Achieving balanced UV SHG responses, optical band gaps and birefringence in rare earth compounds Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu)

Abstract

A series of rare earth iodate sulfate UV compounds, Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu), have been successfully synthesized by the hydrothermal method at 200 °C. These isostructural compounds all crystallize in the noncentrosymmetric space group P2₁2₁2₁ (no. 19) and feature a neutral three-dimensional Ln(IO₃)(SO₄) framework which is composed of 2D cationic Ln[SO₄]⁺ layers bridged by anionic [IO₃]⁻ trigonal pyramids through sharing corner oxygen atoms. Under 1064 nm laser irradiation, Y(IO₃)(SO₄)·3H₂O exhibits a second-harmonic generation (SHG) with an efficiency of 0.7 × KDP@1064 nm. Furthermore, Y(IO₃)(SO₄)·3H₂O has a moderate birefringence (0.118@532 nm) and a large band gap (4.60 eV) and may be a potential UV nonlinear optical material. For Eu(IO₃)(SO₄)·3H₂O, it emits intense photoluminescence peaks at 594 nm and 617 nm when excited under 393 nm near-ultraviolet light, showing promising applications as red phosphors of white-LEDs. The current study elucidates that the incorporation of highly anisotropic lone-paired (IO₃)⁻ units into highly isotropic (SO₄)²⁻ sulfate groups can achieve balanced SHG responses, optical band gaps and birefringence, facilitating the development of novel iodate sulfate crystals for UV nonlinear optical applications.

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Introduction

Rare earth compounds have multifunctional applications in luminescence, ceramic materials, catalysts, nonlinear laser crystals, etc.^1–5 From an atomic viewpoint, rare earth elements have fully occupied outermost and second outermost layers filled with electrons, and empty or singly occupied 5d orbitals, and the internal 4f orbitals have an increasing number of electrons as the atomic number increases. Additionally, rare earth elements exhibit flexible coordination capabilities, enabling them to combine with oxygen atoms to form various polyhedral LnO_x compounds (x = 6–10). These structures often exhibit distortions or noncentrosymmetric arrangements, which are highly advantageous for the formation of nonlinear optical crystals.^6–10 The recently synthesized noncentrosymmetric rare earth compounds, such as REI₅O₁₄ (RE = Y and Gd) (14 × and 15 × KDP), Cs₂YB₃O₆F₂ (5.6 × KDP), and Y₃F(SeO₃)₄ (5.5 × KDP), exhibit strong second-harmonic generation (SHG) efficiency, making them promising candidates for nonlinear optical materials.^11–13 Several strategies have been employed to synthesize compounds with noncentrosymmetric structures. These include the introduction of second-order Jahn–Teller (SOJT) distortion d⁰ transition metal (TM) cations (Mo⁶⁺, V⁵⁺, Nb⁵⁺, Ti⁴⁺, etc.), as seen in K₃Nb₃Ge₂O₁₃ and Li₂TiTeO₆;^14,15 the use of stereochemically active lone-pair (SCALP) cations (Bi³⁺, Pb²⁺, etc.), as in BiO(IO₃) and Cs₂Bi₂O(Ge₂O₇);^16,17 the incorporation of strongly electronegative F⁻ anions, as in CaCe(IO₃)₃(IO₃F)F and [GaF(H₂O)][IO₃F];^18,19 and the mixing of different types of anionic groups, as in Cd₂(IO₃)(PO₄) and AgBi(SO₄)(IO₃)₂.^20,21

