[M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga and In): metal iodate–selenate nonlinear optical materials with a hexagonal tungsten oxide-type topology

Abstract

Exploration of new iodates is crucial due to their splendid applications in the nonlinear optical (NLO) field. Two new IIIA metal iodate–selenates, namely, [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga and In), have been successfully synthesized using the hexagonal tungsten oxide (HTO) framework as an ideal structural template. Isostructural [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga and In) crystallize in the polar space group P6₃mc. Their structures feature _∞[MO₂(OH)₂]³⁻ (M = Ga, In) layers composed of corner-shared circular M₃O₆(OH)₉ trimeric units based on MO₂(OH)₄ octahedra. The SeO₄ groups are capped above half of the trimeric units of the layers while the IO₃ groups are capped below the remaining trimeric units of the layers. Both compounds exhibit moderate phase-matchable second-harmonic generation (SHG) responses (2.9× and 3.8× KH₂PO₄) under 1064 nm laser radiation and the same band gap of 3.38 eV. Results of theoretical calculations revealed that their SHG effects originated from the synergistic effect of MO₆, IO₃ and SeO₄ groups. So far reports on iodate–selenates are scarce and this work provides a new perspective to design new NLO materials through polar HTO-type topologies and a mixed-anionic strategy.

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Introduction

Second-order NLO materials are widely applied in the field of versatile laser technology.^1–7 The key determinant of constructing NLO materials is attaining non-centrosymmetric (NCS) structures, which account for only one-fifth of all crystals.⁸ Metal iodates, a significant class of NLO materials, appear to be favoured by their high possibilities to obtain NCS structures and large SHG coefficients due to the stereochemically-active-lone-pair (SCALP) electrons on I⁵⁺.^9,10 Aligning the SCALP electrons of IO₃ groups in the same direction may result in materials with strong SHG effects.^10–14 It is known that combinations of iodate groups and other NLO-active groups via a mixed-anionic strategy can result in more structural diversities and the integration of the favourable properties of multiple groups.^15–17 Recently, π-conjugated anions (e.g., NO₃⁻ and BO₃³⁻) have been introduced into an iodate system to form new compounds such as Sc(IO₃)₂(NO₃) (4× KDP, 4.15 eV) and Be₂(BO₃)(IO₃) (7.2× KDP, 4.32 eV).^18,19 And non-polar tetrahedral anions (e.g., PO₄³⁻ and SO₄²⁻) have been incorporated to widen the band gaps of materials as exemplified by Cd₂(PO₄)(IO₃) (4× KDP, 4.04 eV) and Nb₂O₃(IO₃)₂(SO₄) (6× KDP, 3.25 eV).^15,20 However, the syntheses of mixed-anionic crystals are challenging as different anions usually compete for metal coordination sites and are prone to forming simple monoanionic compounds.²¹

Until now, some achievements have been made in the iodate–sulfate systems, such as AgBi(SO₄)(IO₃)₂ (3.9× KDP, 3.40 eV), Na₇(IO₃)(SO₄)₃ (0.5× KDP, 4.83 eV) and Ce(IO₃)₂(SO₄) (3.5× KDP, 2.42 eV), manifesting the superiority of the iodate–sulfate system.^22–24 Both selenium and sulfur can adopt a tetrahedral geometry with oxygen atoms.^21,25 Although many iodates and selenates have been reported, reports on iodate–selenates are rare. The NLO-active iodate–selenates reported include Bi₄O(I₃O₁₀)(IO₃)₃(SeO₄) (1.1× KDP, 3.79 eV), Th(IO₃)₂(SeO₄) (no SHG or bandgap data available) and Ln(IO₃)(SeO₄)(H₂O)₂·H₂O (Ln = Gd, Dy, Ho, Er, Tm, Yb, Lu, and Y) (0.05–0.7× KDP, no bandgap data available).^21,26 The f-element-bearing iodate–selenates represent 3D frameworks composed of LnO₆(H₂O)₂/ThO₉ monomers, IO₃ and SeO₄ groups.²¹ Owing to the approximately opposite orientations of IO₃ groups in these materials, their SHG effects are relatively weak.

