is meaningful to figure out the contribution of each functional unit to the SHG efficiency origin of PbCdF(SeO3)(NO3) theoretically. From the results of the band structure (Figure S7), we can find that PbCdF(SeO3)(NO3) is an indirect-band-gap solid because its valence band (VB) maximum is locating at the Y point but the conduction band (CB) minimum is situated between G and Z points. The calculated band gap is 3.47 eV, much smaller than the experimental result of 4.42 eV. The gap underestimation can be attributed to the GGA-PBE functional, which shifts the conduction bands downward intrinsically.23 So, a scissor operator of 0.95 eV was used to correct the band gap during the optical properties calculation of PbCdF(SeO3)(NO3).

Figure 4. Pseudo-3D network of PbCdF(SeO3)(NO3) connected by secondary Pb···O bonds.

structure.14a In these compounds, the nitrate groups are either unbonded free planar triangles or connected by lead atoms only with the Pb−O distances larger than 2.7 Å. Compared with NO3 groups, the SeO3 groups present stronger capability of coordination, which are either pentadente or hexadentate ligands in these structures. IR and UV−vis−NIR Spectra. IR spectrum of PbCdF(NO3)(SeO3) shows that the sample is transparent from 4000 to 1700 cm−1 (Figure S3). The broad and strong absorption bands in the range of 1442−1295 cm−1 and 882−823 cm−1 can be assigned to the characteristic absorption of the NO3− groups. The stretching vibrations ν(Se−O) of the selenite groups appear at 741−688 cm−1, and absorption bands from 437 to 493 cm−1 belong to the bending vibrations ν(O−Se− O) of the SeO3 units.22 UV−vis−NIR diffuse reflectance spectra studies indicate that the sample of PbCdF(NO3)(SeO3) is almost transparent in the range of 2500−500 nm with an optical band gap of 4.42 eV (Figure 5), which is much larger than that of the reported Pb2(SeO3)(NO3)2.13b Thermal Gravimetric Analysis. From the TGA curves of CdPbF(NO3)(SeO3) (Figure S4), we can find that this compound can be stable to 385 °C. It undergoes two steps of weight loss when the temperature continues to surge. The

Figure 5. UV−vis−NIR diffuse reflectance spectra of PbCdF(SeO3)(NO3). D

DOI: 10.1021/acs.inorgchem.8b02044 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. (a) Oscilloscope traces of the SHG signals for the sieved samples (70−100 mesh) of PbCdF(SeO3)(NO3). (b) Curves of measured SHG signals with different particle sizes of PbCdF(SeO3)(NO3).

spherically distributed around the Pb atoms, which reveals that no obvious stereochemical lone-pair activity in the lead atoms of PbCdF(SeO3)(NO3) can be observed. The nonlinear optical properties of PbCdF(SeO3)(NO3) were calculated further. PbCdF(SeO3)(NO3) is crystallized in polar space group Pca21, falling within the point group of mm2. Seeing that Kleinman’s symmetry, the three independent SHG tensors d15, d24, and d33 are calculated to be 9.21 × 10−9 esu, 9.63 × 10−9 esu, and 1.00 × 10−8 esu, respectively. These values are much higher than the experimental result (∼ 2.6 × KDP). Compared to powder SHG measurements, theoretical calculations are based on large periodic crystals which may result in higher SHG response. The frequency-dependent refractive indices curves of PbCdF(SeO3)(NO3) are displayed in Figure S9. The results follow the order of nz > ny > nx in the low energy range (0−2 eV), and the calculated birefringence is 0.055 at 1064 nm, which is large enough to acomplish the phase matching in SHG progression. To investigate the SHG origin of PbCdF(SeO3)(NO3), the spectral decomposition and the SHG density analyses of the largest tensor d33 were implemented. As shown in the bottommost panel of Figure 7, the topside of VB (−3.3 to 0 eV) and the underpart of CB (