I⁵⁺ ions may exhibit significant optical anisotropy due to the presence of lone-pair electrons, which strongly influence the SHG and birefringence properties of crystals. Sulfates and phosphates typically exhibit shorter UV absorption edges, which are significantly favorable for achieving large band gaps in compounds.^22–24 In rare earth iodate sulfates, due to the rich coordination environment of rare earth elements, many structures have been found such as Ln(IO₃)(SO₄) (Ln = La–Gd, except Pm), Ln₂(IO₃)₃(SO₄)OH·3H₂O (Ln = Sm, Eu, Dy), and Ce(IO₃)₂(SO₄).^25–27 In several previous studies, heterovalent substitution has been recognized as an efficient way to synthesize non-centrosymmetric structures, for instance, from Pb₂TiOF(SeO₃)₂Cl to Pb₂GaF₂(SeO₃)₂Cl,^28,29 from CeF₂(SO₄)·H₂O to Ce(IO₃)₂(SO₄),²⁷ and from α- and β-Ba₂[VO₂F₂(IO₃)₂](IO₃) to α- and β-Ba₂[GaF₄(IO₃)₂](IO₃).^30,31 Building on the known crystal structures based on the above strategy, we experimentally introduced lone-paired [IO₃⁻] groups into rare earth sulfates via the hydrothermal method, and successfully synthesized a new series of rare earth iodate sulfate compounds Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu) with noncentrosymmetric structures and large UV absorption edges. Among these structures, Y(IO₃)(SO₄)·3H₂O exhibits a wide band gap of 4.6 eV, a large SHG efficiency of 0.7 × KDP@1064 nm, and type-I phase matching, fully achieving balanced UV SHG efficiency and birefringence. In this study, the synthesis, crystal structures, infrared and UV–vis–NIR spectra, and fluorescence spectra of Ln(IO₃)(SO₄)·3H₂O will be reported and discussed. This study demonstrates that combining large anisotropic [IO₃⁻] groups with isotropic sulfates is an effective strategy for achieving a balance between UV SHG efficiency and birefringence properties.

Results and discussion

Crystal structure

Crystallographic data and refinement results are summarized in Table S1,† while atomic coordinates and bond valence sums (BVS) are provided in Table S2.† Selected bond lengths and bond angles are listed in Table S3.† The phase purity of the synthesized materials Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu) was confirmed by XRD data comparison (Fig. S1†). EDS results show that the crystals contain elements and proportions consistent with the chemical formula (Fig. S2†). The crystal structures of the six crystals are isostructural, exemplified here by Y(IO₃)(SO₄)·3H₂O, which crystallizes in the noncentrosymmetric space group P2₁2₁2₁ (19). The asymmetric unit of Y(IO₃)(SO₄)·3H₂O consists of one Y atom, one I atom, one S atom, ten oxygen atoms, and six hydrogen atoms. The I⁵⁺ cation is connected to three oxygens atoms, forming a distorted trigonal pyramid with bond lengths from 1.787(4) to 1.804(4) Å (Fig. 1a). The S⁶⁺ cation is connected to four oxygen atoms to form a [SO₄] tetrahedron with bond lengths from 1.449(4) to 1.485(4) Å. The Y³⁺ cation is coordinated by eight oxygen atoms to form the [YO₈] polyhedron with Y–O bond lengths from 2.262(4) to 2.450(4) Å. The overall structure is composed of three structural units, i.e., [IO₃], [SO₄], and [YO₈], with each [YO₈] polyhedron linked to three [IO₃] trigonal pyramids, three [SO₄] tetrahedra, and two H₂O through sharing corner oxygen atoms with the monodentate method (Fig. 1b). It is observed along the a-axis that free water molecules are trapped in the cavity structure formed by four [YO₈], two [IO₃], and two [SO₄] units. The calculated BVS of Y, I, and S in Y(IO₃)(SO₄)·3H₂O are +3.337, +5.268, and +6.095, respectively, closely matching their formal values and confirming the structural rationality of Y(IO₃)(SO₄)·3H₂O. The local dipole moments were calculated to analyze the geometric distortion of [IO₃], [SO₄], and [LnO₈] units for compounds Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Eu) (Table S4†). For Y(IO₃)(SO₄)·3H₂O, the calculated dipole moments of [IO₃], [SO₄], and [YO₈] units are 27.8823 D, 1.1702 D, and 4.3600 D, respectively. For Gd(IO₃)(SO₄)·3H₂O, the dipole moments of [IO₃], [SO₄], and [GdO₈] units are 27.8973 D, 1.1840 D, and 3.6735 D, respectively. For Eu(IO₃)(SO₄)·3H₂O, the dipole moments of the [IO₃], [SO₄] and [EuO₈] units are 27.7063 D, 1.2442 D, and 4.7849 D, respectively. Clearly, the distortions of the trigonal pyramidal [IO₃] and tetrahedral [SO₄] units are similar in these compounds, while the distortions of the [LnO₈] units follow the trend [EuO₈] > [YO₈] > [LnO₈], consistent with the SHG efficiency measurements.

Fig. 1 The coordination environment of [SO₄], [IO₃], and [YO₈] units (a) and the crystal structure of Y(IO₃)(SO₄)·3H₂O viewed down the a-axis (b).

