Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂: a new iodate with a very large band gap and optical anisotropy

Abstract

Centimeter-sized large crystals of a new magnesium iodate, Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂, have been successfully grown by a facile aqua-solution method. The title compound exhibits an interesting structural feature, namely, [(IO₂(OH))₂(IO₃)]⁻ pseudo-chains interacting with [Mg(H₂O)₆]²⁺ octahedra via the hydrogen bonds. Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ possessing a large band gap of 4.10 eV exhibits the largest birefringence (0.230 @1064 nm) among the iodate systems containing alkali, alkaline earth, or rare earth metal cations, indicating that the compound is a promising birefringent material that could be applied in the ultraviolet region. The electron density difference map indicates that the great birefringence of the material is attributed to the synergistic effect of the iodate groups and the coordinated water molecules. This work also successfully provides a new method for functional crystals with large birefringence via a facile aqua-solution method.

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Introduction

Iodate is an important family of multifunctional materials that can be applied to various industrial processes such as photonics, optical communications, medical lasers, and data storage fields.^1–4 Many functional iodate materials with macroscopic noncentrosymmetric structures revealing interesting properties have been discovered with the aid of stereochemically active lone pair electrons.⁵ In addition, iodates can exhibit broad transmittance windows up to mid-infrared regions, and high-quality large crystals of iodates are often easily grown. Thus, a number of famous nonlinear optical (NLO) materials such as α-HIO₃, α-LiIO₃, and KIO₃ as well as newly designed iodates satisfying the ever-growing requirements of the industry have been continuously discovered in the past several decades.^6–26 In general, basic building units observed from the metal iodates are IO₃ and IO₄ polyhedra. The units are also able to generate dimers, trimers, and even more complicated groups such as I₂O₅, I₃O₈, I₄O₁₁, I₃O₉, etc.^3,14,26 Further extension via strong intermolecular interactions can also result in a variety of iodate groups with various frameworks.²⁶ The variable iodate building blocks not only enrich the structural chemistry, but also provide many interesting compounds with outstanding optical properties including a large SHG response and birefringence.^27–36

It is well known that birefringence is an important parameter for viable NLO and birefringent materials. To design viable materials with large birefringence, several building blocks with large polarizability and anisotropy such as the coplanar groups (BO₃, B₃O₆, C₃O₃N₃, etc.), lone pair cations (Pb²⁺, Bi³⁺, I⁵⁺, etc.), and d⁰ transition metal cations (Nb⁵⁺, Mo⁶⁺, etc.) with second-order Jahn–Teller effect have been recommended.^23,37,38 The higher density and the optimal arrangement of the building blocks through suitable interactions such as hydrogen bonds in the structures also could boost the optical anisotropy of the target compounds. Guided by the above-mentioned strategy, a variety of promising NLO and birefringent crystals have been designed thus far. Specifically, several iodates with large birefringence such as Bi(IO₃)F₂ and Ba₂[VO₂F₂(IO₃)₂]IO₃ have been constructed by introducing lone pair cations or d⁰ transition metal cations.^8,11 Unfortunately, however, most of the materials possess quite narrow band gaps. Thus, to broaden the band gaps, iodates containing main group metal cations such as Ba₂[GaF₄(IO₃)₂]IO₃, LiGaF₂(IO₃)₂, and Ba[InF₃(IO₃)₂] have been prepared, where the combination of iodates and distorted octahedra groups was proven to be an effective way to design interesting birefringent materials with large band gaps.^39–41 In this paper, we have explored the Mg²⁺ cation containing iodate system to discover new birefringent materials. By doing so, a new magnesium iodate, Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂, has been successfully obtained via the facile aqua-solution method at room temperature. Hydrogen bonding interactions between [Mg(H₂O)₆]²⁺ octahedra and iodate groups could generate a novel structural feature.⁴² In addition, the coordinated aqua ligands may also be expected to contribute to improving the birefringence.⁴³

Experimental section

Reagents

Mg(NO₃)₂·6H₂O (Yakuri, 99%) and HIO₃ (Alfa, 99%) were used as reagents as received.

