LiRE(SO₄)₂ (RE = Y, Gd, Eu): noncentrosymmetric chiral rare-earth sulfates with very large band gaps

Abstract

Three new ternary lithium rare earth metal sulfates, LiRE(SO₄)₂ (RE = Y, Gd, and Eu), have been synthesized via high temperature solid-state and solvothermal reactions. The reported materials crystallizing in the noncentrosymmetric (NCS) nonpolar tetragonal space group, P4̄n2 (no. 118) exhibit three-dimensional structures consisting of REO₈, LiO₄, and SO₄ groups. Ultraviolet-visible diffuse reflectance spectra and density functional theory calculations indicate that the title materials exhibit very large band gaps of ca. 6.7 eV. The frameworks of the sulfates reveal high thermal stabilities of up to 873–1073 K depending on the constituting rare earth cations. Powder second-harmonic generation (SHG) measurements reveal that the sulfates with NCS structures have mild SHG responses of ca. 13 × α-SiO₂ and are non-phase matchable. Photoluminescence experiments on LiEu(SO₄)₂ revealing a bright orange emission show multiple peaks originating from f–f transitions. Detailed synthetic procedures, crystal structures, and full characterization studies of the new sulfates are presented.

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Introduction

Materials with noncentrosymmetric (NCS) structures are the most essential components for the second-order nonlinear optical (NLO) properties modulating and controlling frequency of solid-state lasers. Several crystalline materials such as KH₂PO₄,¹ KTiOPO₄,² and β-BaB₂O₄¹ have been extensively utilized to generate second-harmonic lights with various wavelengths. However, discovering functional NLO materials that can be applied to a specific range, e.g., deep ultraviolet (DUV) area with shorter wavelengths of less than 200 nm, is an ongoing challenge. To generate a novel coherent DUV light, crystals must have wide band gaps (<200 nm of UV cutoff edge), moderate birefringence (Δn ≥ 0.053), and strong second harmonic generation (SHG) response. A few effective functional groups to fulfill the conditions originating from the structural anisotropy and electron density distribution include materials with well-aligned π-conjugated planar moieties such as BO₃³⁻, NO₃⁻, and CO₃²⁻.^3,4 A number of representative compounds consisting of π-conjugated planar groups with large birefringence and wide UV-cutoff window such as K₂BeBO₃F₂,⁵ β-BaB₂O₄, LiB₃O₅,⁶ KSrCO₃F,⁷ and Rb₂Na(NO₃)₃⁸ have been known thus far. Unfortunately, however, the materials are not widely utilized due to some critical drawbacks.⁹ For example, the benchmark DUV NLO material, K₂BeBO₃F₂ not only exhibits a severe layering habit, but reveals an intrinsic toxicity attributable to beryllium.¹⁰ Meanwhile, crystals composed of non-π-conjugated tetrahedral groups such as PO₄³⁻ and SO₄²⁻ should be attractive potential DUV NLO materials because the constituting tetrahedra revealing lower interaction with other external groups can offer more stable framework structures with large band gaps.^11–13 In addition, well-ordered tetrahedra also facilitate the crystallization of macroscopic NCS structures.^14–20 With these ideas in mind, we have successfully synthesized pure polycrystalline samples and crystals of three ternary lithium rare-earth double sulfates, LiRE(SO₄)₂ (RE = Y, Eu, and Gd) by solid-state and hydrothermal reactions. In this paper, detailed characterizations on the novel NCS sulfate materials exhibiting very short UV-cutoff edges are presented along with their electronic structure calculations and photoluminescence properties.

Experimental section

Materials

LiNO₃ (99%, Alfa Aesar), Li₂SO₄·H₂O (99%, Alfa Aesar), Y₂(SO₄)₃·8H₂O (99.9%, Alfa Aesar), Eu₂(SO₄)₃·8H₂O (99.9%, Alfa Aesar), Gd₂(SO₄)₃·8H₂O (99.9%, Alfa Aesar), HNO₃ (60%, Samchun), and H₂SO₄ (95%, Samchun) were used without further purification.

