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Structural Modulation of Nitrate Group with Cations to Affect SHG Responses in Re(OH)2NO3 (Re=La, Y, and Gd): New Polar Materials with Large NLO Effect after Adjusting pH Values of Reaction Systems Yunxia Song, Min Luo, Chensheng Lin, and Ning Ye Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b05119 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 5, 2017

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Chemistry of Materials

Structural Modulation of Nitrate Group with Cations to Affect SHG Responses in RE(OH)2NO3 (RE=La, Y, and Gd): New Polar Materials with Large NLO Effect after Adjusting pH Values of Reaction Systems Yunxia Song †,‡ Min Luo, *,† Chensheng Lin† and Ning Ye*,† †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: A series of rare-earth hydroxide nitrate crystals (La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3) have been synthesized through adjusting pH values of reaction systems under the subcritical hydrothermal condition. All the titled compounds were isostructural with the noncentrosymmetric space group P21 (No. 4) with layer structure, containing [REO9] (RE=La,Y, and Gd) polyhedra in each layer. The polyhedra were stacked on top of each other and further connected with zigzag strings of edge sharing to form infinite corrugated sheets that parallel to the a-c plane. The [NO3] groups that presented two different orientation (A and B) project into the space between the layers. In this study, the angle θ between two different orientation [NO3] groups was defined. With the decrease of ionic radii from La3+, Gd3+ to Y3+, the θ was increased, which led to different second harmonic generation (SHG) effects on lanthanide hydroxide nitrates. The powder SHG measurements revealed that La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were phase-matchable in the visible and UV region and feature large SHG responsed that are approximately 5, 5.5 and 5.6 times that of KH2PO4 (KDP), respectively. Additionally, these title compounds had wide transparent regions from UV to near IR and larger birefringence, suggesting that these crystals were promising UV NLO materials. And their electronic structures and optical properties were calculated based on DFT methods.

Introduction Ultraviolet (UV) Nonlinear optical (NLO) crystals,1-3 which are able to generate UV coherent light through cascaded frequency conversion, have become increasingly important because of their promising applications in laser science and technology.4, 5 6-10 In the past several decades, searching for new UV NLO materials11-13 with high NLO coefficients and broad UV transparency window have attracted considerable attention. To data, most of reported UV NLO crystals with excellent properties are borates, such as β-BaB2O4 (BBO),1 LiB3O5 (LBO),2 CsB3O5 (CBO),3 KBe2BO3F2 (KBBF),14 Sr2Be2B2O7(SBBO),15 K2Al2B2O7 (KABO)16 and Ba4B11O20F (BBOF).17 Among them, BBO and LBO are well-known as commercially available UV NLO materials. According to the anionic group theory, the planar [BO3]3‑ anionic group withπorbital, owning a moderate birefringence and a large microscopic second-order hyperpolarizabilitiesβ18, is deemed to the most favorable basic structure unit in the borates system. Analogous to the [BO3]3 ‑ anionic group, the [CO3]2−and [NO3]− anionic groups with the similar planar triangle units are expected to be good NLO-active anionic groups as well. For example, ABCO3F (A=K, Rb, Cs; B=Mg, Ca, Sr, Ba)10, 19, Na8Lu2(CO3)6F2, Na3Lu(CO3)2F220 and Ca2Na3(CO3)3F21 have been synthesized and proven to be promising UV NLO materials recently. However, up to present, UV NLO materials containing [NO3]- groups have been rarely reported22-25 although the theoretical calculation by Li25 indicated that the microscopic second-order susceptibility of the [NO3]- group

