Dramatically improved optical anisotropy by realizing stereochemically active lone pairs in a sulfate system, K₂SO₄·HIO₃

Abstract

Obtaining sulfate-based materials with large birefringence is of great challenge due to the weak polarizability anisotropy of the constituting spherical SO₄²⁻ tetrahedron. Here, we have implemented a polyhedron of stereoactive lone pair cations, IO₃⁻ to the system to tackle the issue. Centimetre-sized single crystals of an alkali metal sulfate iodate, K₂SO₄·HIO₃, have been readily grown by an accessible solution–evaporation method. Optical measurements on K₂SO₄·HIO₃ single crystals reveal that the reported material exhibits a 70-fold increase in birefringence (ca. 0.14 at 589.3 nm) compared to that of the singular sulfate, K₂SO₄. In addition, K₂SO₄·HIO₃ exhibits a blue-shifted UV cutoff edge of 275 nm, 25 nm shorter than that of α-HIO₃, highlighting the key role of the SO₄²⁻ tetrahedron in broadening the transmittance window.

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Introduction

Birefringence (Δn) refers to an optical property of an optically anisotropic material with a refractive index that varies with the polarization and direction of propagation of light. As an important optical functionality, birefringence is of great significance for specific optical materials. For instance, a sufficient Δn is inevitably required for nonlinear optical crystals to achieve a necessary phase matching condition.^1–4 In addition, birefringent materials can be used to effectively isolate light through modulation such as polarization, and are widely used in various scientific and industrial fields such as advanced scientific instruments, laser industry, optical communication, and polarimetry.^5–11 Commonly applied birefringent materials, including calcite, YVO₄, TiO₂, β-BaB₂O₄, and MgF₂ remain inherently deficient. For example, β-BaB₂O₄ suffers from phase transition problems during crystal growth processes, while MgF₂ has small birefringence. Therefore, developing new birefringent materials is becoming a research hotspot.^12–16

For NLO crystals, it is generally believed that too much birefringence creates a walk-off effect, which often results in lower conversion efficiency. However, the double refraction represents a more powerful and efficient device; therefore, the stronger the better when it comes to birefringent materials. From the viewpoint of the structure–property relationship, birefringence is strongly correlated with the anisotropy of polarizability of structural units. Anionic moieties with π-conjugated planar configurations such as BO₃³⁻, CO₃²⁻, NO₃⁻, B₃O₆³⁻, and H_xC₃N₃O₃^3−x usually exhibit strong polarization anisotropy due to the large distinction between in-plane and out-of-plane optical anisotropy. Thus, the groups have been widely employed to design superior birefringent materials.^17–23 Contrary to the groups mentioned above, some non-planar and/or non-π-conjugated functional groups, especially the tetrahedrally coordinated SO₄²⁻ and PO₄³⁻ units, are often used in the synthesis of optically functional materials attributable to their exceptional ability to create large band gaps. However, because the polarization anisotropy is much smaller, it is difficult to fabricate useful birefringent materials using only polyhedra with non-planar moieties.^24–29

One well-accepted strategy is to introduce stereochemically active lone pair (SCALP) metal cations such as Sn²⁺ or Sb³⁺, which can readily form off-centered polyhedra and induce larger birefringence. For example, by incorporating Sn^II–O–X polyhedra into phosphates, the Pan group discovered that the resulting materials, Sn₂PO₄X (X = Cl, Br, and I), exhibit enhanced birefringence of greater than 0.2 at 1064 nm.³⁰ Also, Zou and coworkers reported that Sb³⁺ containing phosphates and sulfates, i.e., K₂SbP₂O₇F and CsSbSO₄F₂, respectively, show large enhancements in birefringence.^31,32

In addition to Sn²⁺ and Sb³⁺, the SCALP cation, I⁵⁺, which tends to form strongly distortive IO₃⁻ ions, may also trigger large birefringence.^33,34 More importantly, the single crystal growth ability of iodates has been significantly confirmed.³⁵ Although several IO₃⁻ containing phosphates or sulfates for NLO materials have been discovered, studies on using such modules exclusively to design phosphate- or sulfate-based birefringent materials still receive less attention.^36–38 Driven by these insights, a new birefringent material, namely, K₂SO₄·HIO₃, has been successfully prepared by reacting HIO₃ with potassium sulfate, which exhibits significantly improved birefringence when compared to the singular sulphate. In this work, we demonstrate elaborate investigations of single crystal growth, structural description, optical characterization, and density functional theory (DFT) calculations to elucidate the structure–property relationship.

