Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Li6M(SeO3)4 (M = Co, Ni, and Cd) and Li2Zn(SeO3)2: Selenites with Late Transition-Metal Cations Hongil Jo,† Suheon Lee,‡ Kwang Yong Choi,‡ and Kang Min Ok*,† †
Department of Chemistry and ‡Department of Physics, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea S Supporting Information *
ABSTRACT: A series of lithium metal selenites, Li6M(SeO3)4 (M = Co, Ni, and Cd) and Li2Zn(SeO3)2, were synthesized by hydrothermal and solid-state reactions. Li6M(SeO3)4 is composed of Li+ cations, MO6 octahedra, and SeO3 polyhedra, while Li2Zn(SeO3)2 consists of Li+, Zn(Li)O4 tetrahedra, and SeO3 polyhedra. Isostructural Li6Co(SeO3)4 and Li6Ni(SeO3)4 crystallize in the rhombohedral space group R3̅, forming a three-dimensional distorted cubic lattice. Li2Zn(SeO3)2 crystallizes in the orthorhombic space group Pbam and reveals a layered structure in the bc plane. Li6Cd(SeO3)4 revealing a unidimensional structure crystallizes in the polar non-centrosymmetric space group C2, attributed to the parallel alignment of distorted CdO6 octahedra. The direct-current magnetic susceptibility measurements unveil that Li6Co(SeO3)4 is a canted antiferromagnet with TN = 25 K, while Li6Ni(SeO3)4 undergoes an antiferromagnetic transition at TN = 54 K, having a negligible canted moment. The weak ferromagnetism observed in Li6Co(SeO3)4 indicates the significance of spin−orbit coupling, bringing about anisotropic exchange interactions. Li6Cd(SeO3)4 reveals a second harmonic generation (SHG) efficiency of 10 × α-SiO2. Dipole moment calculations on Li6Cd(SeO3)4 indicate that the cooperative interaction of CdO6 and SeO3 is responsible for the observed SHG properties. Band gaps of the compounds are enlarged as atomic number increases. The effect of late transition-metal cations with different coordination numbers on the framework structures and the subsequent physical properties will be also discussed.
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INTRODUCTION Exploring novel functional materials has been one of the most important tasks to synthetic chemists.1−6 Among many, maingroup metal cations such as Pb2+, Sb3+, Bi3+, I5+, etc. have attracted gigantic interest attributable to the asymmetric environment that is arising from the stereochemically active lone pair (SCALP) electrons.7−11 Especially, a number of metal oxides and oxyhalides containing Se4+ have been newly discovered.12−17 Selenites with SCALP have revealed unique characteristics such as antiferromagnetic property,18−20 selective ion-exchange,21 multiferroicity,22 and nonlinear optical properties.23−28 Selenite compounds have also exhibited diverse structural moieties such as open frameworks29 and variable dimensional structures.30 Among abundant structural variations, the centricity change has been significantly influenced by a variety of metal cations attributed to the cation size effect.31−33 Thus far, a few lithium transition-metal selenites have been reported. For example, Li5Mn(II)4Mn(III)(SeO3)8,34 LiFe(SeO3)2,35 and Li2Co3(SeO3)436 exhibit magnetic properties, Li6(Mo2O5)3(SeO3)6 reveals nonlinear optical (NLO) properties,37 Li2Cd3(SeO3)4 shows a transformation reaction,38 and LiSc(SeO3)2·xH2O16 exhibits a reversible single-crystal-tosingle-crystal transformation. Recently, we synthesized a series of lithium-rich transition-metal selenites, Li13M(SeO3)8 (M = © XXXX American Chemical Society
Sc, Cr, Mn, and Fe), and reported their structures and properties.39 Especially, Li13Mn(SeO3)8 revealed an interesting phase transition at low temperature due to the Jahn−Teller effect. Herein, we expand the selenite series using late transition metals such as Co, Ni, Zn, and Cd and successfully discovered four new selenites, namely, Li6Co(SeO3)4, Li6Ni(SeO3)4, Li2Zn(SeO3)2, and Li6Cd(SeO3)4. Detailed synthetic methods, crystal structure determination, and characterization such as magnetic and NLO properties for the title compounds are presented.