From a structural evolution perspective, rare earth sulfates Y(IO₃)(SO₄)·3H₂O can be considered derivatives formed by introducing an [IO₃] group into the parent structure of Y₂(SO₄)₃·8H₂O.³² As is evident from Fig. 2, Y₂(SO₄)₃·8H₂O has a two-dimensional neutral layer structure consisting of [YO₈] and [IO₃] functional units as well as crystal water. There are two types of macrocyclic components in the two-dimensional layer of Y₂(SO₄)₃·8H₂O including a larger eight-membered ring composed of four [SO₄] tetrahedra and four [YO₈] polyhedra and a smaller four-membered ring composed of two [SO₄] tetrahedra and two [YO₈] polyhedral alternatively arranged along the a-axis direction. However, for the title compound Y(IO₃)(SO₄)·3H₂O, with the participation of [IO₃], the two kinds of macrocyclic Y–S–O rings converge into only one kind of six-membered ring, consisting of three [SO₄] tetrahedra and three [YO₈] polyhedra. The [IO₃] trigonal pyramids further bridge the adjacent two-dimensional Y(SO₄) layers into a three-dimensional framework of Y(IO₃)(SO₄)·3H₂O. It is noteworthy that the introduction of [IO₃] into Y₂(SO₄)₃·8H₂O not only increases the dimensionality of the crystal structure but also facilitates the structural conversion from centrosymmetric to noncentrosymmetric space groups.

Fig. 2 Crystal structure comparison of Y₂(SO₄)₃·8H₂O (a) and Y(IO₃)(SO₄)·3H₂O (b).

Infrared and UV–vis–NIR spectra

The infrared spectra of Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu) are presented in Fig. S3.† All compounds have six characteristic absorption bands observed around 3300–3600, 1620–1650, 1100–900, 870–770, 660–500, and 500–400 cm⁻¹, showing a variety of symmetric and antisymmetric vibrations of O–H, S–O, and I–O bonds. Two absorption bands in the ranges of 3300–3600 and 1620–1650 cm⁻¹ are attributed to the O–H bonds from hydroxyl groups, indicating the presence of crystal water molecules in the crystals. The absorption band at 1100–900 cm⁻¹ is assigned to the symmetric stretching ν₁ modes of [SO₄]. The absorption spectrum in the range of 870–770 cm⁻¹ can be attributed to the antisymmetric stretching ν₃ mode of [IO₃]. The range of 660–500 cm⁻¹ is attributed to the antisymmetric bending ν₄ mode of [SO₄], and the absorption spectra in the range of 500–400 cm⁻¹ can be attributed to the combined action of [IO₃⁻] and [SO₄]. These assignments are consistent with previous literature.³³

The UV–vis–NIR diffuse reflectance spectra of Ln(IO₃)(SO₄)·3H₂O (Ln = Er, Ho, Dy, Eu) display characteristic electronic excitation, which is assigned and labeled in their respective spectra (Fig. S4†).^34–36 The band gaps of Ln(IO₃)(SO₄)·3H₂O (Ln = Er, Ho, Dy, Eu) were determined to be 4.51 eV (Dy), 4.51 eV (Er), 4.52 eV (Ho), and 4.42 eV (Eu) by making tangent lines at the absorption edges (Fig. S5†). As shown in Fig. 3, the band gaps of Y(IO₃)(SO₄)·3H₂O and Gd(IO₃)(SO₄)·3H₂O are 4.6 eV and 4.55 eV, respectively. These values are higher than those of most iodate-based materials, such as Bi₂O(SO₄)(IO₃)₂ (2.40 eV),³⁷ KBi(IO₃)₃(OH) (3.5 eV),³⁸ KBi(SeO₄)(IO₃)Cl (3.9 eV),³⁹ and Bi(IO₃)(SO₄) (3.91 eV).³³ This demonstrates that Y(IO₃)(SO₄)·3H₂O and Gd(IO₃)(SO₄)·3H₂O are promising candidates for UV optical materials.

Fig. 3 UV–Vis–NIR diffuse reflectance spectra for Gd(IO₃)(SO₄)·3H₂O and Y(IO₃)(SO₄)·3H₂O.

