Photoconversion Mechanisms and the Origin of Second-Harmonic

Jun 1, 2017 - Photoconversion Mechanisms and the Origin of Second-Harmonic Generation in Metal Iodates with Wide Transparency, NaLn(IO3)4 (Ln = La, Ce...
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Photoconversion Mechanisms and the Origin of Second-Harmonic Generation in Metal Iodates with Wide Transparency, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) and NaLa(IO3)4:Ln3+ (Ln = Sm and Eu) Seung-Jin Oh,† Hyung-Gu Kim,† Hongil Jo,† Tae Gil Lim,‡ Jae Soo Yoo,‡ and Kang Min Ok*,† †

Department of Chemistry and ‡School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea S Supporting Information *

ABSTRACT: Four new metal iodates, namely, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu), and a series of NaLa(IO3)4:Ln3+ (Ln = Sm and Eu) solid solutions were synthesized through hydrothermal reactions. The structures of the title compounds are similar to that of NaY(IO3)4 crystallizing in the acentric monoclinic space group Cc. The iodate materials reveal layered structures composed of LnO8 square antiprisms and IO3 polyhedra, in which each layer is connected by the I···O interactions. NaLa(IO3)4 suggests a great potential as a matrix for optical source attributed to its acentricity and broad transparency from visible to mid-IR region. The photoluminescence properties depending on the concentration of Sm3+ reveal that NaLa(IO3)4:Sm3+ undergoes a self-quenching relaxation over 7 mol % of Sm3+ by dipole−quadrupole interactions. Attributable to the asymmetric coordination environment of Ln3+, stronger electric dipole transitions compared to magnetic dipole transitions were observed for both compounds. In addition, the materials exhibit strong second-harmonic generation (SHG) responses and are type I phase-matchable. The structural origin of the SHG properties for the reported iodates is elucidated.

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INTRODUCTION Metal iodates revealing a wide transparency in the range of visible and infrared can be applicable to excellent sources for optical materials with transparent windows and self-emitting materials, such as second-harmonic generation (SHG) and photoluminescence (PL).1−3 A number of the metal iodates exhibiting diverse coordination interactions between metal cations and iodates tend to crystallize in polar space groups. Especially, the stereochemically active lone pair (SCALP) electrons on iodates act as structure-directing building blocks as well as fundamental polyhedra required for large SHG response.4−13 For example, metal iodates crystallizing in noncentrosymmetric (NCS) structures such as α-LiIO3 and NaY(IO3)4 reveal strong SHG responses with high optical damage thresholds.14 Among many, the PL properties for lanthanide metal cations have been of great interest as light sources for the light-emitting diodes (LED).15,16 Since the emission spectra are closely related to the transparency region of the host matrices of metal iodates, PL properties of Nd3+and Yb3+-doped Y(IO3)3 and AgGa(IO3)4 have been recently reported.17 However, discovering the detailed PL properties and understanding the energy transfer mechanisms in acentric metal iodates system are ongoing challenges. Herein, we present the syntheses, structures, and characterization of four NCS sodium lanthanide iodates, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu), and Sm3+- and Eu3+-doped solid solutions with various concentrations, that is, NaLa1−xLnx(IO3)4 (Ln = Sm and Eu; x = 0−1). Detailed structural analyses for all of the reported iodate materials were performed to understand the © 2017 American Chemical Society

influence of the various lanthanide cations on the origin of SHG behaviors and PL mechanisms.

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EXPERIMENTAL SECTION

Synthesis. NaIO3 (Wako, 99.5%), La(NO3)3·6H2O (Alfa Aesar, 99.9%), Ce(NO3)3·6H2O (Alfa Aesar, 99.5%), Sm(NO3)3·6H2O (Alfa Aesar, 99.9%), Eu(NO 3 ) 3 ·6H 2 O (Alfa Aesar, 99.9%), HNO 3 (SAMCHUN, 60.0%), and deionized water were used as reagents. Single crystals of NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) were synthesized under hydrothermal conditions. An excess amount of NaIO3 (10.00 mmol, 1.979 g) was mixed with Ln(NO3)·6H2O (1.00 mmol; 0.411 g for La, 0.434 g for Ce, 0.445 g for Sm, and 0.446 g for Eu) and 10 mL of deionized water in 23 mL Teflon-lined autoclaves. To increase the crystallinity, 1.0 mL of 60.0% HNO3 was added to each autoclave as a mineralizer. After they were tightly sealed, the autoclaves were heated to 200 °C for 72 h and gradually cooled to room temperature at a rate of 6 °C h−1. The products were filtered, washed with deionized water, and dried in air for 1 d. Tiny colorless platelets, yellow needles, colorless needles, and pale yellow needles for NaLa(IO3)4, NaCe(IO3)4, NaSm(IO3)4, and NaEu(IO3)4 were obtained in 94%, 91%, 92%, and 90% yields, respectively, based on the corresponding lanthanide nitrates. Sm3+- and Eu3+-doped solid solutions were also obtained through the similar hydrothermal reactions. Solutions (10 mL, 0.10 M) containing stoichiometric mixtures of La(NO3)3 and Ln(NO3)3 (Ln = Sm and Eu) and 10.00 mmol of NaIO3 (1.979 g) were introduced into the 23 mL autoclaves with Teflon liners. The sealed autoclaves were heated to 200 °C for 72 h and slowly cooled to room temperature at a rate of 6 °C h−1. White Received: February 28, 2017 Published: June 1, 2017 6973