was larger than those of [BO3]3- and [CO3]2- groups. Therefore, it is necessary to systematically explore novel UV NLO materials in the nitrate system. Since most nitrates are susceptible to hydrolysis, it is a great challenge to find insoluble nitrates NLO materials.22-25, 27 Although metal nitrates can be easily dissolved in water, most of the metal hydroxides are insoluble. Therefore, a synthetic method was proposed in this study that the insoluble metal hydroxide nitrates could be obtained through adjusting the pH values in the reaction systems of metal nitrates solution. Besides the planar triangle [NO3]− group, a general strategy for synthesizing new nitrate UV NLO materials is to introduce the rare-earth metal ions according to the following reasons. First, the NLO materials containing the rare earth ions with closed d or f electron shells may have a wide transparent region.28 Secondly, a distorted rare-earth oxide polyhedron with large hyperpolarizability in the NLO materials contributes to improve the SHG responses.29 Thirdly, rare-earth hydroxide nitrates generally have the layer architecture, such as Ln2(OH)5(NO3)·xH2O (Ln =Y, Gd–Lu)30, 31 Ln8(OH)20(NO3)4·nH2O (Ln =Gd–Lu, Y, n∼1.5) . The layer structure feature is the most conducive to produce large SHG effect and birefringence in terms of the NLO crystal. Therefore, a series of acentric nitrates crystals RE(OH)2NO3 (RE=La, Y, Gd) as nitrates UV NLO materials were produced through adjusting pH values of nitrate systems under subcritical hydrothermal conditions for the first time. Results also showed that rare earth cations had an influence on the ar-

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rangement of [NO3] − groups in these compounds, which affected their SHG responses. Experimental Section Reagents NaNO3 (AR, 99.0%), NaOH (AR, 96.0%), KNO3 (AR, 99.5%) and KOH (AR, 98.0%), La(NO3)3·6H2O (AR, 99.0%), Y(NO3)3·6H2O (AR, 99.0%), Gd(NO3)3·6H2O (AR, 99.0%) were purchased from Adamas-beta. Synthesis Crystals of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were synthesized hydrothermally under subcritical condition32. In the synthesis of La(OH)2NO3，the reaction mixture of NaNO3 (0.5525 g, 0.0065 mol)，NaOH (0.5 g, 0.0125 mol), La(NO3)3·6H2O(2.165g, 0.05mol) and H2O (5.0ml) were sealed in an autoclave with a Teflon liner (23 mL). After the heating at 220°C for 5 days, the mixture was slowly cooled down to ambient temperature at a rate of 3°C h−1. Similarly, single crystal of Gd(OH)2NO3 was synthesized with a mixture of Gd(NO3)3·6H2O (2.7082 g, 0.006 mol), KNO3 (0.758 g, 0.0075 mol), KOH (0.701 g, 0.0125 mol) and H2O (5.0 ml). Y(OH)2NO3 was synthesized from mixture of Y(NO3)3·6H2O (1.9145 g, 0.005 mol), NaNO3 (0.6375 g, 0.0075 mol)，NaOH (0.5 g, 0.0125 mol) and H2O (5.0 ml) in an autoclave with a Teflon liner (23 mL) at 180°C for 5 days, followed by slowly cooling to room temperature at 3°C h−1. In these reaction systems, the pure title compounds were obtained when the final pH values were consistently about 4-6. The entire reaction products were washed with deionized water and ethanol and then dried in the air. Colorless transparent, needle-like crystal of La(OH)2NO3 were obtained with the yield of about 72%. Colorless transparent, block-shaped crystal of Gd(OH)2NO3 and Y(OH)2NO3 were obtained with the yield of about 83% and 89%, respectively. Single Crystal Structure Determination Single crystal Xray diffraction data of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were collected at room temperature on a Rigaku Mercury CCD diffractometer with graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å). A transparent block of crystal was mounted on a glass fiber with epoxy for structure determination. The intensity data sets were corrected with the ωscan technique. The data were integrated with the CrystalClear program. The intensities were corrected for Lorentz polarization, air absorption, and absorption attributable to the variation in the path length through the detector faceplate. Absorption corrections were also applied based on the Multiscan technique. The structures were solved with direct methods, refined with difference Fourier maps and full-matrix least-squares fitting on F2 with SHELXL-97.33 The positions of hydrogen atoms were determined by geometrically calculated and all atoms were refined with anisotropic thermal parameters. In addition, the PLATON34 program was used for checking all the structures. The details of the crystallographic data and structure refinement information for La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were listed in Table 1. Atomic coordinates and isotropic displacement coefficients were listed in Tables S1−S3 and bond lengths were listed in Tables S4−S6.