Fluorescence

Due to the imperative necessity of f-block cations in the field of photoluminescence, we investigated the excitation and emission spectra of the compounds Ln(IO₃)(SO₄)·3H₂O (Ln = Eu, Dy, Ho, Er) (Fig. 4). Under 393 nm near-ultraviolet excitation, Eu(IO₃)(SO₄)·3H₂O exhibits two emission peaks: the strongest at 594 nm and the next strongest at 617 nm, corresponding to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺ ions, respectively (Fig. 4a).⁴⁰ The excitation spectrum of Dy(IO₃)(SO₄)·3H₂O displays multiple absorption peaks at 324 nm (⁶P_3/2), 350 nm (⁶P_7/2), 365 nm (⁶P_7/2), 386 nm (⁴F_7/2 + ⁴I_13/2), and 449 nm (⁴I_15/2) when the emission is monitored at 578 nm. However, under 350 nm excitation, only two main emission bands are observed: the strongest emission peak at 578 nm (⁴F_7/2 → ⁶H_13/2) and a secondary emission peak at 490 nm (⁴F_9/2 → ⁶H_15/2) (Fig. 4b). In the case of Ho³⁺ ions in Ho(IO₃)(SO₄)·3H₂O, a strong and narrow emission peak at 396 nm is observed when monitored at 439 nm emission wavelength (Fig. 4c).⁴¹ Similarly, under 470 nm near-ultraviolet excitation, the Er³⁺ ions in Er(IO₃)(SO₄)·3H₂O exhibit a strong and narrow emission peak at 517 nm (Fig. 4d).^42,43

Fig. 4 Excitation and emission spectra for Ln(IO₃)(SO₄)·3H₂O (Ln = Eu (a), Dy (b), Ho (c), Er (d)).

SHG properties

The SHG responses of the six crystals were measured under 1064 nm radiation using the conventional Kurtz–Perry method. As shown in Fig. 5 and Fig. S6,† although all six crystals crystallize in noncentrosymmetric space groups, the SHG effect for Er(IO₃)(SO₄)·3H₂O, Dy(IO₃)(SO₄)·3H₂O, and Ho(IO₃)(SO₄)·3H₂O is too weak to be measured. In contrast, Y(IO₃)(SO₄)·3H₂O and Gd(IO₃)(SO₄)·3H₂O exhibit their largest SHG effect at particle sizes of 100–150 μm, with SHG intensities of 0.7 × KDP and 0.9 × KDP, respectively. Meanwhile, Eu(IO₃)(SO₄)·3H₂O reaches its maximum SHG intensity of about 1.1 × KDP at particle sizes of 180–250 μm. However, neither Gd(IO₃)(SO₄)·3H₂O nor Eu(IO₃)(SO₄)·3H₂O can achieve phase matching, as indicated by the trend of SHG intensities versus particle sizes. The SHG intensities of Y(IO₃)(SO₄)·3H₂O increase with the increase of particle sizes and substantially remain unchanged at larger size ranges, suggesting the phase-matching ability of the crystal. Calculations of dipole moments reveal that the [IO₃] group makes a prominent contribution to the SHG response of the crystals. The calculated dipole moment of the [EuO₈] polyhedron is 4.7849 D, largest among the three crystals (Ln = Y, Gd, Eu) and the experimentally determined SHG intensity of Eu(IO₃)(SO₄)·3H₂O is also the highest. These results highlight the significant contribution of the [LnO₈] polyhedra to the SHG responses in the Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Eu) systems, which should not be overlooked. Given that the SHG effect and band gap are two critical optical parameters for evaluating nonlinear optical materials, we compared the SHG responses and band gaps of Y(IO₃)(SO₄)·3H₂O and Gd(IO₃)(SO₄)·3H₂O with some selected iodate sulfate crystals (Table 1).

Fig. 5 SHG intensity for samples Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd and Eu) and standard KDP with different particle sizes.

Table 1 Comparison of the space groups, SHG, and band gaps of iodate or sulfate anionic groups

Crystal	Space group	SHG response (×KDP)	Band gaps (eV)
Bi2O(SO4)(IO3)2 ^37	P21/n	0	2.40
Ba2Ce(IO3)8(H2O)^44	Pna21	0.2	2.44
BaZr(IO3)6 ^45	P1̄	0	3.1
KBi(IO3)3(OH)^38	P1̄	0	3.5
Bi(IO3)(SO4)^33	P21/c	0	3.91
CdBi(IO3)(SO4)2 ^33	P21/c	0	4.03
Rb2SO4·SbF3 ^46	P212121	0.3	4.15
NaGaI3O9F^47	P21/c	0	4.27
K2SO4·SbF3 ^46	P212121	0.1	4.44
Na7(IO3)(SO4)3 ^48	P212121	0.5	4.83
Gd(IO 3 )(SO 4 )·3H 2 O	P212121	0.8	4.55
Y(IO 3 )(SO 4 )·3H 2 O	P212121	0.7	4.6

