Designing a novel perovskite-type KCd(NH₂SO₃)₃ with deep-ultraviolet transparency and strong second-harmonic generation response

Abstract

Designing deep-ultraviolet (DUV) nonlinear optical (NLO) materials that simultaneously achieve DUV transparency and strong second-harmonic generation (SHG) remains a significant challenge. Here, we overcome this hurdle by harnessing the versatility of the perovskite (ABX₃) structure to develop a novel DUV NLO crystal. Specifically, we have successfully synthesized a non-centrosymmetric Cd-containing sulfamic compound, KCd(NH₂SO₃)₃, inspired by the perovskite architecture. The strategic incorporation of SO₃NH₂⁻ groups at the X-sites not only induces distorted [CdO₃N₃] octahedra, thereby enhancing the SHG response, but also ensures an ultrawide bandgap, which enables excellent DUV transparency. Consequently, this material exhibits a strong SHG response, approximately 1.1 times that of KH₂PO₄ (KDP), with a DUV cutoff edge below 190 nm. Furthermore, large transparent single crystals (20 × 17 × 5 mm³) can be readily grown via a simple aqueous solution method, underscoring its potential for practical application. This innovative approach offers new design strategies for DUV NLO materials, overcoming the traditional trade-off between SHG response and UV transparency.

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Prof. Min Luo received his B.S. in metallurgical engineering from Central South University, China, in 2011, and his Ph.D. from the Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences in 2016. In the same year, he started his independent career as an assistant professor at FJIRSM. From 2020 to 2022, he was an associate professor at FJIRSM and was promoted to full professor in 2023. He is a recipient of the National Natural Science Foundation of China Excellent Young Scientists Fund. His current research interests include the design, synthesis, and crystal growth of new nonlinear optical materials.

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Introduction

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals are pivotal in generating DUV lasers through direct frequency doubling technology, finding critical applications in advanced instrumentation, micromachining, and biogenetic engineering.^1–6 Essential criteria for these crystals include strong second-harmonic generation (SHG) performance (>1 × KDP) and a short ultraviolet absorption edge (λ_cut-off < 200 nm).⁷ However, these requirements often impose competing constraints, presenting a long-standing challenge in designing materials that achieve both high SHG efficiency and short UV absorption edge. Currently, KBe₂BO₃F₂ (KBBF)³ is the only material suitable for DUV applications, but its reliance on toxic BeO and its layered growth habit limits its broader applicability.⁸ This underscores the vital need to develop novel, practical DUV NLO crystals.

Perovskites (ABX₃), with their flexible lattice structures, inherent stability, and adaptable design capabilities,⁹ have excelled in various domains such as photovoltaics,¹⁰ catalysis,^11,12 and superconductivity.^13,14 Prominent perovskite-type oxides, including BaTiO₃ (λ = 400 nm),¹⁵ LiNbO₃ (λ = 300 nm),¹⁶ and LiTaO₃ (λ = 280 nm),¹⁷ are classic commercial NLO materials, distinguished by their significant SHG effects and their ability to be grown as large crystals. Their robust SHG performance stems from the second-order Jahn–Teller (SOJT) effect,¹⁸ where the distortion of [BX₆] (B = Ti⁴⁺, Nb⁵⁺, Ta⁵⁺) octahedra formed by d⁰ transition metals (TMs) provides active units for SHG. Research, including studies by Ok et al.,¹⁹ has revealed that these distorted octahedra enhance polarizability, promoting non-centrosymmetric (NCS) structures necessary for effective SHG. However, distorted octahedra constructed from d⁰ TMs suffer from d–d transitions that significantly limit the bandgap, preventing classic perovskites from accessing the DUV region.²⁰ Thus, designing distorted octahedra within the perovskite framework to achieve DUV capabilities while maintaining strong SHG presents an intriguing and innovative research avenue.

To tackle this challenge, d¹⁰ transition metals, such as Zn²⁺ or Cd²⁺, emerge as ideal B-site candidates, as they not only induce polar displacements conducive to forming distorted polyhedra but also, owing to their lack of d–d transitions, mitigate red shifts in the absorption edge.^21–23 To further amplify octahedral distortion for enhancing SHG responses and expanding the bandgap, we strategically employed X-site ligands, guided by several considerations. First, according to ligand field theory,²⁴ the coordination of different ligands with central atoms affects electronic configurations, leading to ligand field splitting. This process modifies the length and strength of metal–ligand bonds, creating favorable conditions for further distortion. Second, a greater electronegativity difference between the ligand and the central atom promotes band expansion and reduces the negative impact of TMs on the bandgap.^25,26 Based on these principles, we identified NH₂SO₃⁻ containing mixed anions with strong electronegativity as the optimal X-site ligand. Additionally, NH₂SO₃⁻ features ultra-large HOMO–LUMO gaps (approximately 8.17 eV),²⁷ which should be conducive to further expanding the band gap.^28,29

Furthermore, ensuring the geometric stability of the perovskite structure is paramount, as excessive octahedral distortion can result in structural instability. The octahedral factor (μ) and tolerance factor (t) play crucial roles in evaluating structural stability: stable octahedral formations necessitate a range of 0.44 < μ < 0.90.³⁰ In light of this, we chose Cd²⁺ (μ = 0.48) as the B-site cation, as Zn²⁺ (μ = 0.37) could potentially lead to structural collapse. The tolerance factor, t, serves as an indicator of structural behavior, where a range of 0.89 < t < 1.00 indicates a stable cubic phase. When t is in the range of 0.81 to 0.89, the cubic phase structure can distort into lower-symmetry space groups while maintaining overall structural stability, while t < 0.81 suggests the possibility of collapse.^31,32 Notably, the cubic phase is unsuitable for NLO applications due to its lack of birefringence. Therefore, we identified K (t = 0.88) as the most appropriate A-site cation for NLO materials, ensuring both structural stability and the desired optical properties.