Single crystal growth

Single crystals of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ were grown by the aqua-solution method at room temperature. Mg(NO₃)₂·6H₂O (1 mmol), HIO₃ (3 mmol), and 5 mL of distilled water were mixed in a dish and thoroughly stirred using a glass rod to dissolve the reagents. Transparent needle like crystals were grown for a week in 75% yield based on Mg(NO₃)₂·6H₂O once the water slowly evaporated at room temperature.

Measurements

The single crystal X-ray diffraction data of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ were collected at room temperature via a Bruker D8 QUEST diffractometer with a graphite-monochromated MoKα radiation source (λ = 0.71073 Å) at the Advanced Bio-Interface Core Research Facility, Sogang University. Data reduction and absorption correction were made using the SAINT and SADABS programs, respectively.^44,45 The crystal structure was solved using the OLEX2 package.⁴⁶ The program PLATON was applied to validate if any higher symmetry is missing there.⁴⁷ Crystallographic data, structure refinement information, atomic coordinates, equivalent isotropic displacement parameters, and bond valence sums of all atoms except for H atoms are listed in the ESI.† Powder X-ray diffraction data were collected on a Rigaku Mini Flex 600 diffractometer using Cu Kα (λ = 1.5406 Å) radiation with 40 kV and 15 mA at room temperature. The well-ground polycrystalline sample mounted on a sample holder was scanned in the 2θ range of 5–70° at a scan speed of 20° min⁻¹ and a scan step width of 0.02°. The measured diffraction pattern of the title compound matched well with the calculated data generated from single crystal X-ray diffraction (Fig. 1).

Fig. 1 Experimental and calculated powder XRD patterns of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂. Inset: The picture of an as-grown large crystal.

Infrared (IR) spectra were recorded on a Thermo Scientific Nicolet iS50 FT-IR spectrometer in the spectral range of 500 to 4000 cm⁻¹. The ground sample was made to be in contact with the diamond attenuated-total-reflectance crystal.

Ultraviolet-visible (UV-vis) diffuse-reflectance spectra for Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ were obtained on a Lambda 1050 scan UV-vis-NIR spectrophotometer over the spectral range of 200–700 nm at room temperature. The reflection spectra were converted to the absorbance data using the Kubelka–Munk function.⁴⁸

Thermogravimetric analysis (TGA) was performed using a SCINCO TGA-N 1000 thermal analyzer. The ground polycrystalline sample was loaded into an alumina crucible and heated to 900 °C at a rate of 10 °C min⁻¹ under flowing air. Differential scanning calorimetry data were also obtained on a TA DSC-Q2000 from room temperature to 350 °C at a heating rate of 10 °C min⁻¹ under flowing nitrogen.

First-principles density functional theory calculations were performed using the CASTEP package.⁴⁹ The band structure, density of states, and the optical properties were calculated by using the Perdew–Burke–Ernzerhof generalized gradient approximation (GGA).^50–52 A plane-wave cut-off energy of 925 eV was chosen. To obtain a good convergence of electronic structures and optical properties, the dense k-point sampling less than 0.03 Å⁻¹ was adopted.

Results and discussion

Crystal structure

Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ crystallizes in the monoclinic space group, P2₁/c. In an asymmetric unit, one [Mg(H₂O)₆]²⁺, four IO₂(OH), and two IO₃ units exist (Fig. S1, ESI†). The unique Mg²⁺ cation is coordinated by six H₂O molecules to form the [Mg(H₂O)₆]²⁺ octahedron, in which the Mg–O distances are in the range of 2.036–2.110 Å. While both I(1) and I(3) are linked with two O atoms and one OH⁻ to form IO₂(OH) groups, I(2) is connected by three O atoms to form the IO₃ trigonal pyramid. The distances between I and O atoms observed in IO₂(OH) and IO₃ groups range from 1.780 to 1.913 Å. Long atom contacts such as I(1)⋯O(7) [2.453 Å], I(2)⋯O(9) [2.611 Å], and I(3)⋯O(11) [2.461 Å] are also observed. Considering those longer interactions, [(IO₂(OH))₂(IO₃)]⁻ infinite tubules that are running along the c-axis are monitored (Fig. 2a and b). Finally, hydrogen bonding interactions between the [Mg(H₂O)₆]²⁺ octahedra and [(IO₂(OH))₂(IO₃)]⁻ tubules complete the whole structure of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ (Fig. 2c and d). Bond valence sum calculations on the Mg²⁺, I⁵⁺, OH⁻, and O²⁻ result in values of 2.14, 5.12–5.23, 1.28–1.34, and 1.68–2.03, respectively.^42,53