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Theoretical analysis

To understand the microscopic mechanisms underlying the optical properties of Y(IO₃)(SO₄)·3H₂O, theoretical calculations have been carried out using the first principles method within the framework of plane-wave-based pseudopotential density-functional theory (DFT). The calculated band structure of compound Y(IO₃)(SO₄)·3H₂O in Fig. 6a reveals an indirect band gap of 4.034 eV, which is slightly lower than the experimental value of 4.60 eV obtained from UV–vis–NIR diffuse reflectance spectra. This discrepancy arises from the shortcoming of the conventional DFT-GGA-PBE functional, which tends to lower the conduction band energy levels, leading to an underestimation of the calculated band gap.^49–52 From the total density of states (TDOS) and partial density of states (PDOS) of Y(IO₃)(SO₄)·3H₂O, it is evident that the top of the valence band (VB) is primarily contributed by the I-5s5p and O-2p orbitals. In contrast, the bottom of the conduction band (CB) is mainly contributed by the I-5p, O-2p, and Y-4d orbitals (Fig. 6b). Since the optical properties depend closely on the electron transitions from the VB to the CB, the results of theoretical analysis suggest that the synergistic interaction between the [YO₈] polyhedra and [IO₃] trigonal pyramids is the primary origin of the optical absorption in Y(IO₃)(SO₄)·3H₂O.

Fig. 6 Calculated band structure (a) and DOS (b) of Y(IO₃)(SO₄)·3H₂O.

The birefringence of Y(IO₃)(SO₄)·3H₂O is significantly enhanced by the introduction of highly anisotropic [IO₃] units. For comparison, the calculated birefringence of Y₂(SO₄)₃·8H₂O is only 0.004 @ 532 nm, whereas the refractive index of Y(IO₃)(SO₄)·3H₂O is calculated to be 0.118 @ 532 nm (Fig. 7). Furthermore, the birefringence of Y(IO₃)(SO₄)·3H₂O exceeds those of most metal sulfate materials, such as RbY(SO₄)₂·4H₂O (0.045 @ 546 nm),⁵³ Hg₃O₂SO₄ (0.100 @ 546 nm),⁵⁴ and RbSbSO₄Cl₂ (0.110 @ 1064 nm).⁵⁵ The sufficiently large birefringence of 0.118 at 532 nm for Y(IO₃)(SO₄)·3H₂O explains its phase-matchable SHG responses.

Fig. 7 Calculated refractive index for Y(IO₃)(SO₄)·3H₂O crystals.

Conclusions

In summary, we successfully synthesized a series of rare earth iodate sulfates Ln(IO₃)(SO₄)·3H₂O (Ln = Y, Gd, Er, Ho, Dy, Eu) with noncentrosymmetric structures. Among these, Eu(IO₃)(SO₄)·3H₂O exhibits a large SHG response of 1.1 × KDP at particle sizes of 180–250 μm, while Gd(IO₃)(SO₄)·3H₂O and Y(IO₃)(SO₄)·3H₂O show SHG responses of 0.9 × and 0.7 × KDP at particle sizes of 100–150 μm, respectively. The three-in-one combination of rare earth, iodate, and sulfate functional units results in a comprehensive enhancement of both the SHG effect and optical band gap. The findings of this work provide a feasible design strategy for exploring new nonlinear optical materials that simultaneously exhibit large SHG responses and wide band gaps. Although the other three compounds, Ln(IO₃)(SO₄)·3H₂O (Ln = Er, Ho, Dy), do not display SHG responses, they demonstrate interesting fluorescence properties and can serve as host crystals of rare earth dopants, making them promising candidates for next-generation lighting materials.

Data availability

Crystallographic data have been deposited at the CCDC with deposition numbers 2431211–2431216.† The data supporting this article have been included as part of the ESI,† including synthesis and property characterization; details of crystallographic refinement; atomic coordinates and BVS; bond lengths and angles; dipole moments; PXRD patterns; EDS; infrared spectroscopy; UV–vis–NIR diffuse reflectance; and SHG intensity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is supported by the Natural Science Foundation of Shandong Province (ZR2020ME021), the Program of High-level Talents Research Funding of Yulin Normal University (G2020ZK20 and G2020ZK21), and the Project of the Cultivation of Middle-aged and Young Backbone Teachers in Guangxi Colleges and Universities (1000 Plan).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, property characterization and crystallographic data. CCDC 2431211-2431216. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt00613a

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