Article pubs.acs.org/IC
New Series of Polar and Nonpolar Platinum Iodates A2Pt(IO3)6 (A = H3O, Na, K, Rb, Cs) Bing-Ping Yang,* Chun-Li Hu, Xiang Xu, and Jiang-Gao Mao* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China S Supporting Information *
ABSTRACT: A new series of platinum iodates, namely, α-(H3O)2Pt(IO3)6, β-(H3O)2Pt(IO3)6, and A2Pt(IO3)6 (A = Na, K, Rb, Cs), have been synthesized. Interestingly, among these six stoichiometrically identical compounds, α-(H3O)2Pt(IO3)6 is polar, whereas other compounds are nonpolar and centrosymmetric. They all consist of zero-dimensional [Pt(IO3)6]2− molecular units separated by H3O+ or A+ cations. All of the lone electron pairs of the IO3− groups are aligned and almost point to one direction for α-(H3O)2Pt(IO3)6, whereas IO3− groups are located trans to each other in other compounds. The material, α(H3O)2Pt(IO3)6, exhibits very strong second harmonic generation (SHG) effects, approximately 1.2 × KTiOPO4 (KTP), and is phase-matchable. Thermogravimetric analysis, elemental analysis, infrared spectra, UV−vis spectra, nonlinear optical properties, and theoretical calculations are also reported.
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INTRODUCTION Nonlinear optical (NLO) crystals are of broad interest because they are an important part in optical devices such as second harmonic generators, broadband electro-optic modulators, and tunable parametric oscillators.1−3 Some useful NLO materials have been discovered, such as LiB3O5 (LBO), β-BaB2O4 (BBO), KTiOPO4 (KTP), KH2PO4 (KDP), LiNbO3 (LN), AgGaSe2, and ZnGeP2.4−8 According to the macroscopic property and microscopic structure relationships, the presence of asymmetric structural units plays a key role in the formation of noncentrosymmetric (NCS) and polar compounds. Asymmetric structural units include NCS π-conjugated groups ([BO3]3−, [NO3]−, [CO3]2−, etc.),9−14 cations with lone electron pairs (Pb2+, Bi3+, Te4+, I5+, etc.),15−18 d0 transition metal with second-order Jahn−Teller distortion (Mo6+, V5+, Ti4+, etc.),19 and chalcogenides’ polar pyramidal units (e.g., [AsS3]3− and [SbS3]3−).20 The strategy of introducing multiple asymmetric units into a single structure to achieve new materials with a high NLO coefficient has been taken and worked.21−28 A few iodates with strong second harmonic generation (SHG) response have been synthesized using this strategy.29−34 In contrast to d0 transition metal and lanthanide iodates, platinum iodate is less resolved. The most common oxidation © XXXX American Chemical Society
states for platinum cations in complexes are +2 and +4. The corresponding coordination modes are square-planar geometry and octahedral geometry. Gold(III) and palladium(II) iodates with polar structure have been reported recently.35,36 For instance, RbAu(IO3)4, α-NaAu(IO3)4, and α-CsAu(IO3)4 exhibit strong SHG responses somewhat greater than those of KTP. BaPd(IO3)4 produces a moderate SHG signal of ∼0.4 × KTP. These d8 metals are in a square-planar geometry, hinting that the square-planar coordination mode may be beneficial to the NCS structure. To the best of our knowledge, the only example of platinum iodate is PbPt(IO3)6(H2O), which has previously been synthesized.37 PbPt(IO3)6(H2O) exhibits a strong SHG response of ∼8 × KDP and contains polar [Pt(IO3)6]2− units. Our efforts in the monovalent cation platinum iodate system achieved six compounds: α-(H3O)2Pt(IO3)6, β-(H3O)2Pt(IO3)6, and A2Pt(IO3)6 (A = Na, K, Rb, Cs). Most important of all, the polar material, α-(H3O)2Pt(IO3)6, exhibits very strong SHG effects, approximately 1.2 × KTP, and is phase-matchable. Herein we report their synthesis, structures, thermal properties, spectroscopy measurements, nonlinear optical properties, and theoretical calculations. Received: December 10, 2015
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DOI: 10.1021/acs.inorgchem.5b02859 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Crystallographic Data for α-(H3O)2Pt(IO3)6, β-(H3O)2Pt(IO3)6, and A2Pt(IO3)6 (A = Na, K, Rb, Cs)
a
compound
α-(H3O)2Pt(IO3)6
β-(H3O)2Pt(IO3)6
Na2Pt(IO3)6
K2Pt(IO3)6
Rb2Pt(IO3)6
Cs2Pt(IO3)6
formula fw crystal system space group a/Å c/Å α/deg γ/deg V/Å3 Z Dcalc/g·cm−3 μ(Mo Kα)/mm−1 θmax/deg completeness/% GOF on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) Flack param.
