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Two Polar Molybdenum(VI) Iodates(V) with Large Second Harmonic Generation Responses Yahui Li, Guopeng Han, Hongwei Yu, Hao Li, Zhihua Yang, and Shilie Pan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00521 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019
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Chemistry of Materials
Two Polar Molybdenum(VI) Iodates(V) with Large Second Harmonic Generation Responses Yahui Li,†,‡,# Guopeng Han,†,‡,# Hongwei Yu,†,# Hao Li,†,‡ Zhihua Yang,† Shilie Pan*,† †CAS
Key Laboratory of Functional Materials and Devices for Special Environments; Xinjiang Technical Institute of Physics & Chemistry, CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Two polar molybdenum(VI) iodates(V), NH4[MoO3(IO3)] (1) and KRb[(MoO3)2(IO3)2] (2), have been prepared by hydrothermal method. They crystallize in non−centrosymmetric (NCS) space groups Pna21 and Cc, respectively. Structurally, it is interesting that Compound (2) has exclusive one dimensional [IO3]− chain based on corner−sharing [IO4]3− units in all iodates(V). As for the properties, both compounds exhibit large second harmonic generation responses of 4.7 × KH2PO4 (KDP) for Compound (1) and 8.5 × KDP for Compound (2) with phase-matching behavior. These results are in good agreement with the ones of the theoretical calculations. In addition, both compounds also possess the large birefringences with Δn = 0.083 @1064 nm and 0.146 @1064 nm, respectively. These results indicate that the title compounds will have potential application as the nonlinear optical materials.
1. INTRODUCTION
In recent years, a host of efforts have been focused on exploring a variety of new non−centrosymmetric (NCS) and polar functional materials with piezoelectricity, pyroelectricity, and ferroelectricity properties, especially good nonlinear optical (NLO) performances.1−15 During the past few decades, several well−known NLO crystals, such as KH2PO4 (KDP),16 KTiOPO4 (KTP),17 LiNbO3,18 β−BaB2O4 (BBO),19 LiB3O5 (LBO),20 AgGaX2 (X = S, Se)21−25 and ZnGeP226 have been commercialized even if some drawbacks have limited their application. Herein, in order to better satisfy the wide application of lasers, the efforts for searching new compounds as NLO crystals have been made. The iodates, as a class of promising NLO materials in visible (Vis), near− and mid−infrared (IR) regions, have become a hot−pot during the past decades. A series of high−performance metal iodates, such as NaI3O8,27 ABi2(IO3)2F5 (A = K, Rb and Cs),28 Bi(IO3)F2,29 α−Cs2I4O11,30 β−Cs2I4O11,31 α−AgI3O8,32 β−AgI3O8,32 PbPt(IO3)6(H2O),33 BaPd(IO3)4,34 RbAu(IO3)4,35 and Bi3OF3(IO3)436 have been explored. Among all known iodates, there are only two types of coordination geometries of the I5+ cation, i.e., [IO3]− and [IO4]3− units, if the weak I−O interactions (the I−O bond lengths > 2.4 Å) are neglected. The “isolated” [IO3]− units are common, and they are observed in PbPt(IO 3)6(H2O),33 ABi2(IO3)2F5 (A = K, Rb and Cs),28 K2Au(IO3)5,37 Bi(IO3)F2,29 Ba(MoO2)2(IO3)4O,38 etc. The neutral dimeric [I2O5] units formed by two corner−sharing [IO3]− units could also be observed in Rb3(IO3)3(I2O5)(HIO3)4(H2O)39 and HIO3(I2O5).40 Compared with the normal [IO3]− unit, the [IO4]3− unit is relatively rare. There are two types of the [IO4]3− configurations in known iodates up to now, namely the “isolated” [IO4]3− unit and the trimerical [I3O8]− unit. The “isolated” [IO4]3− unit can only be found in these compounds
as follows, i.e., Ln((MoO2)(IO3)4(OH)) (Ln = La, Eu, Sm, Nd),41,42 M((MoO2)6(IO4)2O4)(H2O) (M = Ba, Sr),43,38 Ag4(UO2)4(IO3)2(IO4)2O244 and Bi2(IO4)(IO3)3.45 The trimerical [I3O8]− unit is also a rare species. Prominent representatives of iodates containing the [I3O8]− trimer are NaI3O8,27 AgI3O8,32 α−Cs2I4O11.30 It is worth mentioning that there are two forms of the [I3O8]− units. One is the “isolated” [I3O8]− unit observed in NaI3O8,27 α−AgI3O8,32 β−AgI3O8,32 Rb2I6O15(OH)2·H2O,31 and so on; Another one is the two dimensional (2D) infinite [I3O8]− layer with 6−member rings observed in α−Cs2I4O11.30 In short, to our best knowledge, there are four types of the I−O units mentioned above, i.e., the “isolated” [IO3]− units, the “isolated” [IO4]3− units, the [I2O5] units and the [I3O8]− units. The finite types of the I−O units lead to the limited structural diversity of iodates. Therefore, we should devote ourselves to explore new I−O units to enrich the structural types of iodates. In addition to exploring new I−O units, there is another common strategy to enrich the structural chemistry of iodates and obtain the excellent NLO materials, that is, introducing d0 transition metal cations (Mo6+, W6+, V5+, Nb5+, Ta5+, Ti4+) into iodates. This method is prone to synthesize NCS compounds with large second harmonic generation (SHG) responses as the d0 transition metal cations are prone to produce large second−order Jahn−Teller effects. A vast of iodates containing a d0 transition metal cation with excellent NLO properties, such as A2Ti(IO3)6 (A = Li, Na),46,47 Cs(VO)2O2(IO3)3,48 NaVO2(IO3)2(H2O),49 AMoO3(IO3) (A = Rb, Cs)50 and NdMoO2(OH)(IO3)4,42 have been reported. In this work, two NCS molybdenum(VI) iodates (V), NH4[MoO3(IO3)] (1) and KRb[(MoO3)2(IO3)2] (2), have been successfully synthesized by mild hydrothermal method. It is interesting to find a unique configuration of the four−coordination I−O units, i.e., 1D [IO3]− chain based on