DOI: 10.1021/acs.inorgchem.7b00531 Inorg. Chem. 2017, 56, 6973−6981

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Inorganic Chemistry Table 1. Crystallographic Data for NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) formula FW (g mol−1) space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρcalc (g cm−3) λ (Å) T (K) R(F)a Rw(F02)b a

NaLa(IO3)4 861.51 Cc (No. 9) 31.656(6) 5.6710(11) 12.906(3) 90.54(3) 2316.8(8) 8 4.940 0.610 00 100 0.0300 0.0778

NaCe(IO3)4 862.72 Cc (No. 9) 31.7033(10) 5.6788(2) 12.8794(5) 90.728(3) 2318.58(14) 8 4.943 0.710 73 298 0.0295 0.0519

NaSm(IO3)4 872.96 Cc (No. 9) 31.4748(12) 5.6225(2) 12.7071(4) 90.927(4) 2248.44(14) 8 5.158 0.710 73 298 0.0418 0.0757

NaEu(IO3)4 874.56 Cc (No. 9) 31.376(3) 5.6082(4) 12.6829(10) 90.981(6) 2231.4(3) 8 5.206 0.710 73 298 0.0470 0.1226

R(F) = ∑∥F0| − |Fc∥/∑|F0|. bRw(F02) = {∑w[(F0)2 − (Fc)2]2/∑w[(F0)2]2}1/2. in platinum crucibles to 900 °C at a rate of 10 °C min−1 under flowing argon. Scanning Electron Microscopy/Energy Dispersive Analysis by X-ray (SEM/EDX). The atomic ratios of the reported materials were determined by utilizing a Hitachi S-3400N and a Horiba Energy EX-250. The atomic ratios were calculated based on the contents of the corresponding lanthanide cations (see the Table S3). Second-Harmonic Generation (SHG) Measurements. The SHG responses were measured by using a modified Kurtz-NLO system with a Q-switched Nd:YAG solid-state laser (1064 nm radiation).26 Since the SHG responses are dependent on the particle size, the reported materials were ground and sieved into the following size ranges: 0−20, 20−45, 45−63, 63−75, 75−90, 90−125, 125−150, 150−200, and 200−250 μm. Polycrystalline α-SiO2 with the 45−63 μm particle size range was used as a reference material to compare the SHG efficiency. Photoluminescence (PL) Measurements. The PL excitation and emission properties were investigated by a photomultiplier tube with a xenon lamp (PSI, Korea) at room temperature. All the spectra were recorded by varying the concentration of Ln3+. The excitation spectra were measured in the range of 200−500 nm to obtain charge transfer band (CTB), and specific absorption peaks of Sm3+ and Eu3+ cations were observed at ca. 598 and 644 nm for NaLa(IO3)4:Sm3+ and at ca. 613 nm for NaLa(IO3)4:Eu3+. The emission spectra under 402 and 393 nm excitations were measured in the ranges of 540−680 and 540−740 nm for NaLa(IO3)4:Sm3+ and NaLa(IO3)4:Eu3+, respectively. The photoluminescence lifetimes for NaLa0.93Sm0.07(IO3)4 and NaEu(IO3)4 were measured by using an in-house photoluminescence system that includes an Nd:YAG laser with a 355 nm excitation wavelength, a 400 nm cutoff filter, and a CCD sensor. The decay curves are recorded for 598 nm for NaLa0.93Sm0.07(IO3)4 and 613 nm for NaEu(IO3)4 emissions.