To obtain polar iodate–selenates, we chose the hexagonal tungsten oxide (HTO) framework as an ideal structural template. The structures of most HTOs feature layers consisting of corner-sharing MO₆ octahedra of d⁰-transition-metals (d⁰-TM), and these layers can be further capped by SeO₃²⁻, TeO₃²⁻ or O₃PCH₃²⁻ from one or both sides, such as A(VO₂)₃(SeO₃)₂ (A = K, Rb, Cs, Tl, and NH₄), A₂(MoO₃)₃(TeO₃) (A = Cs and NH₄), A₂(WO₃)₃(PO₃CH₃) (A = Rb, Cs, and NH₄).^27–35 It is known that d⁰-TM cations can be replaced by Ga³⁺ cations through aliovalent substitution.³⁶ For instance, α- and β-Ba₂[GaF₄(IO₃)₂]IO₃ were designed from α- and β-Ba₂[VO₂F₂(IO₃)₂]IO₃ through the replacement of V⁵⁺ + 2O²⁻ → Ga³⁺ + 2F⁻, and their structures remain unchanged.³⁶ In principle, Ga³⁺/In³⁺ cations can also be introduced in place of d⁰-TM cations maintaining the HTO-type structures as exemplified by AGa₃F₆(SeO₃)₂ (A = Rb and Cs) (P6₃mc) and AGa₃(SeO₄)₂(OH)₆ (A = Na, K, and Rb) (R3̄m).^37–39 In the structures of AGa₃F₆(SeO₃) (A = Rb and Cs), the GaO₂F₄ units replace the VO₆ units in A(VO₂)₃(SeO₃)₂. However, HTO-type iodates have rarely been reported, such as α-, β-, and γ-Cs₂I₄O₁₁.^40–42 The possible reason may be that d⁰-TM cations with high valence states favour more negatively charged anions such as SeO₃²⁻ and TeO₃²⁻ rather than IO₃⁻ anions. Thus, we considered using Ga³⁺/In³⁺ with low valence states to design HTO-type iodates. IO₃ groups with SCALP electrons exhibit large microscopic polarizations in favour of strong SHG effects. SeO₄ groups are also applied to enrich the diversity of HTO structures and to make IO₃ groups aligned on the same side of the HTO layer.

Based on the above ideas, two new HTO-type iodate–selenates, namely, [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1 and In 2), have been synthesized. 1 and 2 show a balanced performance of considerable SHG responses (2.9× and 3.8× KDP) under 1064 nm laser radiation and wide band gaps (3.38 eV). Their SHG effects are larger than those of iodate–selenates reported previously. Herein, we report their syntheses, crystal structures, optical properties and theoretical calculations.

Experimental section

Syntheses

Ga₂O₃ (99.99%, Adamas), In₂O₃ (99.99%, Adamas), Bi₂O₃ (99.99%, Adamas), I₂O₅ (99%, Adamas), and H₂SeO₄ (40%, Aladdin) were purchased and used without further purification. Crystals of [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1, In 2) were grown utilizing the hydrothermal method. Starting materials composed of Bi₂O₃ (0.1398 g, 0.30 mmol), Ga₂O₃ (0.5623 g, 3.00 mmol)/In₂O₃ (0.8329 g, 3.00 mmol), I₂O₅ (0.3338 g, 1.00 mmol), H₂O (2 mL) and H₂SeO₄ (1 mL, 2.76 mmol) were placed in 23 mL Teflon-lined autoclaves, heated to 230 °C for 6 h, held at 230 °C for 2 days, and cooled to 30 °C at 3 °C h⁻¹. The initial and final pH values of the reaction media were <1.0. Clubbed colourless crystals of 1 and 2 were collected after filtration. The yields of 1 and 2 were about 60% based on I of I₂O₅. Bi₂O₃ might be an essential mineralizer and only an amorphous phase was obtained without the addition of Bi₂O₃. The mineralization of Bi₂O₃ or its salts formed during the reaction may help in improving the crystal qualities. The purities of compounds 1 and 2 were confirmed by PXRD (Fig. S1†).