Synthesis

Polycrystalline samples were prepared via conventional solid-state reactions. Stoichiometric amounts of Li₂SO₄·H₂O and RE₂(SO₄)₃·8H₂O (RE = Y, Gd, and Eu) were thoroughly mixed using an agate mortar and pestle. The well-ground reaction mixtures were heated at 600 °C for 12 h and cooled to room temperature by turning off the furnaces. For LiGd(SO₄)₂, an additional 12 h heating and intermediate regrinding were required to synthesize the pure product. Large single crystals of the title compounds were grown by hydrothermal reactions. 1 mmol of RE₂(SO₄)₃·8H₂O and 4 mmol of LiNO₃ were placed in 18 mL Teflon liners with 0.2 mL of HNO₃, 0.5 mL of H₂SO₄, and 0.5 mL of distilled water. After tightly sealing, the autoclaves were heated to 230 °C for 72 h and cooled to room temperature at 6 °C h⁻¹. Millimeter-sized large crystals of LiY(SO₄)₂, LiGd(SO₄)₂, and LiEu(SO₄)₂ were isolated via vacuum filtration in 89.5%, 80.3% and 84.2% yields, respectively, based on RE₂(SO₄)₃·8H₂O.

Measurements

Single crystal X-ray diffraction (SCXRD) data were collected at room temperature using a Bruker D8 Quest diffractometer with a graphite monochromated radiation source (Mo Kα, λ = 0.71073 Å) at the Advanced Bio-interface Core Research Facility, Sogang University. A colorless prism-shaped crystal for LiY(SO₄)₂ (0.038 mm × 0.127 mm × 0.206 mm), a colorless prism-shaped crystal for LiGd(SO₄)₂ (0.058 mm × 0.070 mm × 0.103 mm), and a pale red prism-shaped crystal for LiEu(SO₄)₂ (0.072 mm × 0.180 mm × 0.189 mm) were used for the data collections. After integrating the collected data using the SAINT program,²¹ absorption corrections were applied using the SADABS program.²² The structures were solved using SHELXS-2013²³ and refined using SHELXL-2013²⁴ software implemented in the WinGX-2014 program.²⁵

Powder X-ray diffraction (PXRD) patterns were obtained on a Rigaku Miniflex 600 (Cu Kα, λ = 1.54056 Å) at room temperature. Well-ground polycrystalline samples mounted on sample holders were scanned in the 2θ range of 5–70° with a scan speed of 20° min⁻¹ and a scan step width of 0.02°.

Ultraviolet-visible (UV-vis) diffuse reflectance spectra were obtained from 185 to 900 nm with 1 nm intervals using a PerkinElmer Lambda 1050 spectrophotometer.

Infrared (IR) spectra for the title compounds were obtained using a Thermo Scientific Nicolet iS50 FT-IR spectrometer with an attenuated total reflection accessory in the range of 400–4000 cm⁻¹.

Thermogravimetric analysis (TGA) data were collected using a SCINCO TGA-N 1000 thermal analyzer. Polycrystalline samples were loaded in alumina crucibles and heated from 30 °C to 900 °C at a rate of 10 °C min⁻¹ under the flowing argon gas.

Density functional theory (DFT) calculations were performed using a Quantum Espresso program package.²⁶ The ultrasoft pseudopotential and Perdew–Burke–Ernzerhof (PBE) functional²⁷ with the nonlinear core correction²⁸ were used to calculate electronic structures of lithium rare-earth metal double sulfates. The kinetic energy and charge density cutoff were set to 46.65 and 419.85 Ry, respectively, for LiY(SO₄)₂, 122.08 and 1098.76 Ry for LiEu(SO₄)₂, and 116.606 and 1049.45 Ry for LiGd(SO₄)₂. The self-consistent function convergence thresholds for the reported compounds were set to about 10⁻⁶ Ry. The 4 × 4 × 6 k-point grids of the Brillouin zone were used for all compounds.

Powder SHG measurements were performed by using a modified Kurtz NLO system.²⁹ Ground polycrystalline samples were sieved into the particle size range of <20, 20–45, 45–63, 63–75, 75–90, 90–125, 125–150, 150–200, and 200–250 μm to investigate the correlation between SHG responses and particle sizes. The sieved samples were packed into the respective capillaries (i.d. = 1.8 mm and o.d. = 3.00 mm) and irradiated through a DAWA Q-switched Nd:YAG laser (1064 nm). The SHG light (532 nm) was collected to a Hamamatsu photomultiplier tube and detected using a Tektronix TDS-2012C oscilloscope.