polycrystalline samples of NaLa1−xSmx(IO3)4 (x = 0, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 1) and pale yellow polycrystalline samples of NaLa1−xEux(IO3)4 (x = 0, 0.05, 0.10, and 1) were obtained in phase pure forms. All the synthesized NCS materials were deposited to Noncentrosymmetric Materials Bank (http://ncsmb.knrrc.or.kr). Single Crystal X-ray Diffraction (SCXRD). SCXRD data for NaLa(IO3)4 were obtained by using synchrotron radiation (λ = 0.610 00 Å) with an ADSC Quantum 210 CCD diffractometer on 2D Supramolecular Crystallography (2D-SMC) Beamline, Pohang Accelerator Laboratory (PAL; Pohang, Republic of Korea) at 100 K.18 A tiny platelet of NaLa(IO3)4 (0.160 × 0.360 × 0.260 mm3) was loaded for data collection with an omega scan width of Δω = 1° and an exposure time of 1 s for each frame. HKL 3000sm19 was used for the unit cell refinements, data reductions, and absorption corrections. The SCXRD data for NaLn(IO3)4 (Ln = Ce, Sm, and Eu) were obtained on a Bruker SMART BREEZE diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.710 69 Å) at room temperature on a 1 K CCD area detector. Crystals of NaCe(IO3)4 (0.511 × 0.101 × 0.012 mm3), NaSm(IO3)4 (0.530 × 0.141 × 0.030 mm3), and NaEu(IO3)4 (0.740 × 0.170 × 0.024 mm3) were loaded for data collection with a narrow-frame method (an exposure time of 10 s frame−1; scan widths of 0.30° in ω). All the data were integrated using the SAINT program.20 The data correction was conducted for polarization, absorption, and Lorentz factor. Absorption corrections on hemisphere of data were performed using the program SADABS.21 All of the crystal structures were determined and refined using SHELXS97 and SHELXL-97, respectively,22,23 in the software package, WinGX.22−24 Crystallographic information for the reported materials are reported in Table 1 and Table S1. Powder X-ray Diffraction (PXRD). PXRD measurements for all of the reported materials were taken by a Bruker New D8-Advance at room temperature with 40 kV and 40 mA using Cu Kα radiation (λ = 1.5418 Å). The diffraction patterns were collected for the 2θ angle in the range of 5−110° with a scan step size of 0.02° and step times of 0.1−2.0 s. The structures of solid solutions were refined using the single crystal data of NaLa(IO3)4 as a starting model. The structure refinements were conducted via GSAS program using the Rietveld method.25 Detailed refinement data and Rietveld plots are found in the Table S2 and Figure S1, respectively. Infrared Spectroscopy (IR). IR spectra were recorded using a Thermo Scientific Nicolet iS10 ATR-FTIR spectrometer in the range from 400 to 4000 cm−1 at room temperature. UV−vis Diffuse Reflectance Spectroscopy. UV−vis diffuse reflectance spectra were measured on a Varian Cary 500 scan UV− vis−near-IR spectrometer. The data were collected in the range of 200−2500 nm at room temperature. Thermogravimetric Analysis (TGA). TGA was performed using a SCINCO TGA-N 1000 thermal analyzer. The samples were heated

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RESULTS AND DISCUSSION Structures. The analysis of single-crystal structure for NaLn(IO3)4 indicated that the title compounds are crystallizing in the NCS monoclinic space group Cc (No. 9). Since the reported materials exhibit similar structures to those of the previously reported iodates,14,17 only a very brief structural description of NaLa(IO3)4 is presented here. The asymmetric unit of the structure is composed of two La atoms, two Na atoms, and eight I atoms. All of the I5+ cations are coordinated by three O atoms and forming IO3 polyhedra (see Figure S2). All the iodate groups exhibit unsymmetrical environment owing to the SCALP. The I−O bond lengths and O−I−O bond angles are in the range of 1.786(9)−1.842(8) Å and 91.2(3)− 172.7(3)°, respectively. Notice that there are considerably long I···O interactions with the contact lengths in the range from 6974

DOI: 10.1021/acs.inorgchem.7b00531 Inorg. Chem. 2017, 56, 6973−6981

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Inorganic Chemistry 2.449(8) to 3.075(8) Å. The long I···O contacts toward the lone-pair directions result in an octahedral moiety. Each La3+ cation is coordinated by eight oxide ligands in IO3 groups and composing LaO8 polyhedra. The LaO8 polyhedra show a distorted square antiprismatic structure. The overall structure can be described as double sheets that are parallel to the (100) plane with weak I···O interactions between the different iodates groups in the different sheets (see Figure 1a). Each layer has

Figure 2. Variation of the unit cell volume as a function of the samarium content for NaLa1−xSmx(IO3)4.