Fig. 2 [(IO₂(OH))₂(IO₃)]⁻ infinite tubules viewed along the (a) b-axis and (b) c-axis, and the (c) [Mg(H₂O)₆]²⁺ octahedron in Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂. (d) Hydrogen bonding interactions between the [Mg(H₂O)₆]²⁺ octahedron and [(IO₂(OH))₂(IO₃)]⁻ infinite tubules.

IR spectroscopy

The IR spectrum of the reported compound reveals vibrations occurring from Mg–O, I–O, and O–H bonds (Fig. S2, ESI†). Weak bands at ca. 1413 and 823 cm⁻¹ are associated with the Mg–O bonds.^54,55 The bands found at ca. 779, 750, 711, 552, 525, and 517 cm⁻¹ are attributable to the I–O vibrations.^32,42,56 The peaks occurring at ca. 3365 and 1632 cm⁻¹ are responsible for O–H bonds.^42,43

UV-vis diffuse reflectance spectroscopy

The UV-vis diffuse reflectance spectrum suggests that Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ possesses a short cut-off edge of ca. 262 nm (Fig. 3). The cut-off edge is smaller than most iodates containing alkali or alkaline earth, lone pair, and rare earth metal cations such as LiIO₃ (280 nm), RbIO₂F₂ (295 nm), Ba[FeF₄(IO₃)] (281 nm), Bi₂(IO₄)(IO₃)₃ (376 nm), and Ce(IO₃)₂F₂ (452 nm).^{10,12,33,57,58} Hence, the reported iodate with a broad band gap could be a potential birefringent crystal that can be applied in the UV region.

Fig. 3 UV-vis diffuse reflectance spectrum of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂.

Thermal analysis

As seen in the TGA diagram, Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ reveals three thermal decomposition steps (Fig. S3, ESI†). Initially, the title compound exhibits a weight loss of 12.5% between 84 and 227 °C attributed to the loss of eight H₂O molecules from Mg(H₂O)₆²⁺ and IO₂(OH) groups, which are in agreement with the endothermic peaks occurring at 86 to 204 °C in the DSC diagram. And then, 2 equivalents of I₂ and 5 equivalents of O₂ are lost at ca. 344 and 433 °C, calculated (experimental): 56.3% (56.0%). Finally, I₂ and 5/2 O₂ are released between 540 and 710 °C, calculated (experimental): 27.8% (Exp. 27.6%).

Electronic structure calculations

The electronic structure calculations predict a band gap of 2.83 eV for Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂, which is significantly smaller than the experimental value of 4.10 eV (Fig. 4 and Fig. S4, ESI†). The underestimating tendency of the band gap energy obtained by the electronic band structure calculations from the GGA functional attributable to the PBE exchange-correlation energy has been well documented.^59,60 However, the calculations are still useful in identifying important orbital interactions with a reliable trend. As seen from the band structure, the title compound belongs to an indirect band gap compound because the valence band maximum and the conduction band minimum are located at different points. The conduction band minimum and the valence band maximum are dominated by the I 5p and O 2p orbitals, respectively, with small number of mixing from the I 5s orbitals (Fig. 4). Thus, the IO₃ and IO₂(OH) groups occupying the highest valence band and the lowest conduction band mainly determine the optical properties of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂.

Fig. 4 Density of states for Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂.