Single crystal structure determination

Single-crystal X-ray diffraction data of the two compounds were collected on an Agilent Technologies SuperNova dual-wavelength CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 296 and 297 K, respectively. Data reduction was accomplished with CrysAlisPro and absorption corrections based on the multi-scan method were applied to both data sets.⁴³ Both structures were solved with the ShelXT 2014/5 solution program using Intrinsic Phasing methods and by utilizing Olex2 as the graphical interface.^44,45 The structures were refined with ShelXL 2018/3 using full matrix least squares minimization on F².⁴⁶ All of the non-hydrogen atoms were refined anisotropically. O(1) and O(2) were assigned to be hydroxide oxygens based on the requirement of charge balance and BVS calculations. H atoms associated with OH⁻ anions were located at geometrically calculated positions and refined with isotropic thermal parameters. H atoms of the crystal water molecules were not included in the refinements due to difficulty in the determination of their accurate positions. The racemic twin in the crystals was introduced using a matrix −1 0 0 0 −1 0 0 0 −1 that lowered the R-factor. The Flack parameters were refined to be −0.03(3) and −0.04(4) for 1 and 2, respectively, confirming the correctness of their absolute structures.⁴⁷ The structures were checked for missing symmetry elements using PLATON, and none was found.⁴⁸ Crystallographic data and structural refinements of the two compounds are listed in Table 1, and the selected bond distances are listed in Table S1.†

Table 1 Crystallographic data for [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga, In)

a R 1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = {∑w[(Fo)^2 − (Fc)^2]^2/∑w[(Fo)^2]^2}^1/2.
Formula	Ga3ISeH8O14	In3ISeH8O14
Formula weight	647.08	782.38
Crystal system	Hexagonal	Hexagonal
Space group	P63mc	P63mc
T (K)	296.15	297.15
a (Å)	7.2174(5)	7.7587(9)
b (Å)	7.2174(5)	7.7587(9)
c (Å)	12.0476(11)	12.2140(14)
V (Å^3)	543.49(9)	636.75(16)
Z	2	2
D c (g cm^−3)	3.954	4.081
μ (mm^−1)	13.652	10.739
Goodness of fit on F^2	1.064	1.058
Flack factor	−0.03(3)	−0.04(4)
R 1, wR2 [I > 2σ(I)]^a	0.0250, 0.0504	0.0188, 0.0372
R 1, wR2 (all data)^a	0.0291, 0.0520	0.0242, 0.0393

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Powder X-ray diffraction

Powder X-ray diffraction (PXRD) patterns of compounds 1 and 2 were recorded using a Rigaku MiniFlex II diffractometer with graphite-monochromated Cu Kα radiation in a 2θ range of 10°–70° utilizing a scanning step width of 0.02°.

Energy-dispersive X-ray spectroscopy

Microprobe elemental analyses and elemental distribution maps were obtained using a field-emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy-dispersive X-ray spectroscope (EDS, Oxford INCA). The presence of Ga, In, I, and Se was verified by FESEM analyses (Fig. S2†). EDS elemental studies showed average M/I/Se (M = Ga 1, In 2) molar ratios of 2.98 : 1.00 : 1.37 for 1 and 3.05 : 1.14 : 1.00 for 2.

Thermal analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were implemented using a NETZCH STA 449F3 at a heating rate of 10 °C min⁻¹ under a N₂ atmosphere.

Optical measurements

Infrared (IR) spectra of the title compounds were recorded on a Magna 750 FT-IR spectrometer in the range of 4000 to 400 cm⁻¹with KBr pellets.

Ultraviolet-visible-near infrared (UV-vis-NIR) spectra ranging from 2000 to 200 nm were recorded on a PerkinElmer Lambda 950 UV-vis-NIR spectrophotometer. By applying the Kubelka–Munk function,⁴⁹ reflectance spectra were converted into absorption spectra.

Second harmonic generation measurements

Powder SHG measurements were conducted with Q-switch Nd:YAG laser generating radiations at 1064 nm using the Kurtz and Perry method.⁵⁰ Crystalline samples of 1 and 2 in a particle-size range of 210–300 μm were used for SHG measurements. For testing the phase-matchability, crystalline samples of 1 and 2 were sieved into distinct particle-size ranges (45–53, 53–75, 75–105, 105–150, 150–210, and 210–300 μm). Sieved KDP samples in the corresponding particle-size ranges were chosen as references.