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Table 1 Crystal Data and Structure Refinement La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3

of

Formula

La(OH)2NO3

Y(OH)2NO3

Gd(OH)2NO3

Formula Mass (amu)

234.94

184.94

253.28

Crystal System

Monoclinic

Monoclinic

Monoclinic

Space Group

P21

P21

P21

a (Å)

6.495(4)

6.274(8)

6.355(6)

b (Å)

3.967(2)

3.647(4)

3.723(3)

c (Å)

7.784(5)

7.748(10)

7.749(7)

β (°)

99.913(11)

96.18(3)

96.919(18)

V(Å )

197.57(19)

176.3(4)

182.0(3)

Z

2

2

2

ρ(calcd) (g/cm3)

3.949

3.446

4.622

Temperature (K)

296(2) K

296(2)

296(2)

λ(Å)

0.71073

0.71073

0.71073

212

172

226

µ (mm )

10.714

16.432

18.115

R/wR (I>2σ (I))

0.0137/ 0.0293

0.0377/ 0.0907

0.0205/0.0509

R/wR (all data)

0.0148/ 0.0296

0.0393/ 0.0923

0.0213/0.0511

GOF on F2

1.177

3

F(000) -1

1.077 2

1.077 2

2 2

2 2 1

R(F) = Σ||Fo| – |Fc||/Σ|Fo|. wR(Fo ) = [Σw(Fo – Fc ) /Σw(Fo ) ]

Powder X-ray Diff ffraction Powder X-ray diffraction (PXRD) patterns of polycrystalline materials were obtained at room temperature on a Rigaku Dmax2500 powder X-ray diffractometer with Cu Kα radiation (λ = 1.540598 Å) in the angular range of 2θ = 5−65° with a scan step width of 0.05° and a fixed time of 0.2 s. The powder XRD of the pure powder samples of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were in good agreement with the calculated XRD patterns according to the single-crystal models (Figure S1). Thermal Analysis Thermogravimetric analyses (TGA) were performed on a NETZSCH STA449F3 simultaneous analyzer. Reference (Al2O3) and crystal samples (5−15 mg) were enclosed in Al2O3 crucibles and heated from room temperature to 900 °C at a rate of 10 °C/min under a constant flow of nitrogen. Birefringence The birefringence of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 crystals were measured on a Nikon ECLIPSE LV100 POL polarizing microscope, and the wavelength of the light source was 589.6 nm. Because the measurement accuracy of birefringence mainly depended on the thickness of crystal, the seive crystals size ranging from 25 to 62µm was selected. The formula of calculated birefringence was the following, △R (Retardation)=△n×T △R denotes optical path difference, △n represent birefringence, and T means the thickness of crystal.