Results and discussion

Single crystal growth

Single crystals of K₂SO₄·HIO₃ can be grown by slowly evaporating the aqueous solution containing stoichiometric amounts of K₂SO₄ and HIO₃. During the initial growth period, another plate-shaped phase, KH(IO₃)₂, nucleated and grew rapidly. After that, this phase gradually dissolved and disappeared, and the target material, K₂SO₄·HIO₃, began to appear and grew into large single crystals. The powder X-ray diffraction (PXRD) pattern of ground crystals indicates that the main product is K₂SO₄·HIO₃ together with a small amount of KH(IO₃)₂. This phenomenon implies that crystals of KH(IO₃)₂ and K₂SO₄·KHIO₃ grow competitively in the solution. To suppress the growth of the by-product, the initially formed KH(IO₃)₂ crystals were filtered. When the filtrate is evaporated after filtering twice, only single crystals of K₂SO₄·HIO₃ are formed and colourless centimetre-sized block-shaped single crystals can be harvested after about a month of growth (Fig. 1a). The PXRD pattern of the ground bulk crystals of K₂SO₄·HIO₃ is consistent with the simulated one, indicating a pure phase (Fig. 1b).

Fig. 1 (a) Photographs of as-grown single crystals and (b) powder XRD patterns of ground crystals for K₂SO₄·KHIO₃.

Crystal structure

The title compound crystallizes in the centrosymmetric space group, P2₁/m (no. 11) belonging to the monoclinic crystal system (Table S1†). The asymmetric unit consists of two independent K, one I, one S, one H, and seven O atoms (Fig. S1†). As depicted in Fig. 2a, the S atom is connected to O2, O3, O4, and O5 to form a SO₄ tetrahedron with the S–O distances varying from 1.4610(15) to 1.4928(15) Å, and the I atom is linked to O1, O6, and O7 atoms to generate an IO₃ pyramidal group with the I–O lengths varying from 1.7841(14) to 1.9132(15) Å. Obviously, the lengths of both S–O and I–O bonds are in the normal range. The K atom is bonded to four IO₃ pyramids by sharing corners and to three SO₄ groups by sharing two corners and one edge. Disordered hydrogen atoms occupy two sites in H3 and H6 with an occupancy of 0.25 and 0.75, respectively. The H3 atom is linked to the O3 atom of the SO₄ group and the H6 atom is connected to the O6 atom of the IO₃ unit. Hydrogen bonds were calculated using the PLATON program and the results are shown in Table S6† and Fig. 2b. For the H3 atom, the acceptor is the O7 atom with an H⋯A distance of 2.19(8) Å, and the acceptor of the H6 atom is assigned to the O2 atom with a shorter H⋯A length of 1.68(18) Å. Hydrogen bonds hold the SO₄ and IO₃ groups together and are further linked by the K1O₈ and K2O₈ polyhedra to create a three-dimensional structure. The IO₃⁻ and SO₄²⁻ groups alternate in a well-ordered –IO₃⁻–IO₃⁻–SO₄²⁻–SO₄²⁻– arrangement (Fig. 2c).

Fig. 2 The crystal structure of K₂SO₄·HIO₃. (a) Coordination environment of S, I, and K atoms. (b) Hydrogen bonding interactions observed from IO₂(OH) and SO₃(OH) polyhedra. (c) The well-ordered arrangement of IO₃⁻ and SO₄²⁻ groups in the structure.

The bond valence sum (BVS) values for I, S, and K atoms are calculated to be +4.843, +5.995, and +1.13–+1.175, respectively, which are in good agreement with their normal oxidation states. While the BVS values of O1, O3, O4, O5, and O7 are in the normal range (−1.867 to −2.196), those for O2 and O6 are unusually small (−1.795 and −1.548), indicating their connections with hydrogen atoms as evidenced by single crystal XRD.

Ultraviolet–visible (UV-vis) transmittance spectrum

The UV-Vis transmittance spectrum was obtained using a single crystal oriented in the c-direction. As shown in Fig. 3, the UV cut-off edge of the title compound is 275 nm, corresponding to E_g ≈ 4.50 eV. It is noteworthy that the cut-off edge of our compound is 25 nm shorter than that of α-HIO₃, highlighting the key role of the SO₄ group in transparency in the structure. The band gap of K₂SO₄·HIO₃ is comparable to those of compounds with lone pair cations such as [Al(H₂O)₆](IO₃)₂(NO₃), M(IO₃)₂(NO₃) (M = In and Sc), and SnHPO₃F.^39–42 Thus, the title compound could be applied in the UV region.