from the small amount of Sm(IO3)3 impurity. The R2 in the linear fit is calculated to be 0.997 (see Figure 2). The PXRD patterns for NaLa(IO3)4:Sm3+ and NaLa(IO3)4:Eu3+ are shown in Figure S5. The data also indicate the change of the lattice parameters as a result of the decrease of d with the increase of θ in the Bragg equation, that is, λ = 2d sin θ, where d is the interplanar distance, and λ is the wavelength. Thermal Stability. The title materials, NaLa(IO3)4, NaCe(IO3)4, NaSm(IO3)4, and NaEu(IO3)4 are thermally stable up to 574, 513, 578, and 562 °C, respectively (see Figure S6). Above these temperatures, the materials thermally decompose by losing iodate groups in the form of I2 and O2. The decomposed products over 900 °C turned out to be the corresponding lanthanide oxides with some amorphous phases, which were confirmed by PXRD (see Figure S7). IR Spectroscopy. The title compounds do not reveal any absorption up to 10.5 μm and are transparent in the mid-IR region (see Figure 3). The characteristic bands of NaLn(IO3)4 are well-matched with the previous data.6,14,17 The absorption bands occurring in the range of 950−600 cm−1 confirm the existence of IO3 groups. The bands appearing under 500 cm−1 are attributed to the Ln−O vibrations. A closer examination on

Figure 1. Ball-and-stick representations of (a) NaLa(IO3)4 in the acplane and (b) a single-layer in the bc-plane with 6-MRs. (c) Each double layer contains 8-MRs.

six-membered rings (6-MRs) in a sequence of repeated LaO8 and IO3 polyhedra, and the layers are connected by I(8)O3 groups (see Figure 1b). Na+ cations reside in the 8-MR channels to make a charge balance (see Figure 1a,c). Bond valence sum (BVS) calculations reveal the values matching well to the oxidation states of the constituent atoms (see Table S4). The PXRD patterns for all the reported materials are compared with the calculated patterns obtained from the SCXRD data (see Figure S3). As the site of Ln3+ in the structure of NaLn(IO3)4 is replaced from La3+ to Ce3+, Sm3+, and Eu3+, the unit cell parameters change accordingly: while the lengths of the unit cell parameters decrease, the angle β increases, which is attributable to the change of the ionic radii of lanthanide cations (see Table 1). The ionic radii of sixcoordinate La3+, Ce3+, Sm3+, and Eu3+ are 1.032, 1.01, 0.958, and 0.947 Å, respectively.27 The Rietveld refinements clearly show the diminishing trend of the unit cell sizes for Sm3+doped materials and satisfy the Vegard’s law (see Figure 2 and Figure S4).28 Slight deviations observed in the lower Sm contents (0.05 ≤ x ≤ 0.13) may come from the refinements with the short-range of 2θ for convenience. The deviation for high concentration of Sm, especially for x = 0.9, may be derived

Figure 3. IR spectra for NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu). 6975

DOI: 10.1021/acs.inorgchem.7b00531 Inorg. Chem. 2017, 56, 6973−6981

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Therefore, NaSm(IO3)4 is colorless. For NaEu(IO3)4, the absorption tail is elongated up to ∼450 nm, which results in pale yellowish color. The absorption peaks observed in the spectra of NaSm(IO3)4 and NaEu(IO3)4 are due to the transitions of Sm3+ and Eu3+ from 6H5/2 and 7F0, respectively. All the absorption peaks are assigned in Figure 4. Photoluminescence (PL) Properties. The excitation and emission spectra of Sm3+- and Eu3+-doped NaLa(IO3)4 were monitored depending on the transition wavelengths. The peaks for two iodates, namely, NaLa0.93Sm0.07(IO3)4 and NaEu(IO3)4, revealing the strongest intensities, were analyzed (see Figure 5). While the excitation spectrum for NaLa0.93Sm0.07(IO3)4 reveals a broad charge transfer band (CTB) at ∼200−260 nm for 598 nm emission, the CTB was not observed for 644 nm emission (see Figure S9). Detailed descriptions regarding the existence of CTB are provided later in the energy transfer mechanism section. The other excitations arising from the f−f transitions of Sm3+ cations are observed at 342, 363, 372, 402, 440, and 468 nm (see Figure 5a), which can be assigned to the transitions from 6 H5/2 to different energy states (see Table 2).33,34 Figure 5b shows an emission spectrum of NaLa(IO3)4:Sm3+ under 402 nm excitation. The emissions occurring at 562, 598, and 644 nm are assigned to 4G5/2→6H5/2, 7/2, 9/2, respectively.34,35 Among these transitions, 4G5/2→6H7/2 transition is dominant, and 4G5/2→6H9/2 and 4G5/2→6H5/2 transitions are following in order. All the observed peaks of NaLa1−xSmx(IO3)4 (x = 0−1) are assigned and summarized in Table 2. Figure 5c shows excitation spectra for Eu3+-doped NaLa(IO3)4 for 613 nm emission. The peaks observed at 363, 380, 394, and 464 nm can be assigned as excitations from the ground energy state 7F0, to upper energy states, 5D4, (5G2, 5G4), 5 L6, and 5D2, respectively.34 The CTBs for the transitions from O2− to Eu3+ are observed in the region ca. 292 nm, which is consistent with those previous reports.36,37 The emissions occurring at ∼590, 613, and 701 nm are due to the electronic transitions from 5D0 to 7F1, 2, 4, respectively (see Figure 5d).34 The emission of 5D0→7F2 is dominant, which is followed by 5 D0→7F4 and 5D0→7F1. Detailed information on the electronic transitions for NaLa1−xEux(IO3)4 (x = 0−1) is summarized in Table 3. According to the parity selection rule, since the f−f electric dipole (ED) transitions are strongly forbidden, the transition dominantly occurs for magnetic dipole (MD) transitions (ΔJ = 0, ±1). However, if Ln3+ cations are in asymmetric environment, the selection rule is relaxed, and the ED transitions are possible (ΔJ = 0, ±2). The signal is quite sensitive to the coordination environment of Ln3+ cations.38,39 For NaLa(IO 3 ) 4 :Sm 3+ , the ED transitions may be assigned as 4 G5/2→7H9/2, whereas the MD transitions are assigned as 4 G5/2→7H5/2, 7/2. For NaLa(IO3)4:Eu3+, while the transitions 5 D0→7F1 are due to the MD transitions, those of 5D0→7F2,4 are due to the ED transitions. Both compounds reveal that the ED transitions (4G5/2→7H9/2 for Sm3+; 5D0→7F2 for Eu3+) are stronger than those of the MD transitions (4G5/2→7H5/2 for Sm3+, 5D0→7F2 for Eu3+), which suggests the acentric nature of Ln3+ within the host lattice, NaLa(IO3)4.40,41 Optical Electronegativity of Sm3+. As shown in Figure 5c and Figure S9, because CTBs were observed, the optical electronegativity for ions can be calculated by using the following equation suggested by Reisfeld and Jørgensen:42