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UV-Vis Diff ffuse Reflectance Spectroscopy The UV-vis diffuse reflections were measured at room temperature with a powder sample with BaSO4 as the standard of 100% reflectance. Data were collected on a PerkinElmer Lamda-950 UV/vis/NIR spectrophotometer scanning in the range of 200−2500 nm. Reflectance values were converted to absorbance according to the Kubelka−Munk function.35, 36 Second-Harmonic Generation Powder second-harmonic generation (SHG) signals were measured according to the method from Kurtz and Perry.37 The measurements were performed with a Q-switched Nd: YAG laser at the wavelength of 1064 nm and a frequency doubling at 532 nm, for visible and UV SHG, respectively. Since the powder SHG efficiencies depended strongly on particle size, polycrystalline samples were ground and sieved into the following particle size ranges: 25−45, 45−62, 62−75, 75−109, 109−15, and 150−212 µm. KDP and BBO as the standard samples were prepared through grinding and sieving into the identical particle size ranges in the same way. The ground samples were pressed into a 1 mm thick aluminum holders containing the hole with 8 mm in diameter and covered by glass microscope cover slides. Then these samples were put in a antiglare box and irradiated with a laser waves at 1064 and 532 nm, respectively. In order to avoid the affection of background flash-lamp, a cutoff filter was employed. Furthermore, the second harmonic signal was selected by an interference filter (530 ± 10 nm), and then detected with a photomultiplier tube attached to a RIGOL DS1052E 50-MHz oscilloscope. The above procedures were repeated for the reference samples KDP and BBO. The proportion of the second-harmonic intensity signal outputs between the title compounds and reference samples were calculated. All of the experiments were performed without indexmatching fluid. Computational Descriptions Single-crystal structural data of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were employed for the calculations of electronic properties. The electronic structures of all title compounds were calculated with the DFT method38 within CASTEP code in the Material Studio package. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)39 form was used to describe the exchange and correlative potential of electron−electron interactions. The interactions between ionic core and electron were represented by the norm-conserving pseudopotential, and the valence configurations were treated as H: 1s1, O: 2s22p4, N: 2s22p3, La: 5d16s2, Gd: 4f75s25p65d16s2, Y: 4d15s2. The energy cutoff of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were set at 750 eV, 820 eV and 750 eV, respectively. The converge criteria of total energy for all title compounds was set at 1.0 × 10−5eV/atom. According to the Monkhorst-Pack scheme40, the k-points in the first Brilliouin zone of all title compounds were sampled as 2×4×2. The classical anharmonic oscillator (AHO) model41 was generally adopted to calculate the SHG coefficients. The calculated second-order susceptibilities derived from the AHO model were expressed in terms of the first-order susceptibilities in the following equation,

χijk( 2) (−ω3;ω1, ω2 ) = F (2) χii(1) (ω3 )χ (jj1) (ω1 )χkk(1) (ω2 )

ε 2ij (ω) =

pi (k) p j (k) 8π 2h2e2 Σk Σcv ( f c − f v ) cv 2 vc δ[Ec (k) − Ev (k) − hω] 2 m Veff Evc

(2)

In this expression, δ[Ec(k)-Ev(k)-ħω] represents the energy difference between the conduction and valence bands at the k point with absorption of a quantum ħω. fc and fv represent the Fermi distribution functions of the conduction and valence bands, respectively. The scissors operation should be employed to shift up the conduction band energy as a result of the well-known underestimation of band gap in DFT method. Results and discussion

Figure 1 Products obtained at different pH values.

Synthesis La(OH)2NO3 with a space group of P21/m, was dehydrated from the hydrated lanthanum hydroxide nitrate at 50−150 ℃ in a nitrogen environment42. However, the space group of La(OH)2NO3 has been confirmed to be a noncentrosymmetric space group P21 in this study with single-crystal Xray diffraction techniques. In contrast to dehydration synthesis, the hydrothermal method was an efficient alternative method to grow high optical quality lanthanoid nitrate crystals, in particular to grow those with modest thermal stabilities. With NaOH and KOH to adjust pH values of reaction systems, it was found that the products were strongly depended on the pH values of these reaction systems. As shown in Figure 1, only the clear solution was obtained when the final pH value of reaction system was about 1-3. When the final pH value of reaction system was about 4-6, three pure titled compounds could be obtained. When the final pH value of reaction system was about 9-14, white powder of RE(OH)3 (RE=La, Y, Gd) was obtained. Additionally, when the pH values were about 45 and 5-6, the yields of La(OH)2NO3 crystals were about 48% and 72%, the yields of Gd(OH)2NO3 were about 54% and 83%, the yields of Y(OH)2NO3 were about 70% and 89% with a small amount of side products Y4O(OH)9NO3, respectively.

(1)

The first-order susceptibility χ(1)ii in the low frequency region is given by χ(1)(ω)ii = [ε(ω)ii –1]/4π where the dielectric function Figure 2 Crystal structure of Y(OH)2NO3.



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Figure 3 “Flat-lying” arrangement of nitrate groups in Y(OH)2NO3.