Fig. 3 The UV-vis transmittance spectrum measured on a single crystal oriented in the c-direction.

Thermogravimetric analysis (TGA)

To examine the thermal stability of K₂SO₄·HIO₃, TGA was performed. As displayed in Fig. S2,† there are two stages of weight loss. Initially, the title compound thermally decomposes at about 203 °C. The experimental weight loss of ca. 3.03% is consistent with the calculated value (2.57%) assuming a release of 0.5 H₂O. In the second stage, starting at ca. 364 °C, the total experimental weight loss is about 50.03%, assuming a final residue of K₂SO₄ (cal. 48.94%). The thermal decomposition product is identified by PXRD measured after heating the title compound at 700 °C in air (Fig. S3†).

Birefringence

Since K₂SO₄·HIO₃ belongs to the monoclinic crystal system (point group: 2/m), the material belongs to the biaxial crystal system according to linear optical classification; thus, it has three unequal principal refractive indices, i.e., n_x ≠ n_y ≠ n_z. In this case, a (001) oriented single crystal with a size of 4.0 × 2.5 × 1.0 mm³ exhibiting strong (00l) diffractions was used for birefringence measurements by employing an oil immersion technique (Fig. 4a and Table S10†). As seen in Fig. 4b, the maximum and minimum n values are measured as 1.675 and 1.535, respectively. Based on the properties of the refractive index ellipsoid (Fig. 4c), the birefringence of the title compound is determined to be ≥0.14 at 589.3 nm. It should be noticed that the experimental birefringence has about a 70-fold improvement for the reported material compared to K₂SO₄ (Fig. 4d).

Fig. 4 (a) PXRD pattern of a (001)-oriented single crystal slice used for refractive index measurements using the oil immersion method. (b) The smallest and largest refractive indices measured. (c) Two-dimensional projection of the refractive index ellipsoid of a biaxial crystal. Δn equals the difference between the smallest n₁ and the largest n₂. (d) Birefringence plots of K₂SO₄, KSO₃F, [Ag(NH₃)₂]₂SO₄, and K₂SO₄·HIO₃. Crystal structures of (e) K₂SO₄, (f) KSO₃F, (g) [Ag(NH₃)₂]₂SO₄, and (h) K₂SO₄·HIO₃ (black: K, yellow: S, navy blue: I, green: Ag, red: O, silver: F, pale silver: N, and pale pink: H).

It is noteworthy that monovalent metal sulphates have several interesting examples presented to enhance birefringence. The first example is ASO₃F (A = alkali metal cations and NH₄⁺), where the authors attempted to generate a heteroanion, (SO₃F)⁻ structural module, by chemically substituting one oxygen atom in the homoanion (SO₄)²⁻ group with fluorine (Fig. 4e and f).⁴³ DFT calculations indicate that (SO₃F)⁻ possesses a greater polarization anisotropy than (SO₄)²⁻; thus, ASO₃F reveals larger birefringence than A₂SO₄. For the potassium member, for example, the birefringence is enhanced about 10 times from K₂SO₄ to KSO₃F.

Another example is the [Ag(NH₃)₂]₂SO₄ crystal, which has a sharp increase in birefringence compared to the corresponding Ag₂SO₄ (cal. 0.102 vs. 0.012 at 1064 nm).⁴⁴ The unique linear rod structure of [Ag(NH₃)₂]⁺ with overwhelming polarization anisotropy is thought to be the main cause of the improved birefringence (Fig. 4g). It should be emphasized that K₂SO₄·HIO₃, designed by a lone pair-directed strategy, exhibits exceptionally enhanced birefringence. This suggests that our strategy of improving optical anisotropy and enhancing birefringence by introducing IO₃⁻ groups into the sulphate is highly effective.

DFT calculations

To elucidate the origin of the optical properties of K₂SO₄·HIO₃, the band structure, refractive index, and density of states (DOS) based on the DFT theory were determined. As presented in Fig. S5,† the band gap inferred from the band structure is 4.10 eV, which is very close to the experimental value (4.50 eV). From the refractive index dispersion curves (Fig. S6†), the birefringence is determined to be 0.11 at 586 and 593 nm, which is close to the experimental value (0.14 at 589.3 nm). As can be seen in the DOS, the conduction band maximum (CBM) is dominated by the O-2p, I-5p, and I-4d states, while the valence band minimum (VBM) is mainly composed of O-2p, O-2s, I-5p, and S-3p (Fig. 5). Since the optical properties of the compounds are closely related to the behaviour of electrons in the VBM and CBM, it can be concluded that the optical properties of K₂SO₄·HIO₃ are synergistically determined by the IO₃⁻ and SO₄²⁻ groups. Moreover, because the small birefringence of the monosulfate, K₂SO₄, is attributed to the weakly anisotropic SO₄²⁻ tetrahedron, the greatly enhanced birefringence in K₂SO₄·HIO₃ should have originated from the IO₃⁻ group.