the IR spectra suggests that, while the bands for Ln−O bonds reveal the blue shifts, those for I−O bonds exhibit the red shifts (see the inset of Figure 3). The increase of I−O bond distances may come from the decrease of Ln−O bond lengths. As the Ln cations are changed from La to Ce, Sm, and Eu in NaLn(IO3)4, the ionic radii of Ln cations continuously decrease, which results in stronger Ln−O and weaker I−O bonds. The result is consistent with the crystallographic data. The average Ce−O, Sm−O, and Eu−O bond lengths at room temperature are 2.482, 2.430, and 2.420 Å, respectively, whereas the average I− O bond distances for corresponding iodates are 1.811, 1.812, and 1.814 Å, respectively. The effect of increase of atomic weights for Ln atoms is negligible compared with the bond lengths and the strengths. UV−Vis Diffuse Reflectance Spectroscopy. The UV−vis diffuse reflectance spectra for the title compounds are shown in Figure S8. The band gap energy is determined by using the following Kubelka−Munk function:29,30 F (R ) =

(1 − R )2 K = 2R S

where R, K, and S represent the reflectance, absorption coefficient, and scattering coefficient, respectively. By plotting F(R) versus hν, the band gaps, Eg, for NaLa(IO3)4, NaCe(IO3)4, NaSm(IO3)4, and NaEu(IO3)4 were calculated to be 4.16, 2.97, 4.01, and 4.16 eV, respectively. While no recognizable absorption peak was observed for NaLa(IO3)4, NaLn(IO3)4 (Ln = Ce, Sm, and Eu) exhibit substantial absorption peaks due to the transitions on the corresponding lanthanide cations (see Figure 4). For NaLa(IO3)4, the

Figure 4. Absorbance spectra for NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) measured by the UV−vis diffuse-reflectance spectrometer. (inset) Magnified absorption peaks and their assignment in the range of 300− 600 nm for NaSm(IO3)4 and NaEu(IO3)4.

transparency in the visible region accounts for its colorless characteristics. The transparency of NaLa(IO3)4 in the range from visible to mid-IR suggests the material as a potential host for a light source. Larger absorption for NaCe(IO3)4 shown at ∼338 nm (29 589 cm−1) results in the bang gap energy of 2.97 eV, which is originating from the transition of Ce from 4f 1Γ2 to 5d 8Γ2.31,32 The allowed transition results in the large absorption; thus, NaCe(IO3)4 reveals yellow color. Although NaSm(IO3)4 also reveals absorbance in the region of 400−500 nm, the magnitude of absorption is not large enough. 6976

DOI: 10.1021/acs.inorgchem.7b00531 Inorg. Chem. 2017, 56, 6973−6981

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Figure 5. PL spectra of NaLa(IO3)4:Sm3+ and NaLa(IO3)4:Eu3+ solid solutions. (a) The excitation spectra of NaLa(IO3)4:Sm3+ for 644 nm emission, (b) the emission spectra of NaLa(IO3)4:Sm3+ under 402 nm excitation, (c) the excitation spectra of NaLa(IO3)4:Eu3+ for 613 nm emission, and (d) the emission spectra of NaLa(IO3)4:Eu3+ under 393 nm excitation.