Laser-induced damage threshold measurements

Laser-induced damage thresholds (LIDTs) of the crystalline samples of [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1, In 2) was measured using a Q-switched pulsed laser (wavelength, 1064 nm; pulse duration, 10 ns; beam diameter, 1.1 mm; pulse frequency, 1 Hz). An AgGaS₂ (AGS) sample of the same size (150–210 μm) was also measured under the same test conditions as a reference. The LIDTs of samples 1 and 2 were determined when the samples turned black under the lasing with a gradually increasing emission energy.⁵¹

Computational method

Calculations of the electronic structure and optical properties for [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1 and In 2) were performed using CASTEP based on density functional theory (DFT).^52,53 Norm-conserving pseudopotential was used to treat the electron–core interactions, and GGA-PBE was chosen as the exchange–correlation function.^54,55 The following orbital electrons were treated as valence electrons: Ga 4s²4p¹, In 5s²5p¹, I 5s²5p⁵, Se 4s²4p⁴, O 2s²2p⁴ and H 1s¹. The number of plane waves included in the basis sets were determined by a cutoff energy of 750 eV for 1 and 2. The Monkhorst–Pack k-point sampling of 4 × 4 × 2 for 1 and 2 was applied to perform the numerical integration of the Brillouin zone. During optical property calculations, more than 2 times the valence bands for 1 and 2 were applied to ensure the convergence of linear optical properties and SHG coefficients.

The calculations of second-order NLO properties were based on the length-gauge formalism within the independent-particle approximation.⁵⁶ We adopted Chen's static formula, which was derived by Rashkeev et al.⁵⁷ and later improved by Chen's group.⁵⁸ The static second-order NLO susceptibility can be expressed as: χ^αβγ = χ^αβγ (VE) + χ^αβγ (VH) + χ^αβγ (two bands), where χ^αβγ (VE) and χ^αβγ (VH) give the contributions to χ^αβγ from virtual-electron processes and virtual-hole processes, respectively, and χ^αβγ (two bands) gives the contribution to χ^αβγ from the two-band processes. The formulas for calculating χ^αβγ (VE), χ^αβγ (VH), and χ^αβγ (two bands) are given in ref. 58.

Results and discussion

Crystal structure

[M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1 and In 2) crystalize in the polar space group P6₃mc (no. 186) (Table 1 and Tables S1 and S2†). They are isostructural and only the structure of 1 is presented to illustrate their detailed structures. Its asymmetric unit contains one Ga³⁺, one IO₃⁻, one SeO₄²⁻, two OH⁻, and one lattice water molecule. I, Se and the water molecule are on sites with 3m symmetry whereas the hydroxide anions are located on the mirror plane. The Ga(1) atom is octahedrally coordinated by six O atoms from one iodate group, one selenate group and four hydroxide ligands with Ga–O bond lengths ranging from 1.922(2) to 2.067(7) Å. The I(1) atom is coordinated by three O(3) atoms in an IO₃ trigonal pyramidal geometry with I–O bond lengths of 1.807(6) Å. The Se(1) atom is tetrahedrally coordinated by three O(4) and one O(5) atoms with Se–O bond lengths of 1.646(7) and 1.616(11) Å (Table S1†). All of these bond distances are close to those reported previously.^21,59,60 The calculated total BVS values are 3.20, 5.10, and 6.00 for Ga(1), I(1), and Se(1), respectively, revealing that the Ga, I and Se atoms are in the +3, +5 and +6 oxidation states, respectively.⁶¹

