Article Cite This: Chem. Mater. 2019, 31, 2992−3000
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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*,† †
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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 S Supporting Information *
ABSTRACT: Two polar molybdenum(VI) iodates(V), NH4[MoO3(IO3)] (1) and KRb[(MoO3)2(IO3)2] (2), have been prepared by a hydrothermal method. They crystallize in noncentrosymmetric (NCS) space groups Pna21 and Cc, respectively. Structurally, it is interesting that compound 2 has an exclusive one-dimensional [IO3]− chain based on cornersharing [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 results of the theoretical calculations. In addition, both compounds also possess large birefringences (Δn) of 0.083 at 1064 nm and 0.146 at 1064 nm, respectively. These results indicate that the title compounds will have potential applications as nonlinear optical materials. HIO3(I2O5).40 Compared with the normal [IO3]− unit, the [IO4]3− unit is relatively rare. There are two known types of [IO4]3− configurations in known iodates, namely, the “isolated” [IO4]3− unit and the trimerical [I3O8]− unit. The “isolated” [IO4]3− unit can be found in these compounds only as follows, i.e., Ln((MoO2)(IO3)4(OH)) (Ln = La, Eu, Sm, or Nd),41,42 M((MoO 2 ) 6 (IO 4 ) 2 O 4 )(H 2 O) (M = Ba or Sr), 43,38 Ag4(UO2)4(IO3)2(IO4)2O2,44 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 and α-Cs2I4O11.30 It is worth mentioning that there are two forms of [I3O8]− units. One is the “isolated” [I3O8]− unit observed in NaI3O8,27 α-AgI3O8,32 β-AgI3O8,32 Rb2I6O15(OH)2·H2O,31 etc. Another is the two-dimensional (2D) infinite [I3O8]− layer with six-membered rings observed in α-Cs2I4O11.30 In short, to the best of our knowledge, there are four types of 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 I−O units lead to the limited structural diversity of iodates. Therefore, we should devote ourselves to exploring new I−O units to enrich the structural types of iodates. In addition to exploring new I− O units, there is another common strategy for enriching the structural chemistry of iodates and obtaining the excellent
1. INTRODUCTION In recent years, a host of efforts have been focused on exploring a variety of new noncentrosymmetric (NCS) and polar functional materials with piezoelectric, pyroelectric, and ferroelectric properties, especially good nonlinear optical (NLO) performance.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 or Se),21−25 and ZnGeP2,26 have been commercialized even if some drawbacks have limited their application. Herein, to better satisfy the wide application of lasers, new compounds as NLO crystals have been sought. The iodates, as a class of promising NLO materials in visible (Vis), near-infrared, and mid-infrared regions, have become a hot pot over the past several decades. A series of highperformance metal iodates, such as NaI3O8,27 ABi2(IO3)2F5 (A = K, Rb, or Cs),28 Bi(IO3)F2,29 α-Cs2I4O11,30 β-Cs2I4O11,31 αAgI3O8,32 β-AgI3O8,32 PbPt(IO3)6(H2O),33 BaPd(IO3)4,34 RbAu(IO3)4,35 and Bi3OF3(IO3)4,36 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 (I−O bond lengths of >2.4 Å) are neglected. The “isolated” [IO3]− units are common, and they are observed in PbPt(IO3)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 © 2019 American Chemical Society
Received: February 7, 2019 Revised: March 28, 2019 Published: March 29, 2019 2992
DOI: 10.1021/acs.chemmater.9b00521 Chem. Mater. 2019, 31, 2992−3000
Article
Chemistry of Materials NLO materials, that is, introducing d0 transition metal cations (Mo6+, W6+, V5+, Nb5+, Ta5+, and Ti4+) into iodates. This method is prone to synthesizing NCS compounds with large second-harmonic generation (SHG) responses as the d0 transition metal cations are prone to producing strong second-order Jahn−Teller effects. A vast number of iodates containing a d0 transition metal cation with excellent NLO properties, such as A2Ti(IO3)6 (A = Li or Na),46,47 Cs(VO)2O2(IO3)3,48 NaVO2(IO3)2(H2O),49 AMoO3(IO3) (A = Rb or 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 a mild hydrothermal method. Finding a unique configuration of the four-coordinate I−O units, i.e., one-dimensional (1D) [IO3]− chain based on corner-sharing [IO4]3− units in compound 2, which was first found in all inorganic iodates, is noteworthy. 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 (Δn) are 0.083 at 1064 nm for compound 1 and 0.146 at 1064 nm for compound 2. In addition, the syntheses, crystal structures, infrared (IR) spectra, ultraviolet−visible−nearinfrared (UV−Vis−NIR) diffuse reflectance spectra, powder SHG phase-matching curves, and theoretical calculations of these two compounds are presented.
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 with monochromatic Cu Kα radiation (λ = 1.5418 Å) at room temperature. The patterns were taken in the 2θ range from 10° to 70° with a scan step width of 0.02° and a fixed counting time of 1 s per step. The experimental and calculated powder XRD patterns of compounds 1 and 2 are presented in Figure 1. Clearly, they are in agreement with the calculated ones
Figure 1. Experimental and calculated powder XRD patterns of (a) NH4[MoO3(IO3)] and (b) KRb[(MoO3)2(IO3)2]. except for a few impurity peaks in that of compound 1, which mainly come from (NH4)4Mo2O6 (PDF Card 18-0118) and MoO3 (PDF Card 21-0569). Many efforts were made to avoid the byproduct, but they were always unavoidable. 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 (∼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 laser. Polycrystalline samples of compounds 1 and 2 were ground and sieved into the following particle size ranges:
