α-AgI3O8 and β-AgI3O8 with Large SHG Responses: Polymerization

Apr 25, 2014 - The main structural difference between the two isomers lies in the different .... Chuanling Sun , Yuandong Wu , Dajiang Mei , Thomas Do...
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α‑AgI3O8 and β‑AgI3O8 with Large SHG Responses: Polymerization of IO3 Groups into the I3O8 Polyiodate Anion Xiang Xu, Chun-Li Hu, Bing-Xuan Li, Bing-Ping Yang, 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: Two new noncentrosymmetric isomeric silver polyiodates, namely, α-AgI3O8 (Pnc2) and β-AgI3O8 (I4̅), have been synthesized through the hydrothermal reactions of AgNO3 with I2O5. Both isomers exhibit layered structures that are constructed from I3O8− anions interconnected by Ag+ cations. The main structural difference between the two isomers lies in the different stacking fashions of [AgI3O8] layers along the c axis in order to meet the requirements of their space groups. Powder second-harmonic generation (SHG) measurements indicate that αAgI3O8 and β-AgI3O8 are both phase-matchable materials with large SHG responses of approximately 9.0 and 8.0 times that of KH2PO4, respectively. UV−vis−NIR transmission spectra show that the cutoff absorption edges are 328 nm for α-AgI3O8 and 345 nm for β-AgI3O8. Thermal stability studies demonstrate that both isomers are thermally stable up to about 370 °C. Theoretical calculations based on DFT methods for the two AgI3O8 phases as well as the NaI3O8 analogue have been performed.

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INTRODUCTION

Among the numerous metal iodates(V) reported previously, only two types of coordination geometries for the I5+ cation are known (IO3− and IO43−) if the very weak I−O interactions (>2.4 Å) are neglected. Normally the “isolated” IO3− anions are formed whereas the “isolated” IO43− anions have been found in only Ag4(UO2)4(IO3)3(IO4)2O2, Ba[(MoO2)6(IO4)2O4]·H2O, and Bi2(IO4)(IO3)3 so far.14,15 It is noticed that the tellurium(IV) oxide polyhedra such as TeO3, TeO4, and TeO5 groups are able to be condensed into various types of polynuclear anions with isolated cluster units (e.g., zerodimensional Te3O84−, Te4O116−, Te5O136−) or extended structures (e.g., one-dimensional Te4O104−, two-dimensional Te4O92−).16 This raises our curiosity about the possible polymerization of the iodate groups. So far only two types of polynuclear iodine(V) oxide groups, namely, neutral dimeric I2O5 unit and trinuclear I3O8− anions, are known.3a,b,17−19 The neutral dimeric I2O5 units were observed in HIO3(I2O5) and Rb3(IO3)3(I2O5)(HIO3)4(H2O).17 The I3O8− anions were mentioned in KI3O8 and RbI3O8 in 1941, but no structures were proposed.19 Later, the I3O8− anions were confirmed to be present in NaI3O8,3b the two isomeric Cs2I4O11 phases and Rb2(I3O8)(IO3)(HIO3)2(H2O).3a,18 Among these compounds, the NCS NaI3O8 (P4̅) and polar α-Cs2I4O11 (P63) exhibit strong SHG responses which are comparable to α-LiIO3.3a,b NaI3O8 features isolated I3O8− units that are separated by sodium(I) ions whereas α-Cs2I4O11 exhibits a two-dimensional

Metal iodates have been attracting a lots of research attention due to their potential applications as new second-order nonlinear optical (NLO) materials.1−3 The lone pair electrons on the I5+ cation can induce the formation of asymmetric or polar IOx (x = 3, 4) building units, and noncentrosymmetric (NCS) or polar compounds with excellent second harmonic generation (SHG) properties are formed if the polarizations of these asymmetric or polar IOx (x = 3, 4) building units are properly aligned in their unit cells. Simple ternary metal iodates, such as α-LiIO3, have been widely studied as NLO crystal in the last century.4 During the past decade, the combination of transition metal ions with d0 electronic configurations (Ti4+, Nb5+, V5+, and Mo6+, etc.) or cations containing lone-pair electrons such as Pb2+ and Bi3+ with the iodate anions has been found to be a very effective synthetic route for the designing of new NCS metal iodates with better SHG properties due to the constructive addition of polarizations from both types of asymmetric building units,5−12 which led to the discoveries of a large number of new SHG materials including AMoO3(IO3) (A = Li, Rb, Cs),6a,10c A2Ti(IO3)6 (A = Li, Na),7a,b BaNbO(IO3)5,9a A(VO)2(IO3)3O2 (A = K, Rb, Cs, NH4),6b,9b NaVO2(IO3)2(H2O),9c BiO(IO3),11 and PbPt(IO3)6(H2O).12b Recently it has been reported that the introduction of transition metal ions with a square-planar MO4 geometry (M = Pd2+ or Au3+) into the iodates can also form NCS or polar structures with good SHG properties;13 for example, BaPd(IO3)413a and RbAu(IO3)413b exhibit SHG responses of about 0.4 × and 1.3 × KTiOPO4 (KTP), respectively. © 2014 American Chemical Society

Received: March 13, 2014 Revised: April 16, 2014 Published: April 25, 2014 3219

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Figure 1. As-grown small crystals of α-AgI3O8 (a) and β-AgI3O8 (b) and a 0.12 mm-thick plate-shaped crystal of α-AgI3O8 (c) and a 0.6 mm-thick polished crystal of β-AgI3O8 (d) that are used for transmittance spectrum measurements.

hexagonal tungsten oxide-like [I3O8−]∞ layer composed of corner-sharing IO4 groups with additional IO3 groups capping on the same side of the layer via weak I−O interaction (2.441(16) Å). It is expected that the condensation of IO3− or/ and IO43− may increase the densities of I5+ cations in the unit cell, which could have strong effects on the structures and SHG properties of the compounds formed. Hence, it is worthy to explore the reaction conditions needed for the condensation of mononuclear iodine(V) oxide groups and the effects of such condensations on the structures and SHG properties of materials formed. Therefore, we started a research program to explore the A(I)−I(V)−O ternary system (A = K, Rb, Cs, Ag, NH4+) systematically. Two new acentric silver iodates, namely, α-AgI3O8 and β-AgI3O8, were obtained successfully, and both contain trinuclear I3O8− anions and exhibt strong SHG responses. Herein, we reported their syntheses, crystal structures, and optical properties. Theoretical calculations based on DFT methods for the two AgI3O8 isomers as well as the NaI3O8 analogue have also been performed.

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Microprobe elemental analyses were performed on a field emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDS, Oxford INCA). Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were carried out with a NETZCH STA 449F3 unit at a heating rate of 10 °C/min under a N2 atmosphere. The UV−vis−NIR spectra were recorded at room temperature on a PerkinElmer Lambda 950 spectrophotometer with a wavelength range of 190−2500 nm. IR spectra were recorded on a PerkinElmer Spectrum One IR spectrophotometer with a wavelength range of 4000−400 cm−1. A 0.12 mm thick plate-shaped crystal of α-AgI3O8 (Figure 1c) and a 0.6 mm thick polished slab-shaped crystal of βAgI3O8 (Figure 1d) were used for the transmittance spectra measurement. The measurements of the powder frequency-doubling effects were carried out on the sieved samples by means of the modified method of Kurtz and Perry.20 The 1064 nm and a 2.05 μm radiations generated by a Q-switched Nd:YAG solid-state laser were used as the fundamental frequency light, respectively. Crystals of α-AgI3O8 and β-AgI3O8 were ground and sieved into several distinct particle size ranges (25−45, 45−53, 53−75, 75−105, 105−150, 150−210, and 210−300 μm), respectively, and then were pressed between glass slides and secured with tape in 1.2 mm thick aluminum holders containing a hole with diameter of 8 mm. Sieved KH2PO4 (KDP) and KTiOPO4 (KTP) samples with the same size ranges were used as references for the SHG measurements with 1064 nm and 2.05 μm laser radiations, respectively. The ratios of the SHG signals of the title compounds to those of the references were calculated based on the density of second harmonic outputs of α-AgI3O8, β-AgI3O8, and references in the same particle size range of 150−210 μm. Syntheses. Both α-AgI3O8 and β-AgI3O8 were synthesized by hydrothermal reactions of a mixture of AgNO3 and I2O5 in an autoclave equipped with a Teflon linear (25 mL). The loaded compositions are AgNO3 (8.5 mg, 0.05 mmol), I2O5 (2.671 g, 8 mmol) and H2O (1.6 mL) for α-AgI3O8, and AgNO3 (84.9 mg, 0.5 mmol), I2O5 (2.671 g, 8 mmol), and H2O (1.6 mL) for β-AgI3O8. The

EXPERIMENTAL SECTION

Reagents and Instrumentation. All of the chemicals were analytically pure from commercial sources and used without further purification. AgNO3 (≥99.8%) and I2O5 (≥99%) were purchased from Shanghai Shenbo Chemical Co. Ltd. and Sinopharm Chemical Reagent Co. Ltd., respectively. Powder X-ray diffraction (XRD) patterns of ground crystals were collected on a Rigaku MiniFlex II diffractometer using monochromated Cu Kα radiation (λ = 1.540598 Å) at room temperature with a step size of 0.02°. Infrared (IR) spectra were recorded on a Nicolet AVATAR 370 FTIR infrared spectrophotometer as KBr pellets in the range of 4000− 400 cm−1. 3220

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Figure 2. Experimental and simulated powder X-ray diffraction patterns of α-AgI3O8 (a) and β-AgI3O8 (b). The different diffraction peaks between α-AgI3O8 and β-AgI3O8 are marked with a green star. mixture was heated at 210 °C for 72 h, followed by slow cooling to ambient temperature at a rate of 2.0 °C/h. The final pH values of the reaction media are less than 1.0. The reaction products were washed with ethanol and then dried in air. Colorless plate-shaped crystals of αAgI3O8 (Figure 1a) and granular crystals of β-AgI3O8 (Figure 1b) were obtained as single phases in the yields of about 83% and 92% (based on AgNO3), respectively. Their purities were confirmed by powder Xray diffraction (XRD) studies (Figure 2). Furthermore, although the power XRD patterns for α-AgI3O8 and β-AgI3O8 are very similar due to their similar structures, there are also some obvious differences between the two powder XRD patterns: the characteristic diffraction peaks for α-AgI3O8 are located at 18.7, 25.8, 29.0, 32.5, and 33.7° whereas those for β-AgI3O8 are located 15.7, 24.8, and 36.2°. The energy-dispersive spectrometry (EDS) elemental analyses on several single crystals for each compound gave average Ag/I molar ratios of 1:2.86 and 1:3.17 for α-AgI3O8 and β-AgI3O8, respectively, which are in good agreement with those determined from single-crystal X-ray structure analyses. Efforts to synthesize the alkali-metal and ammonium analogues by using a similar technique were tried but were unsuccessful. The growth of the large crystals was performed using a spontaneous nucleation hydrothermal method within a large autoclave. A mixture of AgNO3 (152.9 mg, 0.90 mmol for α-AgI3O8, 849.4 mg, 5.0 mmol for β-AgI3O8), I2O5 (53.4 g, 160 mmol), and H2O (32 mL) was loaded in an autoclave equipped with a Teflon linear (500 mL). The autoclave was then quickly heated to 210 °C, held at the temperature for 72 h, and slowly cooled to ambient temperature at a rate of 1.0 °C/h. Large crystals of α-AgI3O8 (Figure 1c) and β-AgI3O8 (Figure 1d) with a size of several millimeters were obtained successfully. Single-Crystal Structure Determination. Single crystal X-ray diffraction data of both compounds were collected on an Agilent Technologies SuperNova Dual Wavelength CCD diffractometer with a Mo Kα radiation (λ = 0.71073 Å) at 293 K. The data reduction was performed with the program CrysAlisPro, and absorption corrections based on the multiscan method were applied.21 Both structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 using SHELX-97.22 All non-hydrogen atoms were refined with anisotropic thermal parameters. The refined Flack parameters of −0.09(4) for α-AgI3O8 and 0.03(3) for β-AgI3O8 are close to zero, indicative of correctness of their absolute structures.23 The structures were checked for possible missing symmetry elements with PLATON, 24 but none was found. Crystallographic data are summarized in Table 1. Selected bond distances are listed in Table 2. More details on the crystallographic studies are given in the Supporting Information. Computational Details. Single crystal structural data of α-AgI3O8 and β-AgI3O8 as well as NaI3O8 reported previously were used for the theoretical calculations with the total-energy code CASTEP.25 Band structures, density of states (DOS), and optical properties were

Table 1. Crystallographic Data for α-AgI3O8 and β-AgI3O8 α-AgI3O8 616.57 293(2) orthorhombic Pnc2 8.1443(3) 8.1171(3) 11.7005(5) 773.50(5) 4 5.295 14.588 0.0507 1.028 −0.09(4) 0.0206, 0.0455 0.0226, 0.0469

compound Fw T (K) crystal system space group a (Å) b (Å) c (Å) V (Å3) Z ρcalcd (g·cm−3) μ (mm−1) Rint GOF on F2 Flack factor R1, wR2 (I > 2σ(I))a R1, wR2 (all data)

β-AgI3O8 616.57 293(2) tetragonal I4̅ 8.1283(1) 8.1283(1) 23.4603(7) 1550.00(5) 8 5.284 14.560 0.0477 1.031 0.03(3) 0.0199, 0.0392 0.0206, 0.0398

R1 = ∑∥Fo| − |Fc ∥/∑|F o|; wR 2 = {∑w[(F o)2 − (Fc)2]2/ ∑w[(Fo)2]2}1/2.

a

calculated with density functional theory (DFT) using Perdew− Burke−Ernzerhof (PBE) generalized gradient approximation.26 The interactions between the ionic cores and the electrons were described by the norm-conserving pseudopotential.27 The following orbital electrons were treated as valence electrons: Na-2s22p63s1, Ag4s24p64d105s1, I-5s25p5, and O-2s22p4. The numbers of plane waves included in the basis sets were determined by cutoff energies of 765, 765, and 750 eV, respectively, for α-AgI3O8, β-AgI3O8, and NaI3O8, respectively. The numerical integration of the Brillouin zone was performed using Monkhorst−Pack k-point sampling of 3 × 3 × 2, 3 × 3 × 4, and 3 × 3 × 4 for α-AgI3O8, β-AgI3O8, and NaI3O8, respectively. The other parameters and convergent criteria were the default values of CASTEP code. The calculations of linear optical properties were made in terms of the complex dielectric function ε(ω) = ε1(ω) + iε2(ω). The imaginary part of the dielectric function ε2 is given in the following equation:28

ε2ij(ω) =

8π 2ℏ2e 2 m2V

∑ ∑ (fc k

− fv )

cv

pcvi (k)pvcj (k) Evc 2

δ[Ec (k) − Ev(k) − ℏω] The fc and f v represent the Fermi distribution functions of the conduction and valence bands, respectively. The term picv(k) denotes the momentum matrix element transition from the energy level c of 3221

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Table 2. Selected Bond Length (Å) and the Calculated Bond Valences in α-AgI3O8 and β-AgI3O8 α-AgI3O8

β-AgI3O8

d (Å)

s

×2 ×2 ×2 ×2

2.482(4) 2.494(5) 2.789(5) 2.837(5)

Ag(2)−O(5) × 2 Ag(2)−O(1) × 2 Ag(2)−O(2) × 2

2.410(5) 2.470(5) 2.830(5)

I(1)−O(1) I(1)−O(2) I(1)−O(3)

1.789(4) 1.790(4) 1.923(4)

I(2)−O(4) × 2 I(2)−O(3) × 2

1.801(4) 2.054(4)

I(3)−O(5) I(3)−O(6) I(3)−O(7)

1.787(5) 1.787(4) 1.927(5)

I(4)−O(8) × 2 I(4)−O(7) × 2

1.802(4) 2.045(5)

0.160 0.155 0.070 0.061 0.894 0.195 0.166 0.063 0.847 1.769 1.764 1.231 4.764 1.712 0.864 5.153 1.778 1.778 1.218 4.775 1.708 0.883 5.182

bond Ag(1)−O(3) Ag(1)−O(5) Ag(1)−O(6) Ag(1)−O(2) ∑s

∑s

∑s

∑s

∑s

∑s

d (Å)

s

2 2 2 2

2.480(4) 2.496(4) 2.799(5) 2.843(5)

Ag(2)−O(5) × 2 Ag(2)−O(1) × 2 Ag(2)−O(2) × 2

2.416(4) 2.452(4) 2.853(4)

I(1)−O(1) I(1)−O(2) I(1)−O(3)

1.791(4) 1.791(4) 1.925(4)

I(2)−O(4) × 2 I(2)−O(3) × 2

1.806(4) 2.063(4)

I(3)−O(5) I(3)−O(6) I(3)−O(7)

1.792(4) 1.793(4) 1.929(4)

I(4)−O(8) × 2 I(4)−O(7) × 2

1.806(4) 2.049(4)

0.161 0.154 0.068 0.060 0.889 0.192 0.174 0.059 0.851 1.759 1.759 1.225 4.743 1.689 0.843 5.066 1.754 1.745 1.212 4.711 1.689 0.876 5.131

bond Ag(1)−O(3) Ag(1)−O(5) Ag(1)−O(6) Ag(1)−O(2)

× × × ×

performed in order to understand the effects of Ag+ sources, the amount of the starting materials used, reaction temperatures on the compositions and structures of the final products formed (Supporting Information Table S1). On the basis of results from our experiments, the amounts of AgNO3 and I2O5 employed are important. Compared with AgIO 3 , the preparations for α-AgI3O8 and β-AgI3O8 require a greater I2O5/H2O molar ratio. A higher concentration of iodic acid and a lower pH value in the reaction system is required for the formation of I3O8− anions. Different from β-AgI3O8, α-AgI3O8 is more sensitive to the amount of AgNO3 employed and can only be isolated under a very low AgNO3/H2O molar ratio. Furthermore, if the reaction temperature is lowered to 180 °C or the Ag2O is used instead of AgNO3, AgIO3 will be isolated as the sole product. Our results indicate that the condensation of IO3− into the I3O8− anion occurred under very narrow reaction conditions. As mentioned in ref 3b by Phanon and GautierLuneau, the I3O8− in NaI3O8 is formed from the condensation of three anions IO3− in two steps: first polymerize iodic acids by dehydration into iodic anhydride and then condense iodic anhydride with an iodate anion to form a dative bond, leading to I3O8−. Hence, the formation of I3O8− anions is more complex than IO3− and requires particular synthetic conditions, which is consistent with our result that it is easier to obtain AgIO3 than α-AgI3O8 and β-AgI3O8. It is interesting to note that both NaI3O8 and AgI3O8 isomers were obtained in concentrated acid solution (concentrated nitric acid for Naphase and concentrated iodic acid for Ag-phase), which may indicate that acidic environment is favorable for the condensation of IO3− and formation of I3O8− anions. It is also found that the colorless crystals of α-AgI3O8 or βAgI3O8 in water can be easily transformed into polycrystalline samples of AgIO3 based on powder XRD study (Supporting Information Figure S1); hence, water cannot be used to wash crystals of α-AgI3O8 or β-AgI3O8. However, α-AgI3O8 or β-

the conduction band to the level v of the valence band at a certain k point in the Brillouin zones, and V is the volume of the unit cell. The m, e, and ℏ are the electron mass, charge, and Planck’s constant, respectively. The calculations of second-order NLO properties were based on length-gauge formalism within the independent-particle approximation.29 We adopted the Chen’s static formula, which was derived by Rashkeev et al.30 and later improved by Chen’s group.31 The static second-order NLO susceptibility can be expressed as

χ αβγ = χ αβγ (VE) + χ αβγ (VH) + χ αβγ (two bands) where χαβγ(VE) and χαβγ(VH) give the contributions to χαβγ from virtual-electron processes and virtual-hole processes, respectively; χαβγ(two bands) is the contribution to χαβγ from the two-band processes. The formulas given in ref 31 were used for calculations ofχ αβγ (VE), χ αβγ (VH), and χ αβγ (two bands). To ensure the convergence of SHG coefficients, 704, 352, and 156 empty bands were used for the calculations for α-AgI3O8, β-AgI3O8, and NaI3O8, respectively. In addition, DFT-GGA usually underestimates the conduction bands energies, so the conduction bands energy should be shifted upward by adding a scissor operator to match with the measured values of the band gaps.30,31

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RESULTS AND DISCUSSION Hydrothermal reactions of AgNO3 and I2O5 afford two new ternary silver polyiodates, namely, α-AgI3O8 and β-AgI3O8, which crystallize in acentric space group Pnc2 (also polar) and I4̅, respectively. Both isomers feature layered structures in which trinuclear linear I3O8− anions are interconnected by silver(I) ions. It is interesting to point out that both α-AgI3O8 and β-AgI3O8 exhibit large SHG responses which are phasematchable and wide optical windows; hence, they are promising candidates as new SHG materials. Syntheses. Via hydrothermal reactions of the same starting material of AgNO3 and I2O5, α-AgI3O8 and β-AgI3O8, as well as AgIO3 which was reported previously, can be synthesized as single phases. Systematic synthetic experiments had been 3222

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Figure 3. Packing of the 2D metal polyiodate layers along the c axis in α-AgI3O8 (a), β-AgI3O8 (b), and NaI3O8 (c). Figure 4. 2D silver(I) or sodium(I) polyiodate(V) layer paralell to the ab plane in α-AgI3O8 (a), β-AgI3O8 (b), and NaI3O8 (c).

AgI3O8 crystals remain stable in a very concentrated I2O5 aqueous solution. Crystal Structure. α-AgI3O8 crystallizes in the polar orthorhombic space group Pnc2 (No. 30) whereas β-AgI3O8 crystallizes in the acentric tetragonal I4̅ (No. 82). Both compounds feature layered structures in which trinuclear linear I3O8− anions are interconnected by silver(I) ions (Figure 3a,b). Such layered structures are similar to that in NaI3O8 which crystallizes in the acentric tetragonal P4̅ (No. 81) (Figure 3c). For both α-AgI3O8 and β-AgI3O8, the asymmetric unit contains two Ag, four I, and eight O atoms. I(1) and I(3)

occupying the general sites are in a trigonal IO3 pyramidal geometry, whereas I(2) and I(4) located at the twofold rotation axis are four-coordinated by O atoms in a distorted tetragonal pyramid geometry. The lone pair electrons of I5+ cations occupy the open sides of polyhedra. One I(2)O4 and two I(1)O3 form one trinuclear I3O8− via corner-sharing, and so did I(4)O4 and I(3)O3 groups. The I−O distances of the I−O−I bridge (1.923(4)−2.054(4) Å in α-AgI3O8 and 1.925(4)− 2.063(4) Å in β-AgI3O8) are significantly longer than the 3223

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Figure 5. UV−vis−IR transmittance spectra of α-AgI3O8 (a) and β-AgI3O8 (b).

Figure 6. Curves of measured SHG signals with particle sizes of α-AgI3O8 (●) and β-AgI3O8 (○) under laser radiation at 1064 nm (a) and 2.05 μm (b). Oscilloscope traces of the SHG signals for the samples (150−210 μm) of α-AgI3O8, β-AgI3O8, and NaI3O8 as well as KDP at 1064 nm or KTP at 2.05 μm are shown in the insets.

terminal I−O bonds (1.787(4)−1.802(4) Å in α-AgI3O8 and 1.791(4)−1.806(4) Å in β-AgI3O8), which are comparable with those in NaI3O8 which contains only one unique I3O8− anion in its asymmetric unit (1.919(3)−2.046(3) Å for I−O−I bridges and 1.780(4)−1.806(4) Å for terminal I−O bonds).3b In both forms of silver polyiodates, the I3O8− anions are interconnected by silver(I) ions into similar neutral [AgI3O8] layer (Figure 4a,b). Each I3O8 group connects with four Ag+ cations; however, their coordination modes are different. The I3O8 group containing I(1) and I(2) is octadentate, and each terminal I(1)O3 forms a bidentate chelation with one Ag(1) atom and another bidentate chelation with one Ag(2) atom. The I3O8 group containing I(3) and I(4) is hexadentate, and each terminal I(3)O3 forms a bidentate chelation with one Ag(1) atom and also bridges with one Ag(2) atom. Both Ag(1) and Ag(2) are located at sites with twofold rotation symmetry. Ag(1) is eight-coordinated by four I3O8 groups in a bidentate chelation fashion whereas Ag(2) is octahedrally coordinated by two I3O8 groups in a unidentate fashion and two other I3O8 groups in a bidentate chelation fashion. The Ag−O distances are in the range 2.410(5)−2.837(5) Å in α-AgI3O8 and 2.416(4)−2.853(4) Å in β-AgI3O8. Within the 2D layer, it is noticed that the Ag(1)O8 and Ag(2)O6 polyhedra are interconnected via edge-sharing into a Ag−O chain along the a axis (Figure 4a,b). The bond valence sum (BVS) calculations indicate that the Ag and I atoms are all in oxidation states of +1 and +5, respectively, which are consistent with the expected valences (Table 2).32

The main difference between the structures of the two isomers lies in their different stacking fashions for the [AgI3O8] layers along the c axis (−A−B− for α-AgI3O8 and −A−B−A′− B′− for β-AgI3O8), which are due to their different space groups adopted (Figure 3a,b). In α-AgI3O8, as the glide planes occurs, there are two symmetrically operation related layers (A and B) that stacked alternatively along the c axis. In β-AgI3O8, subject to −4 and body-centered translation, there are four symmetry related layers stacked in the −A−B−A′−B′− fashion along the c axis. The more system operation related layers in βAgI3O8 also resulted in the much longer c length which is almost double that for α-AgI3O8. The thickness of the [AgI3O8] layer is about 4.48 Å, and the interlayer opening width is about 1.37 Å in both α-AgI3O8 and β-AgI3O8. In both compounds, the lone pair electrons of the IO4 groups are orientated toward the small intralayer 1D tunnels along the a axis based on Ag2I4 6-membered rings whereas the lone pairs of the IO3 groups are oriented toward the interlayer space. It is interesting to point out that NaI3O8 (P4̅) reported previously also exhibits very similar layered structure to those of the two AgI3O8 isomers.3b Compared with the larger and more flexible Ag+ cation, the smaller Na+ cation occupying −4 sites form isolated NaOn (n = 4, 8) polyhedra. The requirement of the coordinate of Na+ with fourfold symmetry leads to the relatively compact assembly of I3O8− around Na+ cations (Figure 4c). The unit cell contains only one [NaI3O8] layer (Figure 3c); hence, its c axis is only about 1/2 and 1/4 of those of α-AgI3O8 and β-AgI3O8, respectively. 3224

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Figure 7. Total and partial DOS for α-AgI3O8 (a), β-AgI3O8 (c), and NaI3O8 (e) as well as the detailed partial DOS for each different O atom of αAgI3O8 (b), β-AgI3O8 (d), and NaI3O8 (f).

Thermal Stability Studies. Thermogravimetric analysis (TGA) studies indicate that α-AgI3O8 and β-AgI3O8 exhibit quite similar thermal behaviors, and both of them are thermally stable up to about 370 °C (Supporting Information Figure S2), which is comparable with that of NaI3O8. Upon further heating, they display two steps of weight losses. The large weight losses in the temperature ranges of 370−435 °C for α-AgI3O8 and 370−445 °C for β-AgI3O8 correspond to the release of 1.0 I2 and 2.5 O2 molecules per formula unit, which are in agreement with the strong endothermic peaks at 374 °C for α-AgI3O8 and 378 °C for β-AgI3O8 in their DSC diagrams. The observed weight losses of 53.8% for α-AgI3O8 at 435 °C and 53.5% for βAgI3O8 at 445 °C are very close to the calculated values of 54.1%. The second weak weight losses in the temperature ranges of 435−485 and 445−500 °C may be attributed to the release of 1.5 O2 per formular unit for α-AgI3O8 and β-AgI3O8, respectively. DSC diagrams of α-AgI3O8 and β-AgI3O8 exhibit endothermic peaks at about 465 and 489 °C, respectively, which are consistent with their second weight losses. The final residuals for the two compounds at 500 °C were confirmed to

be AgI based on powder XRD study (Supporting Information Figure S3). The total weight losses of 62.1% for α-AgI3O8 and 61.7% for β-AgI3O8 at 500 °C match well with their calculated values of 61.9%. Vibrational Spectra. The IR spectra of α-AgI3O8 and βAgI3O8 are nearly identical and both exhibit a number of absorption bands below 900 cm−1: 808 (s), 767 (vs), 733 (s), 632 (w), 533 (s), and 491 (vs) cm−1 for α-AgI3O8 and 808 (s), 773 (vs), 740 (s), 625 (w), 536 (s), and 491 (vs) cm−1 for βAgI3O8 (Supporting Information Figure S4). The absorption bands at about 808, 767, 733, and 632 cm−1 for α-AgI3O8 and 808, 773, 740, and 625 cm−1 for β-AgI3O8 could be attributed to the stretching mode of I−O vibrations. The I−O bending is observed at about 533 and 491 cm−1 for α-AgI3O8 and 536 and 491 cm−1 for β-AgI3O. These assignments are in agreement with those previously reported.6−14 UV−Vis−IR Transmittance Spectra. The optical transmittance spectra measured on plate crystal of α-AgI3O8 (Figure 1c) and polished crystal of β-AgI3O8 (Figure 1d) in a range of 220−10000 nm are shown in Figure 5. On the basis of our 3225

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SHG Properties. The plots of SHG signals as a function of particle sizes for α-AgI3O8 and β-AgI3O8 measured on a Qswitched Nd:YAG laser of wavelength 1064 nm are shown in Figure 6a. For large particle sizes (150−210, 210−300 μm), their SHG intensities are nearly independent of particle sizes. Features of the curves for α-AgI3O8 and β-AgI3O8 are wellconsistent with phase-matching behavior according to the rule proposed by Kurtz and Perry.20 Comparison of the SHG signals produced by two samples and KH2PO4 (KDP) samples in the same particle range of 150−210 μm reveals that their SHG responses are approximately 9.0 and 8.0 times that of KDP (the inset in Figure 6a) for α-AgI3O8 and β-AgI3O8, respectively. The SHG measurements on the known NaI3O8 were also performed under the same conditions, which showed a much larger SHG response of 18.7 times that of KDP (the inset in Figure 6a). It could be concluded that the SHG response is reduced significantly with the replacement of Na+ by Ag+. Such large differences may be associated with the different cation sizes and the different bonding characters in these compounds, which will be discussed in more detail in the following sections. Since α-AgI3O8 and β-AgI3O8 exhibit high transmittance in the NIR range with the long-wavelength cutoff located at 6.1 μm, it is worthy to investigate their SHG properties in the NIR region. SHG measurements on a 2.05 μm Q-switch laser revealed that α-AgI3O8 and β-AgI3O8 display moderate SHG responses of about 0.47 and 0.42 times that of KTiOPO4 (KTP), respectively, and both are phase-matchable (Figure 6b). Hence, α-AgI3O8 and β-AgI3O8 can be also used in the NIR region. Under 2.05 μm laser radiation, NaI3O8 showed a large SHG response of about 1.58 times that of KTP, which is much larger than those of the two AgI3O8 isomers. To better understand the origin of the SHG efficiency, the local dipole moments of the “isolated” IO3− and IO43− groups and I3O8− trinuclear anions as well as the net dipole moments of the unit cell for α-AgI3O8 and β-AgI3O8 as well as NaI3O8 were calculated from the geometric structure (Supporting Information Table S2). We adopted the calculation methods described earlier by Poeppelmeier and Halasyamani and their co-workers.18a,33 The lone-pair on the I5+ cation is given a charge of −2 and is localized 1.23 Å away from the I5+ cations.34 The local dipole moments of IO3, IO4, and I3O8 groups in the three compounds are calculated to be 15.970−16.005, 11.175− 11.426, and 16.037−16.525 D for α-AgI3O8, 15.756−15.901, 10.938−11.038, and 15.939−16.637 D for β-AgI3O8, and 16.376, 10.871, and 16.894 D for NaI3O8, respectively. The local dipole moments of the IO3 groups are significantly greater than those of IO4, which may be attributed to the lower symmetry of IO3 units. Within a trinuclear I3O8 unit, the x- and y-components of the polarizations of the two IO3 units are canceled out completely and the x- and y-components of the polarizations of the IO4 unit are zero, while z-components of the polarizations of the two IO3 are toward the same direction which is opposite to that of the IO4. Therefore, only the zcomponent of the dipole moment of the I3O8 unit is nonzero, and its value is calculated to be slightly larger than that of a single IO3. The local dipole moments of AgO8 and AgO6 polyhedra in the two AgI3O8 isomers are calculated to be 0.459 and 1.909 D for α-AgI3O8 and 0.504 and 1.990 D for β-AgI3O8, and the local dipole moments of both NaO8 and NaO4 polyhedra in NaI3O8 are zero. The contribution to the SHG responses in the above three compounds may mainly come from the I3O8 groups. The net dipole moments for the unit cells are 0 D for both β-AgI3O8 and NaI3O8 due to their

Figure 8. Calculated refractive indexes for α-AgI3O8 (a), β-AgI3O8 (b), and NaI3O8 (c).

measurements, the short-wavelength transmission cutoffs are 328 nm (3.78 eV) and 345 nm (3.59 eV) for α-AgI3O8 and βAgI3O8, respectively. The IR absorption edge extends to approximately 6.1 μm for both α-AgI3O8 and β-AgI3O8, implying that there is a wide optical window for optical application. Furthermore, compared with β-AgI3O8, the optical band gap of α-AgI3O8 exhibits a blue shift of approximately 17 nm, which may be associated with their slightly different structures. 3226

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Table 3. Experimental and Calculated Optical Data for α-AgI3O8, β-AgI3O8, and NaI3O8 compounds α-AgI3O8 (Pnc2) Eg (eV)

exp. cal. scissor the static SHG coefficient tensors (× 10−9 esu) exp. SHG effects (× KDP at 1064 nm)

β-AgI3O8 (I4̅)

NaI3O8 (P4̅)

3.78 2.43 1.35 d15 = 1.56, d24 = 3.33, d33 = 5.07, d15 = d31, d24 = d32

3.59 2.46 1.13 d14 = 1.10, d15 = 3.95 d14 = d25= d36, d15 = −d24 = d31 = −d32

3.93 3.11 0.82 d14 =13.1, d15 = 1.50, d14 = d25 = d36, d15 = −d24 = d31 = −d32

9

8

18.7

nonpolar space groups adopted, whereas that for the polar αAgI3O8 has a small value of 3.760 D. However, it is found that the SHG response of NaI3O8 is much stronger than those of the two AgI3O8 isomers. Therefore, there should be other factors which also contribute to the SHG effects for the three AI3O8 (A = Ag or Na) compounds, as supported by our results from the theoretical calculations based on DFT methods. Theoretical Studies. To gain further insights on the nonlinear optical properties of the two title compounds (αAgI3O8 and β-AgI3O8) and their stoichiometrically identical NaI3O8, theoretical calculations based on DFT methods were performed. Results of band structure calculations (Supporting Information Table S3 and Figure S6) indicate all three compounds are indirect band gap systems, and the calculated band gaps of α-AgI3O8, β-AgI3O8, and NaI3O8 are 2.43, 2.46, and 3.11 eV, respectively, much smaller than the experimental values (3.78, 3.59, and 3.93 eV, Figure 5 and Supporting Information Figure S5), which is due to the limitation of the DFT methods.35 Because the optical properties are based on the experimental optical gaps, the scissors of 1.35, 1.13, and 0.82 eV are applied in the optical properties calculations, respectively, for α-AgI3O8, β-AgI3O8, and NaI3O8. The bands could be assigned according to the total and partial densities of states as plotted in Figure 7. We will focus on the VB and the CB in the vicinity of Fermi level (about −8 to 7.5 eV), which count for most of the bonding character and are closely related to the optical properties of the compounds. For NaI3O8, the VB near the Fermi level can be divided into two regions (I and II); in the low energy region I (−8 to −2.5 eV), O-2p states overlap fully with I-5p, indicating strong I−O covalent interactions, and the bands in region II (−2.5 to 0 eV) mainly come from O-2p nonbonding states, i.e., the lone-pair electrons states of O atoms. The CB (III, 3 to 7.5 eV) is originated from the antibonding states of O-2p and I-5p. For AgI3O8, I and III regions can still be attributed to the I−O interactions, similar to NaI3O8. However, because of the involvement of the full-filled Ag-4d states, the VB-II of AgI3O8 can be divided into two subregions (II-a and II-b): the energy lower VB-II-a is still originated from the O-2p nonbonding states, while the upper VB-II-b is contributed by the full-filled Ag-4d and the O-2p states that bonded to Ag (O(1), O(2), O(3), O(5), and O(6)), as shown in Figure 7b,d. These electronic state distribution differences in VB-II between NaI3O8 and AgI3O8 can also be clearly observed from the orbital graphs (Supporting Information Figure S9). The linear optical response properties of three compounds were examined through calculating the complex dielectric function ε(ω) = ε1(ω) + iε2(ω). Its imaginary part ε2(ω) can be used to describe the real transitions between the occupied and unoccupied electronic states. The imaginary and real parts of the frequency-dependent dielectric functions show obvious