Three GaO₂(OH)₄ octahedra are interlinked into a circular trimeric unit of Ga₃O₆(OH)₉via vertex-sharing hydroxide ligands. These trimeric units are further interconnected through corner-sharing of the O atoms of hydroxide ligands into a two-dimensional (2D) _∞[GaO₂(OH)₂]³⁻ layer with 3- and 6-membered rings (MRs). The SeO₄ groups are capped above half of the trimeric units through corner-sharing of the O(4) atoms while the IO₃ groups are capped below the remaining half of the trimeric units via O(3) atoms (Fig. S3†), forming a neutral 2D [Ga(OH)₂]₃(IO₃)(SeO₄) infinite layer (Fig. 1a). Finally, along the c-axis, adjoining [Ga(OH)₂]₃(IO₃)(SeO₄) layers in two different orientations are packed alternately and held together in the –A′AA′A– order via hydrogen bonds (O(2)–H(2)⋯O(5), 2.95(1) Å, 159.5(4)°). All the IO₃ groups are arranged in approximately the same direction, and so are the SeO₄ groups. The lattice water molecules act as interlayer spacers and form weak hydrogen bonds (O(1)–H(1)⋯O(1W), 3.09(1) Å, 175.6(3)°), establishing an overall three-dimensional (3D) supramolecular framework (Fig. 1b).

Fig. 1 Views of the [Ga(OH)₂]₃(IO₃)(SeO₄) neutral layer in the ab-plane (a), and the structure of [Ga(OH)₂]₃(IO₃)(SeO₄)·H₂O along the b direction (b). The H atoms of the lattice water molecules have been omitted for clarity.

Apparently, M–O bond lengths lengthen (Ga–O bond (1.922(2)–2.067(7) Å) < In–O bond (2.094(3)–2.205(8) Å)) with the increase of the ionic radii of six-coordinated M³⁺ cations (Ga³⁺ (0.620 nm) < In³⁺ (0.800 nm)).⁶² The interlayer spacing of the adjacent [Ga(OH)₂]₃(IO₃)(SeO₄) (6.02 Å) is slightly shorter than that of [In(OH)₂]₃(IO₃)(SeO₄) (6.11 Å). Accordingly, the cell volumes also follow the sequence of 1 (543.49(9) ų) < 2 (636.75(16) ų). The specific bond lengths and the calculated total BVS values are presented in Table S1.†

Structural comparison

It is interesting to investigate the structural similarities and differences among [Ga(OH)₂]₃(IO₃)(SeO₄)·H₂O (P6₃mc) and other HTO-type materials. We chose Rb(VO₂)₃(SeO₃)₂ (P6₃), RbGa₃F₆(SeO₃)₂ (P6₃mc) and α-Cs₂I₄O₁₁ (P6₃) as the representatives for a detailed comparison.^30,37,40

In the structure of [Ga(OH)₂]₃(IO₃)(SeO₄)·H₂O (1), the HTO-type layers consist of GaO₂(OH)₄ octahedra which undergo an edge (C₂-type) distortion (Δ_d = 0.111) with two short (1.922(2) Å), two normal (1.944(2) Å) and two long (2.000(7), 2.067(7) Å) Ga–O bonds. In Rb(VO₂)₃(SeO₃)₂, the HTO-type layers are composed of VO₆ octahedra which are C₂-type distortive (Δ_d = 1.112). The HTO-type layers in RbGa₃F₆(SeO₃)₂ contain corner-shared GaO₂F₄ octahedra with a corner (C₄-type) distortion (Δ_d = 0.026). In the structure of α-Cs₂I₄O₁₁, the HTO-type layers are constructed by corner-shared IO₅ polyhedra. The magnitudes of the distortions of Ga-based octahedra (GaO₂(OH)₄ and GaO₂F₄) are much smaller than those of second-order Jahn–Teller (SOJT) distorted polyhedra (VO₆ and IO₅), which may be unfavourable for the SHG effects.

The HTO-type layers of compound 1 feature 3- and 6-MRs, which are similar with those of Rb(VO₂)₃(SeO₃)₂ and RbGa₃F₆(SeO₃)₂. But the HTO-type layers in α-Cs₂I₄O₁₁ only consist of 6-MRs. In compound 1, the 3-MRs are capped with IO₃ and SeO₄ groups respectively, and the H₂O molecules are located in the HTO interlayer corresponding to the 6-MRs. As for the structures of Rb(VO₂)₃(SeO₃)₂ and RbGa₃F₆(SeO₃)₂, the 3-MRs are capped with SeO₃ groups and the counter Rb⁺ cations are above and below the 6-MRs. In the structure of α-Cs₂I₄O₁₁, the IO₃ groups are capped above the junction of 6-MRs and the Cs⁺ cations sit above and below the 6-MRs.

The HTO-type materials are divided into two modes: class 1 (“capped” on both sides) and class 2 (“capped” on one side) according to the distribution of groups on the layers. In the structure of compound 1, the SeO₄ and IO₃ groups are capped above and below the HTO-type layers respectively (class 1). Although the SeO₄ and IO₃ groups are distributed on different sides of the HTO-type layers, the small polarizability of SeO₄ tetrahedral units can effectively avoid the cancellation of the polarity caused by IO₃ groups. In the structures of Rb(VO₂)₃(SeO₃)₂ and RbGa₃F₆(SeO₃)₂, HTO-type layers are capped by SeO₃ polyhedra on both sides (class 1) which leads to partial cancellation of the polarization generated by SeO₃ groups. As for the structure of α-Cs₂I₄O₁₁, IO₃ groups are capped on one side of HTO-type layers (class 2), which could enhance the net polarization of IO₃ groups.

Thermal properties

Thermogravimetric analysis (TGA) revealed that [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1 and In 2) exhibited different thermal stability and thermal behaviours (Fig. S4†). Compounds 1 and 2 displayed weight loss in the temperature regions of 360–850 °C and 150–850 °C, respectively. In the process, the two compounds lost the crystal water first, and then lost the water molecule formed in process of the hydroxy condensation. The total mass losses of 1 and 2 were 56.1% and 45.7%, respectively, corresponding to the release of 4H₂O, 0.5I₂, SeO₂, and 1.75O₂ per formula unit (calc. values 56.5% and 46.7%). The residuals of 1 and 2 were determined by the PXRD method to be M₂O₃ (M = Ga and In) (Fig. S5†). The thermal stability of 1 (∼360 °C) is comparable to those of [GaF(H₂O)][IO₃F] (∼350 °C), Rb(VO₂)₃(SeO₃)₂ (∼300 °C), and RbGa₃F₆(SeO₃)₂ (∼370 °C) but is lower than that of α-Cs₂I₄O₁₁ (∼420 °C).^30,37,40,60 The thermal stability of 2 (∼150 °C) is similar to that of [In(IO₃)(OH)(H₂O)](NO₃) (∼150 °C).⁵⁹ For the difference in the thermal stability of 1 and 2, it is speculated that the interlayer spacing of 2 (6.11 Å) is larger than that of 1 (6.02 Å), which leads to the weakening of its interlayer interaction and is easier for the loss of its lattice water molecules.

Optical properties

IR spectra were collected for products 1 and 2. The IR spectra of the isostructural types show bands with similar frequencies and intensities (Fig. S6†). The presence of H₂O molecules in 1 and 2 can be verified by the stretching bands of O–H in the range of 3700–3200 cm⁻¹ and the H–O–H bending mode approximately at 1637 cm⁻¹.^21,63 Bands in the range of 1130–950 cm⁻¹ could be attributed to the asymmetric stretching ν₃ of Se–O. Peaks at 950–823 cm⁻¹ and 823–670 cm⁻¹ could be assigned to the asymmetric stretching ν₃ and the symmetric stretching ν₁ of I–O, respectively. The precise assignment of peaks at a low frequency (630–400 cm⁻¹) is difficult, primarily owing to the overlay between the asymmetric bending ν₄ of Se–O and the symmetric bending ν₂ of I–O in this range (Table S3†).^21,63,64

The UV-vis-NIR experiments indicated compounds 1 and 2 have the same band gaps of 3.38 eV, as shown in Fig. S7.† Their band gaps are larger than those of Nb₂O₃(IO₃)₂(SO₄) (3.25 eV), Ba₃Ga₂(IO₃)₁₂ (3.06 eV), Rb(VO₂)₃(SeO₃)₂ (2.4 eV) and Rb(MoO₂)₂O(IO₄) (3.33 eV).^20,27,30,65 But the band gaps of 1 and 2 are narrower than those of RbGa₃F₆(SeO₃)₂ (3.57 eV), [GaF(H₂O)][IO₃F] (4.34 eV) and Ba[InF₃(IO₃)₂] (4.35 eV).^37,60,66

SHG properties

Powder SHG measurements under 1064 nm radiation indicated that compounds 1 and 2 exhibited moderate SHG responses of 2.9 and 3.8 times that of KDP samples when measured with a particle size range of 210–300 μm and both crystals were type-I phase-matchable (Fig. 2). The SHG responses of 1 and 2 are larger than those of some reported iodates and selenates, such as Y(IO₃)₂F (2× KDP), HBa_2.5(IO₃)₆(I₂O₅) (1.6× KDP), Na₅(SeO₄)(HSeO₄)₃(H₂O)₂ (1.6× KDP), Au₂(SeO₃)₂(SeO₄) (0.43× KDP), Bi₄O(I₃O₁₀)(IO₃)₃(SeO₄) (1.1× KDP) and Gd(IO₃)(SeO₄)(H₂O)₂·H₂O (0.7× KDP).^{21,26,67–70} Compounds 1 and 2 have the largest SHG responses among the iodate–selenates reported previously. Moreover, the SHG responses of 1 and 2 are stronger than that of Rb(VO₂)₃(SeO₃)₂ (40–50× α-SiO₂, non-phase-matchable) but are obviously smaller than those of RbGa₃F₆(SeO₃)₂ (5.6× KDP), α-Cs₂I₄O₁₁ (300× α-SiO₂, non-phase-matchable), Ga(IO₃)₃ and In(IO₃)₃ (≈300× α-SiO₂, or ≈8× KDP).^{30,37,40,71,72} In most cases, the one-sided distribution of the lone-pair polyhedra could improve the SHG effects but the small distortion of Ga/In-based octahedra may lead to weaker SHG effects compared with those of SOJT-distorted polyhedra (e.g., VO₆, IO₅). However, the majority of HTO-type materials are non-phase-matchable (type 1) owing to their small birefringence.^{29–34,37,40,73–75} Compounds 1 and 2 have good phase-matchabilities due to their moderate birefringence. Based on the Inorganic Crystal Structure Database (ICSD), NCS selenates make up one-fifth of the total selenates, while NCS iodate–selenates account for three-fifths of the total iodate–selenates. Although the number of iodate–selenates is relatively small, we may speculate that the introduction of iodate groups in selenates could facilitate the generation of NCS structures.

Fig. 2 The SHG intensity vs. the particle size of compounds under 1064 nm laser irradiation (a) and the measured oscilloscope traces of the SHG signals (210–300 μm) (b). KDP served as the reference.

LIDT measurements

Laser-induced-damage-threshold (LIDT) values are sensitive to some factors, e.g., band gaps, thermal conductivity, thermal expansivity, photoconductivity, and crystal quality.^15,76 Powder LIDT measurements demonstrated that 1 and 2 had high LIDT values of 34.97 and 59.73 MW cm⁻², which are 10 and 17 times that of AGS (3.5 MW cm⁻²), respectively. LIDT values are comparable with those of Bi₄O(I₃O₁₀)(IO₃)₃(SeO₄) (51.73 MW cm⁻²) and RbGa₃F₆(SeO₃)₂ (39.37 MW cm⁻²) showing that 1 and 2 are capable NLO materials with high powder LIDTs.^26,37

Dipole moment calculations

Dipole moment calculations based on the geometric structure of 1 were used to assess the arrangement of GaO₆, IO₃ and SeO₄ groups.^77–80 The space group P6₃mc shows a definite direction of polarization along the c-axis. The local dipole moments of the Ga(1)O₆, I(1)O₃ and Se(1)O₄ groups were calculated to be 0.827 D (Debye), 13.891 D and 0.162 D (Table S4†). For IO₃ or SeO₄ groups, the dipole moment values of the x- and y-component are almost zero and the values of the z-component are effectively additive, which verifies that the IO₃ or SeO₄ groups are aligned in approximately the same direction. Besides, the directions of the z-component of IO₃ and SeO₄ are opposite, which is consistent with the class 1 HTO-type structure. Nevertheless, the values of the z-component of SeO₄ groups are very small and can greatly avoid the cancellation of the polarization of the IO₃ groups. The net local dipole moment value of 1 is 32.396 D and the value of the dipole moment per unit volume of 1 is 0.060 D Å⁻³, which is larger than those of CsVO₂F(IO₃) (0.05 D Å⁻³), Bi₄O(I₃O₁₀)(IO₃)₃(SeO₄) (0.045 D Å⁻³), RbGa₃F₆(SeO₃)₂ (0.01 D Å⁻³), and α- and β-Ba₂[GaF₄(IO₃)₂](IO₃) (0.044 and 0.043 D Å⁻³).^26,36,37,81 The larger value of the dipole moment illustrates the favorable arrangement of NLO-active groups, which is ascribed to the cooperation of polar HTO-type topologies and a mixed-anionic strategy.

