Synthesis, Characterization, and Theoretical Studies of (Pb4O

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Article pubs.acs.org/JPCC

Synthesis, Characterization, and Theoretical Studies of (Pb4O)Pb2B6O14: A New Lead(II) Borate with Isolated OxygenCentered Pb4O Tetrahedra and Large Second Harmonic Generation Response Fangfang Zhang,† Fangyuan Zhang,† Bing-Hua Lei,†,‡ Zhihua Yang,*,† and Shilie Pan*,† †

Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, Chine Academy of Sciences; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: A new lead(II) borate, (Pb4O)Pb2B6O14, has been synthesized by low-temperature hydrothermal method. It crystallizes in the noncentrosymmetric polar space group P1 and structurally features a three-dimensional framework composed of paralleled spiral B6O14 chains connected by the Pb cations and isolated oxygen-centered Pb4O tetrahedra. It exhibits a phase-matching powder second harmonic generation (SHG) effect of about 3.5 times that of KH2PO4 (KDP) at the 1064 nm fundamental wavelength. It also has optical band gap of 3.3 eV and high thermal stability up to 480 °C, which make it a promising candidate for nonlinear optical (NLO) material. In addition, electronic structure and NLO property are studied by the first-principles method, and the calculated NLO coefficients are consistent with the experimental observations. The important contributions of Pb4O tetrahedra and the borate groups to the SHG response have been identified by the SHG-density method.

1. INTRODUCTION Nonlinear optical (NLO) crystals are the key material of the allsolid state laser to generate coherent light through frequency conversion process and thus play crucial roles in the application and development of the laser technology.1−3 It has been a long lasting interest of researchers from material, chemical, and theoretical sciences to explore NLO crystals with excellent comprehensive properties including large second harmonic generation (SHG) effect, wide transparency window, phase matching, high thermal stability, and ease of crystal growth, etc.4−14 Typically, lead(II) borates have been the focus of intensive investigations due to their structural varieties and functionalities. The Pb2+ ion exhibits variable coordination numbers (2 ≤ CN ≤ 10) and easy-to-form diversity of coordination polyhedron with holodirected or hemidirected geometries.15,16 Meanwhile, the boron atoms can coordinate to three and four oxygen atoms forming BO3 triangle and BO4 tetrahedron, respectively, and these units can further polymerize to complicated BxOy architectures.17−20 Moreover, the synergistic effect between the stereoactive lone pairs on Pb2+ and the π-conjugated borate groups that having micro-SHG responses can effectively enhance the macro-SHG in the NLO materials.21−23 These merits of lead borate make it a good system for exploration of new NLO materials with versatile structures and large SHG response. It is worth noting that the Pb2+ cations can form a special kind of anion-centered © XXXX American Chemical Society

tetrahedra, Pb4O, as the Pb−O bonds with the bond valence of 0.50 v.u. are comparable to the Lewis base strength of the O2− anion in tetrahedral coordination.24 Though the important influence of such anion-centered tetrahedra on certain physical properties has been proposed, for example, the structural motifs of Cu4O tetrahedra in Cu5O2(SeO3)2Cl2 evidently determine the anisotropy of thermal expansion,24 their effect on the NLO properties has never been highlighted. From a practical point of view, binary lead borate is superior to the complicated systems owing to the benefit of obtaining high-quality single crystal. Up to date, only four binary lead borate compounds have been reported,25−30 of which three possess optical nonlinearity. The PbB4O7 crystalline was first reported by Bohaty25 and its NLO property reported by Corker and Glazer.26 Afterward large single crystals with high quality were grown via top-seeded melt-growth method. However, it cannot realize phase-matching for SHG due to the very small birefringence.27 The new lead borate, Pb4B2O7, was synthesized by solid-state reaction techniques, and SHG measurements on the powder samples indicate that it exhibits a SHG response of three times that of KH2PO4 (KDP).28 The oxygen- and leaddeficient lead borate Pb1.5(BO2.25)2 with the nonlinearity value Received: April 15, 2016 Revised: May 19, 2016

A

DOI: 10.1021/acs.jpcc.6b03862 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C close to that of α-quartz was obtained by high-temperature, high-pressure hydrothermal synthesis (T = 270−280 °C, P > 70 atm).29 In recent years, low-temperature hydrothermal method (T ≤ 210 °C, spontaneous pressure) has also been used for exploration of new lead borates, which led to a series of hydrous acentric lead borates with the water solvent incorporated into the final structures.30−32 To our best knowledge, no anhydrous binary lead borate was obtained via low-temperature hydrothermal method. In this article, we report the hydrothermal synthesis, characterization, and theoretical studies of a new lead borate, (Pb4O)Pb2B6O14. It structurally features isolated oxygencentered Pb4O tetrahedra and exhibits a SHG response of about 3.5 times that of KDP, the largest one among the reported binary lead borates with phase-matching property. Herein, the synthesis, crystal structure, thermal stability, IR and UV−vis−NIR diffuse reflectance spectroscopies, and SHG property were measured and discussed. In addition, firstprinciples calculations were carried out in order to analyze the relationship between the electronic structure and optical properties.