anisotropy along different dielectric axis directions (Supporting Information Figure S7). The averaged imaginary parts reveal the strongest adsorption peaks of α-AgI3O8, β-AgI3O8, and NaI3O8 at 5.80, 5.71, and 5.50 eV (Supporting Information Figure S8), respectively, which can be mainly assigned to the electronic interband transitions from the O-2p and Ag-4d to I5p and O-2p states. The dispersion curves of refractive indices calculated according to the formula n2(ω) = ε(ω) display strong anisotropy: nx ≈ ny > nz for α-AgI3O8 and no> ne for βAgI3O8 and NaI3O8 (Figure 8). The nx, ny, and nz for α-AgI3O8 at 1064 nm (1.165 eV) are calculated to be 2.199, 2.203, and 1.995, respectively; the no and ne for β-AgI3O8 and NaI3O8 are 2.224 and 2.014, and 2.198 and 1.973, respectively. It is interesting to point out that the birefringence based on our calculation is very large (0.208, 0.210, and 0.225 at 1064 nm for α-AgI3O8, β-AgI3O8, and NaI3O8, respectively), which is favorable for the phase-matchability in the SHG process. The calculated static values for the nonvanishing independent SHG coefficient tensors are listed in Table 3. The highest tensors of α-AgI3O8, β-AgI3O8, and NaI3O8 are 5.07 × 10−9, 3.95 × 10−9, and 13.1 × 10−9 esu, respectively, lower than our experimental measured SHG signals, but the magnitude trend is consistent with the experimental fact: β-AgI3O8 (8 × KDP) < α-AgI3O8 (9 × KDP) ≈ 0.5 × NaI3O8 (18.7 × KDP). The three compounds possess the same anion groups (I3O8−) and similar layered structures, but what causes such a great difference in SHG effects between NaI3O8 and the two AgI3O8 isomers? We investigate the origin of the intrinsic SHG for these compounds, and the nonpolar β-AgI3O8 is chosen as a representative of the two AgI3O8 isomers for our discussion. According to the sum-overstates formalism, the SHG coefficients of crystals are closely related to three factors, which include (1) the optical gap, (2) the band structures in VB and CB, and (3) the momentum matrix elements of optical transition. It is well-known that wider band gap usually leads to smaller SHG effect for a certain compound, but here, the smaller-gap AgI3O8 exhibits lower SHG effects, so the gap is not a key factor to induce the SHG effects difference between NaI3O8 and the two AgI3O8 isomers. To further explore the other two influencing factors, we re-evaluated the hypothetical SHG coefficients when the momentum matrix elements have the same constant value but retain their sign, namely, Pnm = ±0.0001.36 The results indicate that variation of SHG coefficients in NaI3O8 is much larger than that in β-AgI3O8, and the hypothetical SHG coefficients in the other two compounds tend to be close (the hypothetical d15 = d31 = −2.45 × 10−9 esu for β-AgI3O8 and the hypothetical d14 = d25 = d36 = −2.23 × 10−9 esu for NaI3O8). The fact implies that it is the momentum matrix elements of optical transition rather than the band shape which leads to the SHG difference in the similar compounds. 3227

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Figure 9. SHG density for β-AgI3O8 (a: VB, b: CB) and NaI3O8 (c: VB, d: CB).

The momentum matrix elements of optical transition, Pnm, defines the electron transition from the initial state (n) to the final state (m) and is closely related to the electronic states distribution of VB and CB in the vicinity of the gap. As shown in DOS and the orbital graphs above, the main electronic structures difference between β-AgI3O8 and NaI3O8 is located

in VB-II. For NaI3O8, almost the whole VB-II comes from the O-2p nonbonding electronic states (lone pair electrons). For βAgI3O8, the energy lower VB-II-a is still the O-2p nonbonding states, but due to the Ag−O interactions, the upper VB-II-b is contributed by the full-filled Ag-4d and the O-2p states that bonded to Ag. 3228

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Notes

It is generally thought that the closer the states approach to the VB top, the more easily the electrons are excited, but it is not completely the case. The effective optical transition not only requires the initial states and the final states overlap in r space but also requires them to overlap in k space.37 In order to exhibit the contributions of the electronic states to momentum matrix elements (further to SHG effects) more directly, we calculated the SHG effect density in VB and CB based on bandresolved, which can identify the dominant orbitals giving the major contribution to the SHG process (Figure 9).38,39 As shown in Figure 9, the SHG effect density in CB is contributed by the O-2p and I-5p orbitals for both compounds. For NaI3O8, the SHG effect density in VB is concentrated in O atoms, especially in the terminal O-2p lone pair orbitals in VB-II. For β-AgI3O8, the major SHG density in VB is also at terminal O atoms and has little at Ag atoms and the O atoms that bonded to Ag; the SHG density shape is rather alike to the O-2p lone pair orbitals in VB-II-a than the Ag-4d orbitals in VB-II-b. The facts indicate that the SHG effects density in VB is originated from the terminal O-2p lone pair orbitals. However, due to the bonding of Ag to O atoms, the terminal O-2p lone pairs in AgI3O8 are much less than those in NaI3O8, thus contributing less to SHG effects. So it is the participation of Ag and Ag−O bonding in the system that largely lowers the density of SHGactive O-2p lone pairs, thus lowering the SHG response of AgI3O8.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 21231006, 21003127, and 21203197).

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CONCLUSIONS In summary, two new noncentrosymmetric ternary polyiodates, namely, α-AgI3O8 and β-AgI3O8, have been synthesized and structurally characterized. They are prepared by hydrothermal reactions of silver nitrate and a large excess of I2O5. Both isomers exhibit nearly identical neutral [AgI3O8] layers parallel to the ab plane which are composed of I3O8− anions interconnected by Ag+ cations. The packings of such [AgI3O8] layers along the c axis are different in α-AgI3O8 and β-AgI3O8. Both α-AgI3O8 and β-AgI3O8 possess wild optical transmission windows in the ranges of 328−6100 nm and 345− 6100 nm, respectively. Powder SHG measurements using 1064 nm radiation indicate that the two isomers are phase-matchable materials with strong SHG responses of about 9.0 and 8.0 times that of KDP for α-AgI3O8 and β-AgI3O8, respectively. These features make α-AgI3 O8 and β-AgI3O 8 promising new candidates for NLO applications. Our future research efforts will be devoted to the growth of high quality large crystals of the two compounds, more detailed optical property measurements, and explorations of other materials containing polynuclear iodate anions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

X-ray crystallographic files in CIF format, measured powder XRD patterns, TGA and DSC curves, IR spectra, UV−vis transmittance spectrum for NaI3O8, the calculated dielectric function, the calculated molecular orbitals, list of the hydrothermal reaction conditions, dipole moment calculations, and the calculated state energies of the L-CB and H-VB. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

* E-mail: [email protected]. Fax: (+86)591-83704836. 3229

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dx.doi.org/10.1021/cm500898q | Chem. Mater. 2014, 26, 3219−3230