Crystal Structure. RE(OH)2NO3 (RE=La, Y, Gd) are isostructural and crystallizes into a monoclinic crystal system with a noncentrosymmetric space group of P21 (No. 4). Thus, only the structure of Y(OH)2NO3 will be discussed here in detail. As shown in Figure 2, Y(OH)2NO3 exhibited a layer structure where each layer contains identical building-block, namely [YO9] polyhedra. The corners of the coordination polyhedron are occupied by six O atoms from the OH- groups and three O from two [NO3] groups (Figure 2). Y-O bond lengths in the [YO9] polyhedra range from 2.331(11) to 2.621(11) Å. In this structure, [YO9] polyhedra are stacked on top of each other and further connected with zigzag strings of edge sharing to form infinite corrugated sheets that parallel to the a-c plane. The N atoms are coordinated to three O atoms to form planar [NO3] triangles that project into the space between the two layers. The N−O bond distances are from 1.203(10) to 1.269(9) Å and O−N−O bond angles vary between 115.9(7) to 121.4(8) °. There is, however, just a single crystallographically unique NO3- group in this structure. Since through the 21 screw of the space group, two successive [NO3] group layers (A and B) present different orientation (Figure 3). Then, it is possible to define the angle θ between two different orientation [NO3]. It is well known that the effects of cations on the arrangement of [CO3] groups in the alkaline-alkaline earth fluoride carbonate ABCO3F (A = K, Rb, Cs; B = Ca, Sr, Ba) and alkalitransition metal fluoride carbonates ATCO3F (A = K, Rb; T = Zn, Cd) have been discussed.19, 43 Besides, there are quite a few papers describing structural modulation and SHG efficiencies influenced by different cations44-46. Similarly, it was found that the decrease of ionic radii from La3+, Gd3+ to Y3+ implied the increase of θ. The value of the rotational angle θ of [NO3] groups in La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 are 72.863°, 79.755°, 82.144°, respectively. Furthermore, the structural modulation of the anion groups resulted in the difference of macroscopic SHG effects among these three lanthanide hydroxide nitrates. TG Analysis. As shown in Figure S2 of the thermogravimetric analysis (TGA) trace of RE(OH)2NO3 (RE= La, Y, Gd), the thermal behaviors of these materials were quite similar, and all of them were thermally stable up to about 310°C. Upon further heating, all title compounds showed two steps of weight losses. The first weight losses in the temperature range of 310−386℃ for La(OH)2NO3, 310-380℃for Gd(OH)2NO3 and 310−351°C for Y(OH)2NO3 were 7.9% (cal. 7.66%), 7.11% (cal. 7.10%) and 10.34% (cal. 9.73%), respectively, which attributed to the release of 1 mol of water per formula unit from dehydroxylation of the hydroxide layers. The second