The photoluminescence (PL) properties of LiEu(SO₄)₂ were investigated by a Jasco FP-8550 with a xenon lamp at room temperature. The excitation spectrum under 613 nm emission was collected in the range of 200–500 nm to obtain charge transfer bands. And the emission spectrum under 396 nm excitation was collected in the range of 550–750 nm. The photoluminescence quantum yield (PLQY) and lifetime were measured by using an integrating sphere with a 396 nm excitation wavelength. The decay curve was recorded for 613 nm and the single exponential function was used to calculate the lifetime.³⁰

Results and discussion

Structural description

Although it was reported that LiEu(SO₄)₂ crystallized in the polar NCS orthorhombic space group, Pnn2 (no. 34),³¹ our SCXRD analyses indicate that the three title compounds with similar structures crystallize in the nonpolar NCS tetragonal space group, P4̄n2 (no. 118) (Table 1). Therefore, the structure of LiY(SO₄)₂ will be described here as a representative for the whole title compounds. The three-dimensional framework of LiY(SO₄)₂ consists of YO₈ dodecahedra, SO₄ tetrahedra, and LiO₄ tetrahedra. In an asymmetric unit, one Li⁺ cation, one Y³⁺ cation, one S⁶⁺ cation, and two O²⁻ anions are present (Fig. 1). Each Y³⁺ cation is coordinated by four O(1) and four O(2) atoms to form YO₈ dodecahedra with the Y–O(1) and Y–O(2) distances of 2.2921(16) and 2.4449(15) Å, respectively. While S⁶⁺ cation is linked by two O(1) and two O(2) in SO₄ tetrahedral environment with the S–O(1) and S–O(2) lengths of 1.4622(16) and 1.4839(14) Å, respectively, Li⁺ cation interacts with four O(2) with the Li–O(2) contact distance of 1.9172(15) Å in the LiO₄ tetrahedral moiety (Fig. 1a and Table S1, ESI†). Fundamental building blocks (FBBs) of LiY(SO₄)₂ consist of one YO₈ dodecahedron and six corner- and edge-sharing SO₄ tetrahedra through oxygen atoms. The helical structure of FBBs can be easily identified by the corner-shared SO₄ tetrahedra linked in the same positions along the b-axis. Further linkage of six sulfate groups in each chiral unit to another YO₈ dodecahedron results in a pseudo-layered structure where YO₈ and SO₄ units are in an alternately parallel arrangement (Fig. 1b and c). Within the assembled pseudo-layered backbones, eight-membered rings (8-MRs) composed of four YO₈ and SO₄ are observed, in which the rings rotate in the clockwise direction and grow up helically along the c-axis. Li⁺ cations reside in channels constructed by the stacked rings along the c-direction (Fig. 2a). The structure can be also described as an anionic framework composed of LiO₄ and SO₄ tetrahedra with charge balancing Y³⁺ cations (Fig. 2b). The whole crystal constructs an isotropic and nonpolar structure because the helical structure does not make any net polarization, which is consistent with the compound's nonpolar space group. Although LiGd(SO₄)₂ and LiEu(SO₄)₂ reveal the same crystal structures as that of LiY(SO₄)₂, their unit cell volumes are 2.72% and 3.62% larger, respectively, than that of LiY(SO₄)₂. Bond valence sum (BVS) calculations performed on Li⁺, Y³⁺, Gd³⁺, Eu³⁺, S⁶⁺, and O²⁻ for all three compounds result in values of 1.12–1.18, 3.13, 3.26, 3.24, 5.99–6.01, and 2.02–2.07, respectively.

Table 1 Crystallographic data for LiRE(SO₄)₂ (RE = Y, Gd, and Eu)

a R(Fo) = ∑‖Fo| − |Fc‖/∑|Fo|. b R w(Fo^2) = {∑w[(Fo)^2 − (Fc)^2]^2/∑w[(Fo)^2]^2 }^1/2.
Compound	LiY(SO4)2	LiGd(SO4)2	LiEu(SO4)2
fw	287.97	356.31	351.02
Space group	P4̄n2 (no. 118)	P4̄n2 (no. 118)	P4̄n2 (no. 118)
a = b (Å)	7.57390(10)	7.6335(3)	7.6490(2)
c (Å)	5.4854(2)	5.5514(3)	5.57730(10)
V (Å^3)	314.664(14)	323.48(3)	326.312(18)
Z	2	2	2
D calc. (g cm^−3)	3.039	3.656	3.573
λ (Å)	0.71073	0.71073	0.71073
T (K)	298.0(2)	298.0(2)	298.0(2)
R(Fo)^a	0.0096	0.0072	0.0072
R w(Fo^2)^b	0.0273	0.0178	0.0188
Flack x	0.001(5)	0.032(12)	0.004(10)