Table 2. Electronic Transitions of Sm3+-Doped NaLa(IO3)4 absorption from 6H5/2 for 644 nm emission 4

H9/2, 4D7/2, 4H11/2 D3/2, (4D, 6P)5/2, 4H7/2 4 D1/2, 5P7/2, 4L17/2, 4K13/2, 4F9/2 (6P, 4P)5/2, 4L13/2, 4F7/2, 6P3/2, 4K11/2, 4L15/2, 4 G11/2 4 F5/2, 4 M17/2, 4G9/2, 4I15/2 4 G7/2, 4I9/2, 4 M15/2, 4I11/2, 4I13/2 4

wavelength (nm)

energy (cm−1)

342 363 372 402

29 240 27 548 26 882 24 876

440 468

22 727 21 368

5

D4 G2, 5G4 5 L6 5 D2 5

wavelength (nm)

energy (cm−1)

363 380 393 464

27 548 26 316 25 381 21 552

emission from 5D0 under 393 nm excitation

wavelength (nm)

energy (cm−1)

590 613 701

16 949 16 313 14 265

7

F1 F2 7 F4 7

6

H5/2 H7/2 6 H9/2 6

wavelength (nm)

energy (cm−1)

568 598 644

17 606 16 722 15 528

= 1.74.43 By doing so, χopt(O) is calculated to be 2.88. From the calculated value for oxygen and the CTB for NaLa(IO3)4:Sm3+ (222 nm), the OE for Sm3+ can be also calculated to be 1.38. Critical Quenching Concentration and Distance in NaLa1−xSmx(IO3)4. Figure S10 reveals the dependence of the Sm3+ concentration on PL intensities of NaLa1−xSmx(IO3)4. Both excitation and emission spectra reveal that the maximum intensity reaches at x = 0.07 attributed to the quenching effect. As the doping concentration increases, the distance between Sm3+ decreases, and a cross-relaxation occurs between the Sm3+ cations.35,44 As a result, the energy tends to transfer from one state to another instead of to release the radiating light individually when the Sm3+ reaches closer than a critical distance. The critical distance Rc can be estimated through the following equation:

Table 3. Electronic Transitions of Eu3+-Doped NaLa(IO3)4 absorption from 7F0 for 613 nm emission

emission from 4G5/2 under 402 nm excitation

ECTB = [χopt (X) − χopt (M)] × 30 000cm−1

where χopt(M) and χopt(X) represent the optical electronegativity (OE) for the metal center and the coordinated anion, respectively. Since the OE for O2− is varied by every host material, the OE of O2− is calculated using the CTB of NaLa(IO3)4:Eu3+ and the previously reported value of χopt(Eu) 6977

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Figure 6. Energy diagrams and proposed energy transfer mechanisms for (a) NaLa(IO3)4:Sm3+ and (b) NaLa(IO3)4:Eu3+. While the solid line arrows indicate electronic transitions, the curved arrows in dashed line indicate nonradiative relaxations. For NaLa(IO3)4, four types of crossrelaxations are presumed to exist.

I K = x 1 + β(x)Q /3

⎛ 3V ⎞1/3 R c ≈ 2⎜ ⎟ ⎝ 4πxcN ⎠

where I is the intensity of emission, x is the concentration of Sm3+ cations greater than critical concentration (x = 0.07), and K and β are constants for the given host under the same excitation condition. The Q is a constant for multipolar interactions and can have values of 6, 8, and 10, which are corresponding to d−d, d−q, and q−q interactions, respectively.

where V, xc, and N are the volume of the unit cell (Å3), the Sm3+ concentration (mol %), and the number of cation sites per unit cell, respectively.45 For NaLa0.93Sm0.07(IO3)4 system, V = 2336.74 Å3, xc = 0.07 mol %, and N = 8; therefore, the critical distance is ∼19.7 Å. PL for NaLa1−xEux(IO3)4. The quenching effect played a critical role in Sm3+-doped NaLa(IO3)4. However, the more Eu3+ cations exist in the host lattice, the stronger PL properties are observed in Eu3+-doped NaLa(IO3)4. The difference in PL properties between Sm3+- and Eu3+-doped compounds is perhaps originating from the gap of energy states for each lanthanide cations. The energy levels of Sm3+ are closer and more diverse than those of Eu3+, which drives cross-relaxations between the energy levels of Sm3+ cations (see Figure 6). Therefore, the excitation and emission intensities are strongest when all of the La3+ are substituted by the Eu3+ without any shift of peak positions. Energy Transfer Mechanism for Sm3+- and Eu3+Doped NaLa(IO3)4. The energy-level diagrams for Sm3+- and Eu3+-doped NaLa(IO3)4 and the proposed energy transfer mechanisms are provided in Figure 6. Under the UV irradiation, while no CTB for NaLa0.93Sm0.07(IO3)4 was observed in ED transitions, weak CTBs were monitored for MD transitions (see Figure 6). The result indicates that the direct excitation of Sm3+ influences only the emission of ED transition, which is forbidden, and the interactions between Sm3+ cations and the host are very weak. According to the quenching effect, NaLa(IO3)4:Sm3+ undergoes a nonradiative relaxation, and the exchange interaction process cannot occur attributed to the large critical distance. Hence, multipole− multipole interactions should be considered. According to the Dexter’s theory, there are three types of multipolar interactions, that is, dipole−dipole (d−d), dipole−quadrupole (d−q), and quadrupole−quadrupole (q−q) interactions.46,47 The multipolar interactions can be decided by the following equation:

( ) versus log(x

The plot of log

I x Sm3 +

3+

Sm

) shows a straight line

over the critical concentration with the slope of −2.56 and the calculated R2 = 0.996 in the linear fit (see Figure 7).

Figure 7. Logarithmic plot of log(I/x) as a function of log(x) for the emission of NaLa1−xSmx(IO3)4.

Consequently, the Q value is calculated to be 7.68, which is an indicative value for the d−q interactions. Thus, the energy transfer process occurs via d−q interactions between Sm3+ cations, which brings the fluorescent self-quenching. Also, four possible cross relaxations may be assigned as 4G5/2 + 6978

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Inorganic Chemistry H5/2→6F11/2 + 6F5/2, 4G5/2 + 6H5/2→6F5/2 + 6F11/2, 4G5/2 + H5/2→6F9/2 + 6F7/2, and 4G5/2 + 6H5/2→6F7/2 + 6F9/2.35,44 Therefore, the excited energy states generated by CTs from O2− to Sm3+ and Eu3+ through the absorptions under 402 and 363 nm irradiations are relaxed by nonradiative processes to 4 G5/2 and 5D0 levels, respectively. After that, emissions occur by 4 G5/2→6H5/2, 7/2, 9/2 and 5D0→7F1, 2, 4 transitions. However, the processes of CTB for NaLa(IO3)4:Sm3+ make a weak contribution on the light-emitting MD transitions and even do not contribute to ED transitions at all. In addition, attributable to the various and wider energy state gaps for Sm3+ compared to those for Eu3+, the quenching effect occurs through nonradiative relaxations in Sm3+-doped NaLa(IO3)4. Therefore, the increased concentration of Sm3+ brings up selfcross-relaxations by d−q interactions. PL Lifetimes. The decay curves of PL properties were also investigated under 355 nm radiation on those compounds that reveal strong emission bands, namely, NaLa0.93Sm0.07(IO3)4 and NaEu(IO3)4. NaLa0.93Sm0.07(IO3)4 exhibits a strong PL emission at 598 nm, which was measured in a time sequence of 3 μs. Since the relaxations of NaLa0.93Sm0.07(IO3)4 occur through multipolar d−q interactions, the decay curve is not well-fitted with a first-order exponential equation.48 Thus, it is treated as a nonexponential function suggested by Inokuti and Hirayama (I−H) model:44,49 6

(IO3)4, NaCe(IO3)4, NaSm(IO3)4, and NaEu(IO3)4 exhibit SHG efficiencies of 120, 50, 60, and 130 times that of α-SiO2 (see Figure 8). Also, all of the compounds are type I phase-

6

Figure 8. Phase-matching curves (type I) for NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu). The curves are drawn to guide the eye and are not fits to the data.

matchable. The origin of the strong SHG response can be understood by careful examinations of the structure and the net polarization of the materials. The compounds contain asymmetric polyhedra, LnO8 and IO3. Though LnO8 polyhedra exhibit distortions, the degree of the distortion for LnO8 is negligible compared to that of IO3 groups. The calculated dipole moments are summarized in Table S5. Some of the dipole moments cancel each other. For instance, the directions of the dipole moments for I(1)O3 and I(5)O3, I(3)O3 and I(7)O3, I(2)O3 and I(6)O3 polyhedra are opposite to each other (see Figure 9). Thus, the constructive addition of the

⎡ ⎛ t ⎞3/ s ⎤ t ⎢ I(t ) = I0 + A exp − − a⎜ ⎟ ⎥ ⎢⎣ τ0 ⎝ τ0 ⎠ ⎥⎦

where I(t) is the PL intensity at time t, I0, A, and a are the constants, τ0 is the lifetime, and s is the constant that is physically the same as Q. For fitting the curve, s is treated as 8 for d−q interactions. The decay curve is also precisely fitted with a second-order exponential equation due to the nonexponentiality:37 ⎛ −t ⎞ ⎛ −t ⎞ I(t ) = B + A1 exp⎜ ⎟ + A 2 exp⎜ ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

where B, A1, and A2 are the constants, and τ1 and τ2 are the lifetimes for each exponential component. The average lifetime can be derived using the following equation: τavg =