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large weight losses in the temperature range of 386-650, 380750 and 380-650℃ could be ascribed to the release of 0.5 mol of N2O5 per formula unit of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3, respectively. The observed weight losses of 22.97% for La(OH)2NO3 at 386 ℃, 21.06% for Gd(OH)2NO3 at 380 ℃ and 28.53% for Y(OH)2NO3 at 351 °C were very close to calculated values of 22.99%, 21.32% and 29.20% for La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3, respectively. Besides, the total weight losses of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were 30.87%, 28.17% and 38.87%, which matched well with their calculated values (30.65% for La(OH)2NO3, 28.42% for Gd(OH)2NO3 and 38.93% for Y(OH)2NO3). Therefore, the general formula of decomposition reaction for all compounds could be expressed as the following, RE(OH)2NO3 → 0.5RE2O3 + H2O + 0.5N2O5(g) (RE= La, Y, Gd) Optical Properties UV-vis diffusion reflection spectra of all the title compounds were collected (see FigureS3). The calculation methods of absorption (K/S) data, F(R) = (1R)2/2R=K/S, based on the Kubelka–Munk function have been adopted. In this expression, R, K and S represent the reflectance, absorption and scattering, respectively. In the (K/S)versus-E plots, the onset of absorption was provided through extrapolating the linear part of the rising curve to zero. Optical diffusion reflection spectrum revealed that band gaps of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were approximately 4.76, 4.83 and 4.84 eV, with UV cutoff edges of 260, 257 and 256 nm, respectively. These results indicated that all of these nitrate NLO crystals might be potentially applied in the UV region. Birefringence According to the the birefringence measurement of three title compounds on Nikon ECLIPSE LV100 POL polarizing microscope, the Retardation of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were approximately 1173 nm, 1120 nm, and 1333 nm, respectively. Furthermore, the thickness of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3 were 8 µm, 10 µm and 10 µm, respectively (Figure S4). The theoretical calculation values of birefringence with the first-principle method (∆n = | nz − nx |) and the measured values were presented in Table 2. All title compounds had large birefringence; the birefringence of each compound was close to that of BBO (0.124). These birefringence values could ensure the crystals to realize the phase-matching in the UV region as short as possible. In the light of anionic group theory, the larger birefringence of these crystal mainly originated from the contribution of [NO3] – groups. Furthermore, the spatial arrangements for [NO3]– groups in these three rare-earth hydroxide nitrates were similar to that of [BO3]3− in BiB3O6 (BIBO)47. According to the birefringence calculation of BIBO by Lin, such alignment characteristic of anionic groups would give the considerable contribution to the refractive index. Table 2 Calculated and experimental values of the birefringence (∆ n) crystal calculated experimental △n (λ=589.6nm) La(OH)2NO3 0.203 0.146 Gd(OH)2NO3 0.171 0.112 Y(OH)2NO3 0.185 0.133


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Figure 4 (a) SHG measurements of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 ground crystals with a reference of KDP at 1064 nm wavelength (b) SHG measurements of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 ground crystals with a reference of BBO as at 532 nm wavelength.

NLO Properties Plots of the SHG signal as a function of particle size of the ground crystals of RE(OH)2NO3 (RE= La, Y, Gd) with a laser at 1064 and 532 nm as the fundamental waves were shown in Figure 4, in which KDP and BBO samples were used as reference for visible and UV SHG measurement, respectively. The results indicated that these NLO crystals were consistent with phase-matching behaviors in the visible and UV regions according to the rule proposed by Kurtz and Perry. The measurements of second-harmonic signal for La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 in comparison with KDP sample in the same particle range of 150−210µm revealed that their SHG responses were approximately 5, 5.5 and 5.6 times that of KDP, respectively. These values are proportional to the squares of the nonlinear deff coefficients. Since the reported d36 coefficient of KDP was 0.39 pm/V,48 the derived deff coefficients of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were 1.396, 1.464 and 1.478pm/V, respectively. The SHG effects of title compounds were close to that of BBO in the visible region. However, the SHG intensities of three nitrate compounds dramatic were decreased in the UV region, in contrast with BBO sample in the same particle, probably because the UV cutoff edge of RE(OH)2NO3 (RE= La, Y, Gd) were close to 266nm, thus resulting in a part of absorption when powder samples were irradiated with an incident laser at 532 nm. Relationship between structure and NLO properties According to the anionic group theory49-51, the macroscopic SHG coefficients for La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 originate from a geometrical addition of the microscopic second-order susceptibility of the planar [NO3] group. In order to better understand the relationship between the NLO properties and structure, the calculation methods based on the anionic group theory have been adopted, which are described in detail in the Supporting Information. We assume that the title compounds have similar refractive indices, which results in the approximate localized field (F). Hence, according to Eqs. (2) and (4) in the Supporting Information, the NLO coefficient is proportional to density of the [NO3] group (n/V) and the structural criterion(C). In the structure, there are two successive [NO3] group layers (A and B)

present different orientation, forming an angle θ. To further elucidate the relationship between the NLO properties and structure, the relationship between the structural criterion C and the rotational angle θ have been investigated according to the anionic group theory. The space group of RE(OH)2NO3 (RE= La, Y, Gd) belong to class 2, only four independent tensors of second-order sus（2） χ（2） χ χ（2） χ（ 2） ceptibility, namely 112 , 123 , 222 and 233 , should be take into account under the restriction of Kleinman’s symmetry.