Fig. 5 The total and partial DOS of K₂SO₄·HIO₃.

Conclusions

In summary, a new birefringent material, K₂SO₄·HIO₃, containing two types of anionic groups has been discovered. In particular, the birefringence of K₂SO₄·HIO₃ is greater than that of previously developed KSO₃F and [Ag(NH₃)₂]₂SO₄, and is 70 times higher than that of K₂SO₄ attributable to the successful combination of IO₃⁻ units. The finding indicates that sulfates can be a class of very effective systems for improving the optical anisotropy once polyhedra of lone pair cations are successfully introduced. There is no doubt that such a methodology can also be applied to other systems with poor-birefringence, such as phosphates and silicates, to fabricate functional materials with enhanced optical anisotropy.

Experimental

Crystal growth

Potassium sulfate (K₂SO₄, S. P. C. GR Reagent, first grade) and iodic acid (HIO₃, Alfa Aesar, 99%) were used as starting reagents. In typical experiments, stoichiometric amounts of K₂SO₄ (28.4 mmol, 4.94 g) and HIO₃ (28.4 mmol, 5.00 g) were dissolved in 100 mL of deionized water and stirred at 80 °C for 1 h to allow the solution to become clear. The clear solution was continued to heat until 50 mL of concentrated solution was obtained. After filtering twice, the flask containing the solution was sealed with a perforated plastic film and the solution was slowly evaporated at room temperature. Colorless centimeter-sized block-shaped single crystals were harvested in about one month of growth. The PXRD pattern of the ground bulk crystals agrees well with the simulated pattern.

PXRD

PXRD data were collected at room temperature on a Rigaku MiniFlex 600 diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). The 2θ range was 10–60° with a scan step width of 0.02° and a fixed counting time of 5 s per step.

Single-crystal structure determination

The single crystal X-ray diffraction data were collected on a Bruker APEX-II CCD diffractometer utilizing Mo Kα radiation (λ = 0.71073 Å) at 300(2) K at the Advanced Bio-Interface Core Research Facility at Sogang University. Data reduction, cell refinement, and absorption corrections were conducted with the program APEX 4. The structure was solved by the intrinsic phasing method and refined on F² by full-matrix least-squares techniques using the program Olex2.⁴⁵ The structure was checked using the program PLATON and no higher symmetry was found.⁴⁶ Crystal data and detailed structural information are given in Tables S1–S8.†

Refractive index measurements

The refractive index was measured using an oil immersion technique on a gem refractometer equipped with a sodium yellow laser (λ = 589.3 nm). The refractive index was recorded for every 45-degree rotation.

UV-vis spectroscopy

UV–vis transmittance spectra were collected on a JASCO V-660 spectrometer in the wavelength range of 200–800 nm using a (001)-oriented single crystal slice.

Infrared (IR) spectra

IR spectra were recorded on a Nicolet AVATAR 330 FT-IR spectrometer in the wavenumber range of 650 to 4000 cm⁻¹.

TGA

TGA was performed by using a SCINCO TGA-N 1000 thermal analyzer. A 6.6 mg portion of powder sample was loaded in an alumina crucible and heated from 25 to 800 °C at a rate of 10 °C min⁻¹ under flowing air.

Theoretical calculations

The band structure and DOS of K₂SO₄·HIO₃ were calculated by employing a QUANTUM ESPRESSO program based on the DFT. The optical properties of K₂SO₄·HIO₃ were calculated using CASTEP.⁴⁷ The ultrasoft pseudopotential types with nonlinear core correction,⁴⁸ Perdew–Burke–Ernzerhof exchange–correlation function,⁴⁹ and scalar relativistic effect were applied. The valence electrons of K-3p4s, I-4d5p5s, S-3p3s, O2-2p2s, and H-1s were used in the calculations. The k-points of the Brillouin zone were set to be 4 × 4 × 2. The other parameters were set as default.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work 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: CIF; detailed crystallographic data, BVS, measured refractive index data, asymmetric units, TG curves, PXRD patterns, and band structures. CCDC 2237869. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00192j

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