A1τ12 + A 2 τ2 2 A1τ1 + A 2 τ2

The lifetime of NaEu(IO3)4 was measured over 50 μs for 613 nm emission. The decay curve of NaEu(IO3)4 is accurately fitted by the following first-order exponential model: ⎛ −t ⎞ I(t ) = I0 + A exp⎜ ⎟ ⎝ τ0 ⎠

Figure 9. A net-polarization occurring by the sum of each dipole moment for iodate groups in NaLa(IO3)4.

With the above equations, the lifetimes of NaLa0.93Sm0.07(IO3)4 were determined as τ0 = 59.9 and τavg = 80.1 μs by an I−H model and a second-order exponential equation, respectively. Also, the lifetime of NaEu(IO3)4 was calculated to be τ0 = 0.716 ms, which was much longer than that of NaLa0.93Sm0.07(IO3)4. The difference of the decay times may be attributed to the quenching effect for Sm3+. The detailed values and fitted curves are found in Table S7 and Figure S11, respectively. SHG Properties. Since NaLn(IO3)4 crystallize in the NCS space group Cc, the SHG properties were examined. NaLa-

polarizations from I(4)O3 along the approximate [11−1] direction and from I(8)O3 along the approximate [0−1−2] direction is the main contribution on the net dipole moment. The net dipole moment of iodates in NaLn(IO3)4 calculated from the vector sum point to the approximate [10−4] direction, which is consistent with the previously reported result of NaY(IO3)4 (see Table S6).14 Slight difference in SHG efficiencies is observed from the title materials, although they do have the same structures. The difference may come from the 6979

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Inorganic Chemistry Notes

absorption properties of the lanthanide cations. As seen in Figure 4, La3+ and Eu3+ show no significant absorption characteristics at ∼532 and 1064 nm, whereas Sm3+ absorbs at ∼1064 nm region, which is related to 6 H 5/2 → 6 F 9/2 transitions. Ce3+ also absorbs some light due to f−d transitions. Since we measured the SHG signals under 1064 nm for 532 nm, the absorptions associated with these regions have the SHG efficiencies of NaCe(IO3)4 and NaSm(IO3)4 to be lowered compared to those of NaLa(IO3)4 and NaEu(IO3)4.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Chung-Ang University Graduate Research Scholarship in 2016 (for S.-J.O.). This research was also supported by the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT and Future Planning (Grant Nos. 2014M3A9B8023478 and 2016R1A2A2A05005298). We acknowledge Prof. K.-S. Sohn and Mr. M. Kim (Sejong Univ.) in obtaining the data for PL lifetimes.

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CONCLUSIONS Four new lanthanide iodates, NaLn(IO3)4 (Ln = La, Ce, Sm, and Eu) crystallizing in the NCS monoclinic space group Cc, have been successfully synthesized by hydrothermal reactions in high yields. By introducing various lanthanide cations to a prototypic acentric framework, a variety of interesting photonic properties have been investigated. NaLa(IO3)4 revealing a broad transparency in the range from visible to mid-IR may be a promising host material for light source. While NaEu(IO3)4 shows increased PL properties on high concentration of Eu3+, NaSm(IO3)4 exhibits a quenching effect where the PL properties are maximized at 7 mol % of Sm3+. The small gap between the energy states suggests a self-quenching mechanism on NaLa(IO3)4:Sm3+. Attributable to the large critical distance of 19.7 Å, the energy transfer of Sm3+-doped NaLa(IO3)4 occurs through the d−q multipolar interactions. Because of the acentric nature around Ln3+, it is observed that the ED transitions are stronger than those of MD transitions. In addition, the SHG measurements indicate that the materials show moderately strong SHG efficiencies of 50−130 times that of α-SiO2 and are type-I phase-matchable.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00531. Selected bond distances, crystallographic information, atomic ratios, bond-valence sum calculations, calculated dipole moments, net direction of dipole moments, fitted values of decay curves, Rietveld plots, illustrated asymmetric unit and polyhedra, PXRD patterns, variation of unit cell parameters, TGA diagrams, UV−vis diffusereflectance spectra, excitation spectra, PL intensites plotted as a function of Sm3+ concentration, decay curves (PDF) Accession Codes

CCDC 1533686−1533689 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-820-5197. Fax: +82-2-825-4736. E-mail: [email protected]. ORCID

Kang Min Ok: 0000-0002-7195-9089 6980

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