Figure 5 Macroscopic coordinates(X-Y) of the crystal and the microscopic coordinates (x’-y’) of the [NO3] groups. χ（2）

With 233 as an example, the following equation can be obtained according to Eq. 2 in the support information, (2) χ 233 =

F (2) .g 233 .β 111 ([ NO3 ]); V n

g 233 = ∑ [α (21)α (31) 2 −α (21)α (32) 2 − 2α (22)α (31)α (32)] p



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Table 3 NLO Eff ffects of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 Crystals

La(OH)2NO3 Gd(OH)2NO3 Y(OH)2NO3

SHG coefficient (visible) (× KDP) 5.0 5.5 5.6

Structural criterion

C = cos

Densities of the [NO3] (n/V) (Å−3 )

(n/V) ×C (Å−3 )

Angle (θ) of [NO3] groups(deg)

0.804 0.767 0.754

0.0101 0.0110 0.0113

0.00812 0.00843 0.00852

72.863 79.755 82.144

Where α(21) and α(22) denote the direction cosines between the macroscopic coordinate Y axis of the crystal and the microscopic coordinate x’ and y’ axis of [NO3] groups, respectively. Analogously, α(31) and α(32) denote the direction cosines between the macroscopic coordinate Z axis of the crystal and the microscopic coordinate x’ and y’ axis of [NO3] groups, respectively. As shown in Figure 5, the θ presents the angle between two [NO3] groups pointing in different direction; θ1 and θ2 were defined as the angle between the microscopic coordinate x’ and the macroscopic coordinate X and Y axis, respectively. Since two different orientation [NO3] groups are equivalent in the crystallographic sites, OA should be equal to OB. Therefore, the relationship between θ, θ1 and θ2 can be derived according to their geometrical relationships, namely, θ1=90°- and θ2= . In order to simplify the calculation, we assume that macroscopic coordinate Z is completely parallel to microscopic coordinate y’. Combining with above-mentioned geometrical relation ships, we can calculate that, α21 = cos , α22 = 0, α31 = 0, α32 = 1. Consequently, g233 can be expressed as the following, g233=-cos

Similarly,

θ θ g112=sin2 cos ; 2 2 g123 =0; g 222 = cos 3

θ

2 In case of unspontaneous polarization, the structural criterion C is defined as the following, g 2|g | θ c = = 233 = cos n 2 2 The calculated gijk values were listed in Table 4. Calculations of density of [NO3] group (n/V) and the structural criterion (C) of all title compounds were summarized in Table 3. Table 4 Contribution of Diff fferent Geometrical Factors (g) to Structure Factors (C) Crystals (n) g112/n g222/n g123/n g233/n La(OH)2NO3 (n=2) Gd(OH)2NO3 (n=2) Y(OH)2NO3 (n=2)

0.284

0.521

0

0.804

0.315

0.452

0

0.767

0.325

0.428

0

0.754

As listed in Table 3, th arrangement of [NO3] groups gave the greatest contribution to the g233. Therefore, the macroscopic SHG coefficient of title compounds mainly was from the contribution of g233. Furthermore, with the decrease of RE3+