Mechanism analysis

To gain in-depth knowledge of the electronic structures and optical properties of compounds 1 and 2, systematic theoretical calculations were implemented based on the density functional theory (DFT) method. As shown in Fig. S8,† compounds 1 and 2 exhibited indirect band gaps with the calculated values of 4.01 and 3.30 eV, respectively, which were relatively different from those of the measured values from the UV-vis NIR spectra (3.38 eV). The reason may be that the conventional DFT-GGA-PBE functional could not describe the band gaps very accurately.^82–86 Since compounds 1 and 2 showed similar partial density of states (PDOS) graphs (Fig. S9†), compound 2 was picked as the representative for detailed illustration. In the PDOS graphs, the O 2p nonbonding states defined the topmost valence band (VB), and the unoccupied O 2p, I 5p orbitals dominated the bottom-most conduction band (CB). Therefore, the band gap of compound 2 was mainly determined by the O and I atoms.

The calculated birefringence of compounds 1 and 2 were 0.070 and 0.066 at 1064 nm, respectively, which contributed to achieving the phase matching in the SHG process (Fig. S10†). The refractive index curves also reflected that compounds 1 and 2 were both negative uniaxial birefringent materials.

The second-order NLO susceptibilities of 2 were researched considering the restriction of Kleinman's symmetry and the space group (P6₃mc). The largest SHG tensor d₃₂ was 1.79 pm V⁻¹ and was calculated to be ca. 4.6× KDP. To deeply demonstrate the SHG origin of 2, the SHG-weighted electron density (SHG density for short) of d₃₂ was determined as presented in Fig. 3. In the VB, the SHG effect originated chiefly from the 2p nonbonding orbitals of all O atoms. As for the CB, the unoccupied I-5p and O-2p orbitals served as the predominant contributors. Summing the SHG contribution densities of the VB and CB, the accurate SHG-contributed percentages of InO₆, IO₃, and SeO₄ groups were 26.43%, 45.71%, and 13.34%, respectively. The result revealed that the great SHG effect of 2 originated from the synergistic effect of InO₆, IO₃ and SeO₄ groups.

Fig. 3 The SHG density of d₃₂ in the VB (a) and the CB (b) for [In(OH)₂]₃(IO₃)(SeO₄)·H₂O.

Conclusions

In summary, two new IIIA metal iodate–selenates, [M(OH)₂]₃(IO₃)(SeO₄)·H₂O (M = Ga 1 and In 2), were obtained using mild hydrothermal reactions. The structures exhibit HTO-type layers consisting of corner-shared trimeric units M₃O₆(OH)₉ that are capped, above and below, by SeO₄ and IO₃ groups, respectively. The compounds exhibit phase-matchable SHG responses 2.9 and 3.8 times that of KDP, moderate band gaps of 3.38 eV, and high powder LIDTs 10 and 17 times that of AGS; hence they are promising NLO materials. This work provides an effective idea for designing novel NLO materials that are based on the cooperation of polar HTO-type topologies and a mixed-anionic strategy.

Author contributions

Qian-Qian Chen performed the experiments, data analyses, and paper writing. Chun-Li Hu performed theoretical calculations. Bing-Xuan Li performed LIDT tests. Jiang-Gao Mao conducted structural determinations, data analyses and major revisions of the manuscript. All the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 22031009, 21921001, and 21975256).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2244693 and 2244694. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00415e

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