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EXPERIMENTAL SECTION
Synthesis. Li2CO3 (Hagashi, 98%), CoO (Alfa Aesar, 99.998%), NiO (Junsei, 99%), ZnO (Wako, 99.0%), CdO (Aldrich, 99.5%), SeO2 (Alfa Aesar, 99.4%), and HCl (Aldrich, 37%) were used as received. Crystals of Li6Co(SeO3)4, Li6Ni(SeO3)4, Li2Zn(SeO3)2, and Li6Cd(SeO3)4 were grown by hydrothermal reactions. 6.00 × 10−3 mol (0.443 g) of Li2CO3, 1.00 × 10−3 mol of MO (M = Co, Ni, Zn, and Cd), and 4.00 × 10−3 mol (0.444 g) of SeO2 were loaded into Teflon cups with 2 mL (4 mL for Li6Cd(SeO3)4) of deionized water and 0.6 mL of HCl. The reagents were mixed with magnetic stirring bars for 5 min. After they were tightly sealed, the autoclaves were heated to 230 Received: February 3, 2018
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DOI: 10.1021/acs.inorgchem.8b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry °C, dwelled for 3 d, and cooled to room temperature at a rate of 6 °C h−1. Crystals of Li6Co(SeO3)4, Li6Ni(SeO3)4, Li2Zn(SeO3)2, and Li6Cd(SeO3)4 were isolated by filtration in 68, 22, 91, and 90% yields, respectively, based on SeO2, in phase-pure forms. Bulk polycrystalline samples of Li6Co(SeO3)4, Li6Ni(SeO3)4, Li2Zn(SeO3)2, and Li6Cd(SeO3)4 were synthesized by solid-state reactions. 2.00 × 10−3 mol (0.148 g) of Li2CO3, 6.67 × 10−4 mol (0.050 g) of CoO, and 2.67 × 10−3 mol (0.296 g) of SeO2 for Li6Co(SeO3)4, 2.00 × 10−3 mol (0.148 g) of Li2CO3, 6.67 × 10−3 mol (0.050 g) of NiO, and 2.67 × 10−4 mol (0.296 g) of SeO2 for Li6Ni(SeO3)4, 2.00 × 10−3 mol (0.148 g) of Li2CO3, 2.00 × 10−3 mol (0.163g) of ZnO, and 4.00 × 10−3 mol (0.444 g) of SeO2 for Li2Zn(SeO3)2, 2.00 × 10−3 mol (0.148 g) of Li2CO3, 6.67 × 10−4 mol (0.086 g) of CdO, and 2.67 × 10−3 mol (0.296 g) of SeO2 for Li6Cd(SeO3)4 were combined. Each reaction mixture was thoroughly ground and pelletized. The pellets were put into fused silica tubes (height = 20 cm, o.d. = 0.5 cm), evacuated, and sealed. The tubes for Li6Co(SeO3)4 and Li6Ni(SeO3)4 were heated to 500 °C for 24 h, while those for Li2Zn(SeO3)2 and Li6Cd(SeO3)4 were heated to 300 °C for 72 h with intermediate three regrindings. After they were cooled, the products were washed with deionized water and dried in air for 1 d. The phase purity was confirmed by powder X-ray diffraction (PXRD). Noncentrosymmetric Li6Cd(SeO3)4 was deposited to NCS Materials Bank (http://ncsmb.knrrc.or.kr). Structure Determination. Single-crystal X-ray diffraction data were collected through Bruker SMART BREEZE diffractometer (Mo Kα radiation) at room temperature. A purple hexagonal plate (0.011 mm × 0.203 mm × 0.238 mm) for Li6Co(SeO3)4, a yellow hexagonal plate (0.019 mm × 0.092 mm × 0.113 mm) for Li6Ni(SeO3)4, a colorless plate (0.017 mm × 0.056 mm × 0.220 mm) for Li2Zn(SeO3)2, and a colorless needle (0.011 mm × 0.015 mm × 0.108 mm) for Li6Cd(SeO3)4 were selected. The collected data were integrated with the program SAINT.40 An absorption correction via the program SADABS was performed.41 The structure solution was obtained by SHELXS-2013,42 and the crystal structure was refined with SHELXL-2013.43 All calculations were performed by the programs implemented in the software package WinGX-2014.44 The crystallographic information and selected bond distances are summarized in Table 1 and Tables S1−S4. Characterization. PXRD data were collected in the 2θ range of 5− 70° with a step size of 0.02° for 0.1 s. The phase purity for the reaction products was confirmed by comparing the PXRD data with calculated
patterns generated from single-crystal X-ray diffraction (Figures S1− S4). Energy dispersive analysis by X-ray (EDX) was performed through a Horiba Energy EX-250 instrument coupled to a Hitachi S-3400N scanning electron microscope. The ratios are consistent with crystal structure obtained from single-crystal X-ray diffraction (see Table S5). IR spectra were acquired by a Thermo Scientific Nicolet iS10 FT-IR spectrometer. UV−Vis diffuse-reflectance spectra were obtained from a Varian Cary 500 scan UV−Vis−NIR spectrophotometer. Thermal analysis was conducted on a SCINCO TGA N-1000 thermal analyzer up to 900 °C under flowing Ar at a rate of 10 °C min−1. The direct-current (dc) magnetic susceptibilities for polycrystalline samples of Li6M(SeO3)4 (M = Co and Ni) were measured by using a conventional superconducting quantum interference device (SQUID) magnetometer (MPMS-XL7, Quantum Design). The static magnetic susceptibilities were recorded in the temperature range T = 2−300 K with an applied field of H = 1000 Oe for zero-field-cooled (ZFC) and field-cooled (FC) processes. The isothermal magnetization curves were measured in a field range of μ0H = −7 to 7 T at 2 K. Second-harmonic generation (SHG) measurements for Li6Cd(SeO3)4 were performed using a DAWA Q-switched Nd:YAG laser (1064 nm radiation operating at 20 Hz). Polycrystalline Li6Cd(SeO3)4 was graded into different particle size ranges (