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Fig. 1 (a) ORTEP (50% probability ellipsoids) drawing for each polyhedra, YO₈ and SO₄ in the FBB of LiY(SO₄)₂. (b) Polyhedral representation of an alternately repeating pseudo-layer with parallel YO₈ dodecahedra and SO₄ tetrahedra of LiY(SO₄)₂ in the bc-plane (orange, Y; yellow, S; and red, O). (c) Each chiral FBB of LiY(SO₄)₂ is presented.

Fig. 2 (a) Polyhedral representation of LiY(SO₄)₂ in the ab-plane (grey, Li; orange, Y; yellow, S; and red, O). Eight-membered rings (8MRs) composed of four YO₈ and SO₄ are observed. (b) A structural representation of LiY(SO₄)₂ with an anionic framework composed of LiO₄ and SO₄ tetrahedra with charge balancing Y³⁺ cations.

UV-Vis diffuse reflectance spectroscopy

UV-Vis diffuse reflectance spectra of all three title compounds reveal reflectance at ca. 230–245 nm. While LiY(SO₄)₂ and LiGd(SO₄)₂ reflect more than 60% of light from 900 nm to 185 nm, LiEu(SO₄)₂ absorbs ca. 83% of light at 230 nm (Fig. 3). The large band gaps of the compounds may be attributed to the combination of the nonconjugated SO₄ tetrahedra, alkali metal containing LiO₄ tetrahedra, and the polyhedra of rare earth metal cations. The observed red-shift of the UV cutoff edge for LiEu(SO₄)₂ might be originating from the charge transfer between O 2p orbitals and partially filled Eu³⁺ 4f orbitals. Half-filled f⁷ orbitals of trivalent gadolinium cations are much more stable than those of Eu³⁺ with incompletely filled f⁶ orbitals.³²

Fig. 3 UV-Vis reflectance spectra of LiRE(SO₄)₂ (RE = Y, Gd, and Eu).

IR spectroscopy

Three isostructural title compounds exhibit similar IR absorption bands arising from the corresponding bonds (Fig. S2, ESI†). Bands observed at 1000–1150 cm⁻¹ are assigned to S–O bonds of sulfate tetrahedra. And absorption peaks at 650–700 cm⁻¹, 400–500 cm⁻¹, and 600–650 cm⁻¹ may be originating from the RE–O and Li–O bonds, respectively. The assignment of the observed vibrations are consistent with the reported data.³³

TGA

TGA indicates that all three title compounds exhibit no significant weight loss up to ca. 800 °C (Fig. S3, ESI†). However, PXRD data for samples measured after heating to high temperature indicate that LiY(SO₄)₂ and LiRE(SO₄)₂ (RE = Gd and Eu) start decomposing at 650 and 820 °C, respectively. For LiGd(SO₄)₂ and LiEu(SO₄)₂, thermally decomposed main residues turned out to be Gd₂O₂SO₄ and Eu₂O₂SO₄, respectively, along with some Li₂SO₄ based on PXRD (Fig. S4, ESI†).³⁴ Based on the similar PXRD pattern, the major thermal decomposition products of LiY(SO₄)₂ are assumed to be Y₂O₂SO₄ and Li₂SO₄ although the diffraction pattern of Y₂O₂SO₄ has not been reported. Higher decomposition temperatures of LiGd(SO₄)₂ and LiEu(SO₄)₂ might be attributable to the stronger corresponding RE–O bonds. Also, the title compounds are assumed to decompose by losing the volatile Li slowly at high temperature. For example, LiY(SO₄)₂ starts decomposing at 650 °C; however, the framework of the material is still observed if the material is heated at 1000 °C for 2 h. Only after heating the material at 1000 °C for 14 h, a complete decomposition occurs based on the PXRD patterns (Fig. S5, ESI†).

DFT calculations

Calculations using the DFT method have been performed to study electronic structures and band gaps of the title compounds. By doing so, indirect band gaps of LiY(SO₄)₂, LiGd(SO₄)₂, and LiEu(SO₄)₂ were calculated to be 6.00 eV, 5.87 eV, and 4.52 eV, respectively (Fig. S6, ESI†), in which the corresponding UV cutoff edges are 206.8 nm, 211.4 nm, and 274.5 nm, respectively. The underestimation of calculated band gaps in the PBE functional compared to those of experimental values is well-documented.³⁵ LiEu(SO₄)₂ has a significantly narrower band gap than those of LiY(SO₄)₂ and LiGd(SO₄)₂ because of the energy level difference in 4f orbitals for rare earth metal cations. In Eu³⁺ cations, partially filled 4f⁶ orbitals contribute to both valence band maximum (VBM) and conduction band minimum (CBM) and allow f–f transitions (Fig. S7, ESI†). On the other hand, the energy levels of half-filled 4f⁷ orbitals for Gd³⁺ are in the lower position than that of VBM and thus they are much more stable than those of 4f⁶ orbitals for Eu³⁺. The 2p orbitals of O²⁻ mainly contribute to VBM in all three title compounds.

Powder SHG measurements

SHG properties of the title compounds have been investigated using the Kurtz–Perry powder method. The reported materials exhibit SHG responses of ca. 13 times that of α-SiO₂ and are type-I nonphase-matchable at 1064 nm radiation (Fig. 4). It is not surprising that the title materials consisting of alternately aligned REO₈ dodecahedra, SO₄ tetrahedra, and LiO₄ tetrahedra exhibit mild SHG efficiencies because the net moments effectively cancel in the nonpolar space group. The calculated birefringence of LiY(SO₄)₂ is ca. 0.028 at 532 nm, which also supports well the fact that the title compounds are nonphase-matchable at 1064 nm.

Fig. 4 Plots of SHG versus particle size for LiRE(SO₄)₂ (RE = Y, Gd, and Eu).

PL spectroscopy

PL experiments of LiEu(SO₄)₂ reveal a bright orange emission, which corresponds to the strongest emission at 613 nm induced by 396 nm of excitation (Fig. 5). Specifically, the excitation spectrum emitting 613 nm shows peaks at ca. 287, 299, 320, 363, 382, 396, 417, and 466 nm, which can be assigned to f–f transitions of ⁷F₀ → ⁵G₅, ⁷F₀ → ⁵G₄, ⁷F₀ → ⁵G₃, ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G₂, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, and ⁷F₀ → ⁵D₂, respectively. The broad band occurring at ca. 260 nm is charge transfer arising from O 2p and Eu 4f orbitals (Fig. 5a). The emission spectrum induced by 396 nm exhibits peaks at ca. 592, 613, 658, and 699 nm, which is attributable to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄, respectively (Fig. 5b).^33,36 LiEu(SO₄)₂ reveals a strong external PLQY of ca. 39.8% and long lifetime of ca. 3.6 ms once the material is irradiated by 396 nm radiation (Fig. 5c). The observed bright orange light can be depicted in the CIE 1931 diagram with the emissive coordinates of (0.65, 0.35) (Fig. 5d).

Fig. 5 Photoluminescence (a) excitation and (b) emission spectra, (c) PL decay curve, and (d) CIE diagram for LiEu(SO₄)₂.

Conclusions

Pure polycrystalline samples and large crystals of three ternary lithium rare-earth double sulfates, LiRE(SO₄)₂ (RE = Y, Gd, and Eu) have been synthesized by solid-state reactions and solvothermal methods. The reported isostructural materials crystallizing in the NCS nonpolar space group, P4̄n2, reveal 3D chiral frameworks with REO₈, SO₄, and LiO₄ fundamental building units. LiRE(SO₄)₂ reveal very short UV cutoff windows owing to the rigid SO₄ tetrahedra and REO₈ dodecahedra. LiRE(SO₄)₂ exhibit SHG responses of ca. 13 times that of α-SiO₂ and are type-I nonphase-matchable attributable to the constituting nonpolarizable rare-earth cations and non-π conjugated sulfate tetrahedra. The PL spectrum of LiEu(SO₄)₂ revealed a strong fluorescence excited by light with very short wavelengths under 300 nm due to its short UV cutoff edge.

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: Detailed crystallographic data, experimental and calculated PXRD data, SEM-EDX data, IR spectra, TGA diagrams, band structures, and DOS diagrams. CCDC 2209391–2209393. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qm00983h

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