(La, Gd, Y) ionic radii, the angle was increased gradually. The rotation angles of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 are 72.863, 79.755 and 82.144°, respectively. According to the equation c= , structural criterion (C) was smaller when the relative rotational angle between [NO3] groups was greater. The calculated C factors of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 are 0.804, 0.767 and 0.754, respectively. However, although the C factor of La(OH)2NO3 is greater than that of Gd(OH)2NO3 or Y(OH)2NO3, the macroscopic SHG effect of La(OH)2NO3 was smaller than that of the other two, probably because the SHG coefficients depended on not only the structural criterion, but also the density of anionic group. The density of the [NO3] group (n/V) for La(OH)2NO3 is 0.0081, which is lower than that of Gd(OH)2NO3 or Y(OH)2NO3. According to the structural factor (C) and the density of the [NO3] group (n/V), the macroscopic SHG effect of La(OH)2NO3 is the smallest among these three NLO nitrates. The relationship between structure and NLO properties was in good agreement with the SHG measurements (Table 3). Theoretical Calculations The energy band structures of three compounds were plotted in Figure S5. La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 have direct band gaps of 3.168 eV, 3.594 eV and 3.620 eV, respectively. These calculated values were smaller than the experimental value (4.68eV, 4.84eV and 4.83 eV) due to the underestimation of band gap with the DFT method. Because the optical properties depend on the experimental optical gaps, the scissors of 1.512 eV for La(OH)2NO3, 1.246 eV for Gd(OH)2NO3 and 1.21 eV for Y(OH)2NO3 are adopted in the optical properties calculations. The partial densities of states (PDOS) for La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were presented in Figure S6, in which only the valence band (VB) and the conduction band(CB) in the vicinity of Fermi level were shown because the linear and nonlinear optical properties were mainly determined by the states close to the forbidden band. As shown in Figure S6, the VB maximum and the CB minimum were predominately derived from the O 2p and N 2p orbitals, respectively, which indicated that the electron transition was mainly contributed by inside excitation of the [NO3] - group. Therefore, the SHG effect might primarily originate from the contribution of NO3 groups. The calculated static values for independent tensors of second-order susceptibility were plotted in Figure 6. As listed in Table 5, the largest tensor components (d23) of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were 0.949 pm/V, 1.398 pm/V and 1.015 pm/V at 1064nm, respectively, which agreed well with the results predicted by the geometrical factor calculations with the anionic group theory, which further demonstrated that the NLO coefficients of these compounds mainly came from the [NO3] NLO-active groups.


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Figure 6 Calculated frequency-dependent second harmonic generation coefficients of La(OH)2NO3, Y(OH)2NO3 and Gd(OH)2NO3. Table 5 Experimental and Calculated Optical Data of La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 Crystals La(OH)2NO3 Gd(OH)2NO3 Y(OH)2NO3 Eg (eV) exp. cal. scissor the static SHG coefficient tensors(pm/V)

4.680 4.840 3.168 3.594 1.512 1.246 d23=0.949,d25=0.675, d23=1.398,d25=1.094, d22=0.630, d21=0.480, d22=1.034,d21=0.855, d14=d25=d36,d16=d21,d23= d34

Conclusions A series of rare-earth hydroxide nitrates, La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3, have been synthesized and characterized. These crystals are in the same acentric monoclinic space group P21 and exhibit layers structure where each layer contains [REO9] (RE=La,Y, and Gd) polyhedra and [NO3] triangular, featuring two different orientation [NO3] groups in these structures. The powder SHG measurements indicated that La(OH)2NO3, Gd(OH)2NO3 and Y(OH)2NO3 were phase-matchable in the visible and UV regions and had large SHG responses of approximately 5, 5.5 and 5.6 times that of benchmark KH2PO4 (KDP), respectively. The UV-vis diffuse reflectance spectroscopy study on powder samples showed that these compounds had wide transparent regions from UV to near IR. The first exploratory investigations on these rare-earth hydroxide nitrates indicated that they were potential NLO materials. Growing high-quality large crystals of these hydroxide nitrate to measure their more detailed optical property and exploring other UV NLO materials containing [NO3] groups will be the key emphasis of study in the future.

ASSOCIATED CONTENT The CIF data and additional tables and figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was funded by the National Natural Science Foundation of China (Grant Nos. 91222204, 51425205 and U1605245)

4.830 3.620 1.210 d23=1.015,d25=0.760, d22=0.680,d21=0.575,

and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000). We thank Professor Jianggao Mao and Ms. Mingli Liang at FJIRSM for their help with SHG measurements in ultraviolet region.

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A series of nitrates RE(OH)2NO3 (RE=La, Y, Gd) with large SHG effects as UV NLO materials were produced through adjusting pH values of nitrate systems.


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Structural Modulation of Nitrate Group with ... - ACS Publications

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