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corner−sharing [IO4]3− units in Compound (2), which was first found in all inorganic iodates. Meanwhile, both compounds show large SHG responses approximately 4.7 and 8.5 times that of the commercialized KDP, respectively. According to the theoretical calculations, their birefringences are ∆n = 0.083 @1064 nm for Compound (1) and Δn = 0.146 @1064 nm for Compound (2). In addition, the syntheses, crystal structures, IR spectra, ultraviolet−visible−near−infrared (UV−Vis−NIR) diffuse reflectance spectra, powder SHG phase−matching curves, and theoretical calculations of these two compounds have been presented. 2. EXPERIMENTAL SECTION 2.1. Syntheses of Compounds. MoO3 (Aladdin Industrial Co., Ltd., 99.9%), KIO4 (Aladdin Industrial Co., Ltd., 99.8%), NH4F (Tianjin Baishi Chemical Co., Ltd., 96%), Rb2CO3 (Jiangxi Dongpeng New Materials Co., Ltd., 98%) and NaOH (Tianjin Baishi Chemical Co., Ltd., 96%) were used as received from commercial sources without any further purification. Compound (1) was prepared via hydrothermal reaction of MoO3 (0.720 g, 5 mmol), KIO4 (0.69 g, 3 mmol), NH4F (0.259 g, 7 mmol), NaOH (0.120 g, 3 mmol) and 1 mL deionized water in a pouch. Compound (2) was also synthesized by the hydrothermal method using MoO3 (0.288 g, 2 mmol), KIO4 (0.92 g, 4 mmol), NH4F (0.037 g, 1 mmol), Rb2CO3 (0.092 g, 0.4 mmol) and 1 mL deionized water in a pouch. Then these reactions run in 120 mL autoclaves with PTFE liners (40 ml deionized water and 6 pouches in one liner) for 3 days at 220 °C and cooled at a rate of 2.7 °C/h to room temperature. After the reactions, the products were cleaned with deionized water. The syntheses involved a complex redox process in which the high valance I7+ ions in [IO4]− were reduced to the I5+ ions in [IO3]− and [IO4]3−. The similar oxidation−reduction reactions have also been reported in the synthesizing process of many other iodates.50,51 2.2. Structure Determination. Preselected single crystals were placed on a glass fiber with epoxy for data collection. The diffraction data of Compounds (1) and (2) were collected on a Bruker SMART APEX II CCD diffractometer using monochromatic Mo−Kα radiation (λ = 0.71073 Å) at 296(2) K and integrated with the SAINT program.52 The crystal structures were determined by direct methods and refined by full−matrix least−squares fitting on F2 using SHELX53 crystallographic software package. The program PLATON54 confirms that there was no existence of other higher symmetry. It is necessary to note that there are two disordered sites (K(1)/Rb(1) and K(2)/Rb(2)) in Compound (2) with almost same occupancies in both sites according to the single crystal structure data. The relevant crystallographic information of Compounds (1) and (2) were listed in Table S1 in the Supporting Information. The Wyckoff positions, site occupancy factors (S.O.F.), atomic coordinates, equivalent isotropic displacement parameters and selected bond lengths and angles were listed in Tables S2 and S3. 2.3. Powder X−ray Diffraction. The powder X−ray diffraction (XRD) data for Compounds (1) and (2) were collected on a Bruker D2 PHASER diffractometer equipped
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with monochromatic Cu Kα radiation (λ = 1.5418 Å) at room temperature. The patterns were taken in 2θ range from 10 to 70°with a scan step width of 0.02°, and a fixed counting time of 1 s per step, respectively. The experimental and calculated powder XRD patterns of Compounds (1) and (2) are presented in Figure 1. Clearly, they are in agreements with the calculated ones except for a few impurity peaks in that of Compound (1), which mainly come from (NH4)4Mo2O6 (PDF # 18−0118) and MoO3 (PDF # 21−0569). Many efforts were made to avoid the byproduct, but they were always unavoidable.
Figure 1. Experimental and calculated powder XRD patterns of (a) NH4[MoO3(IO3)] and (b) KRb[(MoO3)2(IO3)2], respectively.
2.4. Infrared Measurement. The IR spectra of Compounds (1) and (2) were measured on a Shimadzu IRAffinity−1 spectrometer. The samples were mixed thoroughly with dried KBr (about 6 mg of the sample and 600 mg of KBr), and the IR spectra were collected in the range from 400 to 4000 cm−1 with a resolution of 2 cm−1. 2.5. UV−Vis−NIR Diffuse Reflectance Measurement. The UV−Vis−NIR diffuse reflectance data of Compounds (1) and (2) were collected using a Shimadzu SolidSpec−3700DUV Spectrophotometer with the measurement range extending from 190 to 2600 nm at room temperature. 2.6. Powder Second Harmonic Generation Measurement. The powder SHG measurements of Compounds (1) and (2) were performed by the Kurtz−Perry method.55 The fundamental 1064 nm irradiation was generated by a Q−switched Nd:YVO4 solid−state lasers. Polycrystalline samples of Compounds (1) and (2) were ground and sieved into the following particle size ranges:
