Polar Polymorphism - ACS Publications - American Chemical Society

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Article pubs.acs.org/cm

Polar Polymorphism: α‑, β‑, and γ‑Pb2Ba4Zn4B14O31−Synthesis, Characterization, and Nonlinear Optical Properties Hongwei Yu,†,‡,§ Hongping Wu,†,§ Qun Jing,† Zhihua Yang,† P. Shiv Halasyamani,*,‡ and Shilie Pan*,† †

Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Technical Institute of Physics and Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China ‡ Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204-5003, United States S Supporting Information *

ABSTRACT: Polar materials are critical for a variety of functional properties including ferroelectricity, pyroelectricity, and nonlinear optical behavior. Vital to developing new polar materials is an understanding of how the polarity influences the functional property, i.e., structure− property relationships. At present, structure−property relationships on polar materials have focused on materials with similar structural motifs. Interestingly, there are limited reports on the structure−property relationships of polar polymorphs, likely attributable to the challenge of synthesizing polar polymorphic materials. In this paper, a new strategy for the synthesis of polar polymorphs is presented. By employing this strategy, we report on the synthesis and characterization of the first example of a borate with all polar polymorphs: P1 for α-Pb2Ba4Zn4B14O31 (α-PBZB), Cc for β-PBZB, and P32 for γ-PBZB. In addition, powder second-harmonic generation (PSHG) measurements indicate that the polymorphs are SHG-active and type-I phase matchable. Structure−property relationships are discussed through theoretical calculations.

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INTRODUCTION Polymorphism, the ability of a system to adopt multiple crystal structures while retaining identical chemical composition, is of particular interest for developing new functional materials and understanding structure−property relationships.1−4 As for functionalities, polymorphs typically possess different physical and chemical properties, as their crystal structure varies. As for the study of structure−property relationships, polymorphs have identical chemical compositions but adopt different crystal structures. They are the ideal system for studying the influence of the atomic packing on the functional properties. In our study, we are interested in polar polymorphs. Polar materials are of academic and commercial interest due to the technologically relevant functional properties, including ferroelectricity, pyroelectricity, and nonlinear optical behavior.5−22 The key to develop new polar materials is through understanding structure−property relationships. However, the current studies on polar materials and their structure−property relationships mainly focus on the structures with the same or similar structural motifs but with different chemical compositions. For example, Li’s group and Ye’s group illustrated the effort of cation coordination control of anionic group alignment on SHG effect based on the study of the BaMBO3F (M = Zn, Mg)19 and ABCO3F (A = K, Rb, Cs; B = Ca, Sr, Ba),20 respectively. Halasyamani et al. discovered that the A+ cations have a dramatic influence on macroscopic polarity based on the study of the A2Ti(IO3)6 (A = Li, Na, K, Rb, Cs, Tl) system.10 © XXXX American Chemical Society

Poeppelmeier and co-workers probed the origin of phase matchability of NCS structures based on the study of two polar chemically similar hybrid compounds.12 He et al., through investigating atomic substitution, were able to congruently melt (K1−xNax)2Al2B2O7 (0 < x < 0.6).16 Obviously, all of the studies focus on the materials with the same or similar structure with different chemical compositions. However, investigations based on polar polymorphs are under-investigated. To synthesize polar polymorphs, two problems are apparent: (1) how to synthesize the polar structure efficiently; (2) how to make the polar structure exhibit polymorphs. Although statistically there is a low possibility for inorganic materials to crystallize in a polar space group,17 a series of strategies has been developed, which consists of using π-conjugated planar borate rings to direct noncentrosymmetric borate-based materials,11,20−22 involving cations exhibiting second-order Jahn−Teller (SOJT) distortions to obtain asymmetric building units10,21 or incorporating halide anions with large electronegativites to add to the polarization of structure.12,14,20−22 These strategies have increased the incidence of polarity in any new inorganic material. However, for the synthesis of polar polymorphs, it seems that no strategy is available. In our study, we suggest that cations with flexible coordination environments Received: April 28, 2015 Revised: June 3, 2015

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DOI: 10.1021/acs.chemmater.5b01579 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials Table 1. Crystal Data and Structure Refinement for α-, β-, and γ-PBZB αtemperature (K) wavelength (Å) formula weight crystal system space group unit cell dimensions

Z V (Å3) ρCalcd (Mg/m3) μ (mm) R (int) GOF on F2 final R indicesa [Fo2 > 2σ(Fo2)]a Flack factor extinction coefficient largest diff. peak and hole (e·Å−3) a

β296(2) 0.71073 1872.56 monoclinic Cc a = 7.044(5) Å b = 12.216(8) Å c = 31.625(2) Å β = 92.537(4)°

triclinic P1 a = 7.009(3) Å b = 7.045(3) Å c = 16.085(8) Å a = 78.990(6)° β = 82.573(6)° γ = 60.250(5)° 1 676.3(6) 4.597 21.726 0.0299 0.889 R1 = 0.0400 wR2 = 0.0831 0.021(8) 0.00119(19) 2.362 and −3.131

4 2718.5(3) 4.575 21.621 0.0956 0.922 R1 = 0.0491 wR2 = 0.0713 0.022(5) 0.000152(12) 4.651 and −4.293

γ-

trigonal P32 a = 7.1021(3) Å c = 47.043(4) Å

3 2055.0(2) 4.539 21.452 0.1292 0.945 R1 = 0.0448 wR2 = 0.0775 0.003(4) 0.00181(7) 2.729 and −1.897

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

including three temperature-induced polymorphs and three pressure-induced polymorphs.36,37 However, interestingly, in PBZB polymorphs, they contain two types of asymmetric units, the Pb2+ cations with a lone pair and Zn2+ cations with d10 electronic configuration, and all of these polymorphs crystallize in the polar space groups. As we know, they represent the first examples of borate with all the polymorphs being polar ones. Herein, we report on the synthesis, characterization, and functional properties of the polar polymorphs of PBZB.

and a variable anion framework will be favorable for the formation of the polymorphs, since the variation of the cation coordination or anion framework leads to the formation of multiple crystalline forms. Guided by these ideas, we choose the Pb2+ and Ba2+ cations and a variable zinc-borate anion framework. First, the Pb2+ cations with a lone pair and Zn2+ cations with d10 electronic configuration are both favorable for the formation of polar structures.21,23 More importantly, the Zn2+ cations are generally coordinated by four O atoms to form tetrahedra, which are often found in zinc-borate anion frameworks. Different from the B atoms, the Zn atoms possess low-valence state. Therefore, the ZnO4 tetrahedra in the anion framework can connect each other by corner-sharing or edge-sharing14,24 (the edge-sharing link is scarcely seen for high-valence low coordinated small cations, such as B, Si, P, etc.,25,26 because it will be a violation of Pauling’s third and fourth rules27). Thus, the flexible coordination of the ZnO4 tetrahedra makes the zinc-borate anion framework more variable than that of borates, borosilicates, and borophosphates. In addition, both the Pb2+ and Ba2+ cations have large ionic radii and exhibit flexible coordination environments. For the Ba2+ cation, the coordination number (CN) can generally vary from 6 to 10. For the Pb2+ cations, owing to the repulsion interactions of the lone pairs, their coordination environments are more complicated. These can even be divided into primary and secondary coordination spheres with coordination numbers (CN) ranging from 3 to 9.28,29 Thus, by combining the flexible coordination Pb2+ cations with the variable zinc-borate framework, we successfully synthesized three new polar Pb2Ba4Zn4B14O31 (PBZB) polymorphs. They are referred to as α-, β-, and γPBZB according to their crystallization temperature from low to high. Owing to the rich structural chemistry of borates, the polymorphs seem to be common in borates,30−37 especially for lone-pair-cations containing bortates. For example, in BiB3O6 polymorphs, even six different polymorphs have been observed,