H6PtI6O20 1282.54 hexagonal P63 9.2688(10) 5.2328(6) 90 120 389.33(7) 1 5.470 21.019 26.37 100 1.236 0.0307, 0.0840 0.0336, 0.0859 0.09(4)
H6PtI6O20 1282.54 trigonal R3̅ 11.1638(8) 11.5343(12) 90 120 1244.93(18) 3 5.132 19.720 26.37 100 1.095 0.0303, 0.0742 0.0346, 0.0757 N/A
Na2PtI6O18 1290.47 trigonal R3̅ 11.0206(11) 11.3523(17) 90 120 1194.1(2) 3 5.384 20.601 24.67 100 1.038 0.0480, 0.1105 0.0547, 0.1169 N/A
K2PtI6O18 1322.69 trigonal R3̅ 11.2037(8) 11.3686(13) 90 120 1235.83(19) 3 5.332 20.358 26.36 100 1.156 0.0322, 0.0748 0.0386, 0.0785 N/A
Rb2PtI6O18 1415.43 trigonal R3̅ 11.3670(5) 11.4129(9) 90 120 1277.08(13) 3 5.521 24.892 26.34 100 1.100 0.0288, 0.0738 0.0295, 0.0741 N/A
Cs2PtI6O18 1510.31 trigonal R3̅ 11.5598(5) 11.5807(9) 90 120 1340.19(13) 3 5.614 22.323 26.32 100 1.140 0.0359, 0.0988 0.0377, 0.1002 N/A
R1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = {∑w[(Fo)2 − (Fc)2]2/∑w[(Fo)2]2}1/2.
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the ICSD database (CSD number 430469) (Web site: https://www. fiz-karlsruhe.de/; e-mail: crysdata@fiz-karlsruhe.de). Physical Measurements. Microprobe elemental analyses were measured on a field emission scanning electron microscope (FESEM, JSM6700F). The instrument equipped with an energy-dispersive X-ray spectroscope (EDS, Oxford INCA). The powder X-ray diffraction experiments were run on a Rigaku MiniFlexII diffractometer with Cu Kα radiation in a 5−65° 2θ range, with a step size of 0.05°. Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were carried out on a NETZSCH STA 449F3 apparatus. Al2O3 crucibles were used as containers and references. All samples were heated under a N2 atmosphere in a 30−1000 °C range at a heating rate of 10 °C/min. Infrared spectra were obtained with a Fourier transform infrared spectrometer (Bruker Optics VERTEX 70) using KBr pellets in the range of 4000−400 cm−1. A PerkinElmer UV− vis−NIR spectrophotometer Lambda 950 was used to collect UV− vis−NIR diffuse reflectance spectra. The spectra were recorded in the 300−2500 nm region. Powder second harmonic generation measurements were performed on modified Kurtz and Perry equipment using a 2.05 μm Qswitched laser.43 SHG behavior depended intensely on the particle size, so polycrystalline samples of α-(H3O)2Pt(IO3)6 were ground, sieved, and divided into seven size ranges ( ne in the low-energy region, indicating that it is a negative uniaxial crystal. The birefringence at 2 μm (0.62 eV) is as large as 0.28, and it is very helpful to reach phase-matching in the SHG process for the compound. Furthermore, to explore the SHG response origin of α(H3O)2Pt(IO3)6 from the perspective of the electronic structure, we performed the spectral decomposition of d22 in the static limit (see the bottom-most panel of Figure 7). Clearly, it is the region near the Fermi level (−2.5 to 0 eV in VB and