Table 1. Crystal Data and Structure Refinements for (Pb4O)Pb2B6O14 empirical formula formula weight space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 Z calculated density/g·cm−3 absorption coefficient/mm−1 F(000) crystal size/mm3 θ max /° reflections collected/unique completeness/% GOF on F2 final R indices [I > 2sigma(I)]a R indices (all data)a Flack parameter

2. EXPERIMENTAL SECTION Synthesis. Colorless crystals of (Pb4O)Pb2B6O14 were synthesized by low-temperature hydrothermal reactions of PbO, H3BO3, and KOH via a “Teflon pouch method”.33−35 First, 0.223 g (1.0 mmol) of PbO, 0.185 g (3.0 mmol) of H3BO3, and 0.028 g (0.5 mmol) of KOH were mixed and sealed in FEP Teflon pouch. Then the pouch was put in a 100 mL Teflon-lined autoclave, backfilled with 30 mL of distilled H2O. The autoclave was sealed and heated at 210 °C for 4 days. After the reactions were slowly cooled to room temperature, colorless platy crystals of (Pb4O)Pb2B6O14 were produced in >80% yield based on Pb. Single-Crystal XRD. Single-crystal XRD collection was performed on Bruker SMART APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.710 73 Å) at 296 K. The structure was solved by the direct methods and refined by full matrix least-squares fitting on F2 using SHELX.36 All Pb atoms were refined with anisotropic thermal parameters, and the structure was checked for possible missing symmetry with PLATON.37 Crystallographic data for the title compound are summarized in Table 1. The final refined atomic positions and equivalent isotropic displacement parameters are summarized in Table 2. Bond valence sum (BVS)38 for all atoms was calculated with the following formula: Vi = ∑sij and sij = exp[(d0 − dij)/b], where sij is the valence of bond i−j, and d0 and b are bond valence parameters, with values 1.963 and 0.49 for Pb−O bonds39 and 1.371 and 0.37 for B−O bonds,38 respectively. BVS calculation results are also listed in Table 2. Selected bond lengths and bond angles and anisotropic displacement parameters are listed in Tables S1 and 2 in the Supporting Information. Powder XRD. Powder XRD data were collected on a Bruker D2 PHASER diffractometer with Cu Kα radiation (λ = 1.5418 Å). The 2θ range was 10−70° with a step size of 0.02° and a fixed counting time of 1 s per step. IR Spectroscopy. The IR spectrum was recorded on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer within the range 400−4000 cm−1 by using KBr pellets. UV−Vis−NIR Diffuse Reflectance Spectroscopy. Optical diffuse reflectance spectrum was measured on a Shimadzu

(Pb4O)Pb2B6O14 1548.00 P1 6.9536(13) 7.2026(13) 7.8003(14) 76.248(11) 76.694(11) 73.982(12) 359.03(11) 1 7.160 70.169 642 0.233 × 0.121 × 0.119 27.48 5144/2963 99.2 1.080 R1 = 0.0382, wR2 = 0.0932 R1 = 0.0393, wR2 = 0.0939 0.35(2)

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

Table 2. Atomic Coordinates (×104), Equivalent Isotropic Displacement Parameters (Å2 × 103), and Bond Valence Sums (BVS) for (Pb4O)Pb2B6O14a atoms

x/a

y/b

z/c

Ueq

BVS

Pb(1) Pb(2) Pb(3) Pb(4) Pb(5) Pb(6) B(1) B(2) B(3) B(4) B(5) B(6) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15)

6868(1) 2521(1) 4387(1) 8812(1) 7027(1) 3509(1) 2190(30) 2070(40) 8830(40) 7240(40) 8830(40) 1710(30) 3540(20) 7310(20) 7140(20) 6160(20) 8100(20) − 70(20) 1940(20) 3590(20) 150(20) 3070(20) 1570(20) 1720(20) 310(20) 5880(20) 7670(20)

− 579(1) − 1080(1) 4295(1) 5005(1) 253(1) 4094(1) 960(30) 950(30) 2870(30) − 4180(30) 2470(30) 7920(30) 1070(20) 1240(20) 2015(19) − 2350(20) 5003(19) 2078(18) − 1090(20) 840(20) 2240(20) 1457(19) − 924(19) 6020(20) 2630(20) 3720(20) 4560(20)

117(1) 8678(1) 1818(1) 3371(1) 4971(1) 6765(1) 4930(30) 1690(30) 160(30) 8070(30) 7100(30) 4110(30) 6114(18) 7406(18) 1084(18) 7780(20) − 374(17) 8516(17) 5381(17) 32(18) 5293(17) 3019(17) 2386(18) 4485(18) 1369(19) 4180(20) 6830(20)

9(1) 9(1) 10(1) 11(1) 11(1) 12(1) 3(4) 4(4) 8(5) 10(5) 12(5) 5(4) 7(3) 9(3) 6(3) 15(3) 4(3) 5(3) 9(3) 9(3) 9(3) 5(3) 5(3) 8(3) 13(3) 14(3) 13(3)

2.23 2.13 2.06 1.98 2.16 1.93 2.99 2.98 3.03 2.96 2.99 2.92 2.13 2.00 2.12 2.04 2.03 2.16 1.98 2.03 2.08 2.04 1.96 2.00 1.81 1.96 2.02

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

B

DOI: 10.1021/acs.jpcc.6b03862 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C SolidSpec-3700DUV spectrophotometer at room temperature. Data were collected in the wavelength range 190−2600 nm. Thermal Analysis. Thermal gravimetric (TG) analysis and differential scanning calorimetry (DSC) were carried out on a NETZSCH STA 449F3 thermal analyzer instrument at a temperature range 40−615 °C with a heating rate of 5 °C min−1 in an atmosphere of flowing N2. Powder SHG. Powder SHG measurement was performed on a modified Kurtz-NLO system40 using a pulsed Nd:YAG laser for excitation (1064 nm, 10 ns, 10 kHz). Polycrystalline samples were ground and sieved into distinct particle size ranges (