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EXPERIMENTAL SECTION

Reagents. PbO (Tianjin Hengxing Chemical Reagent Co., Ltd., 99.5%), BaCO3 (Tianjin Hengxing Chemical Reagent Co., Ltd., 99.5%), ZnO (Tianjin Hengxing Chemical Reagent Co., Ltd., 99.5%), and H3BO3 (Tianjin Baishi Chemical Reagent Co., Ltd., 99.5%) were used as received. Synthesis. Crystals of α-, β-, and γ-PBZB were prepared by high temperature solution with an excess amount of PbO and H3BO3 as flux. For α-PBZB, a reaction mixture with 6.696 g (30 mmol) of PbO, 2.360 g (12 mmol) of BaCO3, 1.871 g (23 mmol) of ZnO, and 4.019 g (65 mmol) of H3BO3 were placed in a platinum crucible. The mixtures were melted at 850 °C and held at this temperature for 10 h. Then, the temperature quickly cooled down to 700 °C and was further decreased to 500 °C at a rate of 5 °C/h. Finally, it was quenched to room temperature. Clear, submillimeter size colorless crystals of α-PBZB were obtained (Figure S1 in the Supporting Information). By using the same conditions, the crystals of β- and γ-PBZB were also obtained from a reaction mixture with 6.696 g (30 mmol) of PbO, 2.940 g (15 mmol) of BaCO3, 1.627 g (20 mmol) of ZnO, and 4.019 g (65 mmol) of H3BO3 and a reaction mixture with 2.232 g (10 mmol) of PbO, 4.909 g (25 mmol) of BaCO3, 1.627 g (20 mmol) of ZnO, and 5.565 g (65 mmol) of H3BO3, respectively. Polycrystalline samples of compounds α- and β-PBZB were prepared by conventional solid-state methods. The stoichiometric starting reagents were preheated at 300 °C for 10 h. The temperature was raised to 530 °C for α-PBZB (600 °C for β-PBZB) and held for 72 h with several intermittent grindings, after which pure α- and β-PBZB were obtained. The purity of the samples was characterized on a Bruker D2 PHASER diffractometer equipped with a diffracted beam B

DOI: 10.1021/acs.chemmater.5b01579 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

In this formalism, the total SHG coefficient χ(2) is divided into virtual-electron (VE), virtual-hole (VH), and two-band processes, and it should be noticed that the contribution from two-band process was proved to be zero exactly.48 The formulas for calculating the contribution from VE and VH are as follows,

monochromator set for Cu Kα radiation (λ = 1.5418 Å). The diffraction patterns of α- and β-PBZB were taken from 10° to 70° (2θ) with a scan step width of 0.02° and a fixed counting time of 1 s/step. The calculated and experimental diffraction patterns are shown in Figure S2 in the Supporting Information. For γ-PBZB, although a series of solid-state reactions at different temperatures were tried, we were unable to obtain a pure phase. Single Crystal X-ray Crystallographic Studies. Single crystal structures were determined on an APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K and integrated with the SAINT program.38 Numerical absorption corrections were carried out using the SCALE program for area detector.39 All calculations were performed with programs from the SHELXTL crystallographic software package.40 All atoms were refined using full matrix least-squares techniques, and final least-squares refinement was on Fo2 with data having Fo2 ≥ 2σ (Fo2). The structures were checked for missing symmetry elements by the program PLATON.19 Crystal data and structure refinement information are listed in Table 1. The final refined atomic positions and selected bond distances (Å) and angles (deg) are given in Tables S1 and S2, in the Supporting Information, respectively. UV−vis−NIR and Infrared (IR) Spectra. UV−vis−NIR diffuse reflectance spectra were measured at room temperature with a Shimadzu SolidSpec-3700DUV spectrophotometer in the wavelength range of 190−2600 nm. The reflectance spectra were converted to absorbance with the Kubelka−Munk function.41,42 IR spectra were recorded with a Shimadzu IR Affinity-1 Fourier transform IR spectrometer in the 400−4000 cm−1 wavenumber range using KBr pellets. Thermal Analysis. The thermal properties of the compounds are studied on a NETZSCH STA 449C simultaneous analyzer under flowing nitrogen gas and heated from the room temperature to 1000 °C at a rate of 5 °C/min for α-PBZB. SHG Measurement. Powder SHG were measured by the KurtzPerry method.43 Polycrystalline samples of compounds and KDP were ground and sieved into distinct particle size ranges (