To confirm the longer interactions between the I and O groups (bond lengths ≥ 2.46 Å), the electron localization function was calculated (Fig. 5a).^26,61 The obvious gaps between the I1⋯O7, I2⋯O9, and I3⋯O11 strongly suggest that the interactions between them are intermolecular interactions rather than the normal covalent I–O bonds in these structures.

Fig. 5 (a) Electron localization function diagram and (b) calculated birefringence for Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂.

Birefringence

Based on the plot of calculated birefringence in Fig. 5b, Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ possesses a very large birefringence of 0.230 @1064 nm, which is much larger than those of most reported iodates. In fact, this value is the largest one among the iodate systems containing alkali, alkaline earth, or rare earth metal cations (ESI†). To further investigate the source of the birefringence, the electron density difference map was calculated (Fig. S5, ESI†). By doing so, we found that the strong covalent I–O and O–H bonds existing in IO₃, IO₂(OH), and H₂O should contribute to the large birefringence of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂. Once we substitute Mg²⁺ for the large Ba²⁺ cation and remove In³⁺ in Ba[InF₃(IO₃)₂], we were able to enhance the density of I–O groups from 4.9 × 10⁻³ Å⁻³ to 10.8 × 10⁻³ Å⁻³. Although Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ consists of a weaker distortive octahedra (Δd = 0.15), the compound still possesses a larger birefringence than those of Ba[InF₃(IO₃)₂] (0.172 @1064 nm; Δd(InO₂F₄) = 0.35) and LiGaF₂(IO₃)₂ (0.181 @1064 nm; Δd(GaF₂O₄) = 0.25). The band gap and birefringence for the reported typical iodates are listed in Table 1. As shown in Table 1, Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ exhibits a good balance between the band gap and birefringence. Even compared with several commercial birefringent crystals such as LiNbO₃ (0.0836 @633 nm), α-BaB₂O₄ (0.1222 @532 nm), and CaCO₃ (0.171 @633 nm), Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ exhibits a very attractive birefringence that can be applied to a promising UV birefringent crystal.^63–65

Table 1 Band gap and birefringence (Δn @1064 nm) for typical iodates

Compound	band gap (eV)	Δn @1064 nm	Ref.
RbIO3	4.0	0.063	10
YI5O14	3.82	0.091	62
GdI5O14	4.07	0.092	62
α-AgI3O8	3.78	0.208	27
β-AgI3O8	3.59	0.210	27
NaI3O8	3.93	0.213	26
Ba2[FeF4(IO3)2]IO3	3.9	0.125	57
LiGaF2(IO3)2	4.33	0.181	40
Ba[InF3(IO3)2]	4.35	0.172	41
Bi(IO3)F2	3.97	0.209	8
Ce(IO3)4	2.17	0.039 (@546 nm)	58
Mg(H2O)6[(IO2(OH))2(IO3)]2	4.10	0.230	This work

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Conclusion

Crystals of a new magnesium iodate, Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂, consisting of Mg(H₂O)₆, IO₃, and IO₂(OH) groups have been grown via an aqua-solution method. The reported material reveals an interesting pseudo-3D structural feature that is composed of [Mg(H₂O)₆]²⁺ octahedra and [(IO₂(OH))₂(IO₃)]⁻ tubules through hydrogen bonding interactions. The compound possesses a large band gap of 4.10 eV and a short UV cut-off edge of 262 nm. Interestingly, the synergistic effect of distorted [Mg(H₂O)₆]²⁺, IO₃, and IO₂(OH) groups resulted in a very large calculated birefringence of 0.230@1064 nm. Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂ maintains a good balance between the band gap and birefringence, which successfully enables the material as a potential birefringent crystal in the UV region. We are in the process of introducing F⁻ anions into a similar framework structure to shorten the UV cut-off edge while maintaining the large birefringence.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. 2018R1A5A1025208 and 2019R1A2C3005530).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, IR spectrum, TGA diagram, band structure, and electron density difference map of Mg(H₂O)₆[(IO₂(OH))₂(IO₃)]₂. CCDC 2167789. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2tc01638a

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