Consequently, with these considerations in mind, we successfully synthesized a novel perovskite-type crystal, polyhedral (NH₂SO₃)₃, exhibiting a substantial SHG response and a short UV cut-off edge enabling DUV access. Notably, this material can be easily grown using a simple aqueous solution evaporation method, yielding large single crystals measuring 20 × 17 × 5 mm³. This breakthrough elegantly reconciles the challenge between achieving strong SHG and a short UV absorption edge, thereby setting a new paradigm for the design of DUV NLO materials.

Experimental section

Reagents

H₂O, KOH (Adamas, ≥99.9%), CdCO₃ (Adamas, 99.9%), and HSO₃NH₂ (Adamas, 99.0%) were used to grow the perovskite-type crystals.

Single-crystal growth

First, 0.1 mol each of KOH and CdCO₃, and 0.3 mol of NH₃SO₃ were weighed, following a 1 : 3 stoichiometric ratio. The beaker was placed on a temperature-controlled heated magnetic stirrer, dissolving NH₃SO₃ in 300 ml of water at 50 °C. Once dissolved, KOH and CdCO₃ were gradually added. The mixture was allowed to react for 2 hours, and then the solution was filtered. Next, the filtered solution was put in an oven at 50 °C for evaporation. When crystal grains were formed, the solution was transferred to a 50 °C water bath and cooled at 0.5 °C per day to grow crystals. As the temperature reached 40 °C, the reduced solute concentration slowed down crystal growth, and impurity ions increased, causing defects. At this stage, the crystals were removed.

Single-crystal X-ray diffraction

A PXS5-B1 continuous zoom stereo microscope was used to meticulously select colorless KCd(NH₂SO₃)₃ crystals. The crystal structure of KCd(NH₂SO₃)₃ was determined using single-crystal X-ray diffraction data collected with a Rigaku Mercury CCD diffractometer and graphite-monochromated Mo-Kα radiation (λ = 0.7103 Å). Additionally, the structure was solved using direct methods in OLEX2 with the SHELXTL program.^33,34

Power X-ray diffraction

The grown small crystals were ground into very fine powder and spread evenly on a glass slide specifically designed for testing. To obtain the powder X-ray diffraction data of KCd(NH₂SO₃)₃ using Cu Kα radiation (λ = 1.540598 Å), the test 2θ range was set from 5° to 60°, the scan step width at 0.01° and the scan speed at 0.1° min⁻¹.

Energy-dispersive X-ray spectroscopy analysis

Microprobe elemental analyses were performed on a field emission scanning electron microscope (FESEM, SU-8010) equipped with an energy dispersive X-ray spectrometer (EDS).

Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) was conducted using a Netzsch STA449F3 simultaneous analyzer under flowing N₂ at a heating rate of 10 °C min⁻¹.

Optical properties

At room temperature, the UV-vis-NIR transmission spectral data of KCd(NH₂SO₃)₃, following a polishing process, were recorded by scanning in the wavelength range of 190–800 nm using a Lambda 950 UV-vis-NIR spectrophotometer (PerkinElmer).

Second-harmonic generation measurements

The polycrystalline SHG signals for KCd(NH₂SO₃)₃ were measured on a 1064 nm solid-state laser with KDP crystals as reference samples. The crystals were ground and sieved into six different sizes: 25–45 μm, 45–62 μm, 62–75 μm, 75–109 μm, 109–150 μm, and 150–212 μm.

Theoretical calculations

Density functional theory (DFT)³⁵ calculations were performed to determine the band structure of KCd(NH₂SO₃)₃. The exchange–correlation energy was modeled using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE)³⁶ functional. The valence electron configurations considered were as follows: K (3s² 3p⁶ 4s¹), Cd (4d¹⁰ 5s²), S (3s² 3p⁴), O (2s² 2p⁴), N (2s² 2p³), and H (1s¹). A plane-wave energy cutoff of 600 eV was applied, and the reciprocal space was sampled with a 2 × 2 × 2 Monkhorst–Pack grid. The HSE06 hybrid functional was employed to refine the band gap and density of states (DOS) calculations for KCd(NH₂SO₃)₃. Nonlinear optical (NLO) coefficients (d_ij) were computed using the method proposed by Rashkeev et al., with further developments by Lin et al.³⁷ The dipole was calculated with the B3LYP/def2svp method implemented by Gaussian 09.

Results and discussion

A high-quality, colorless, and transparent single crystal of KCd(NH₂SO₃)₃ was successfully grown via an aqueous solution method, reaching dimensions of up to 20 × 17 × 5 mm³ (Fig. S1†). The crystal quality was further assessed through high-resolution X-ray diffraction, with the full width at half maximum (FWHM) of the rocking curve measured at 93.6′′ (Fig. S2†) being indicative of its high quality. The powder X-ray diffraction (XRD) patterns exhibited excellent agreement with simulated patterns derived from the crystal structure, confirming the phase purity (Fig. S3†). Energy-dispersive X-ray spectroscopy (EDS) analysis revealed the presence of K, Cd, N, S, and O elements in the crystal, with atomic ratio analysis indicating a stoichiometric ratio of K : Cd : N : S : O of approximately 1 : 1 : 3 : 3 : 9 (Fig. S4†), consistent with the chemical formula. Notably, the crystal maintained its transparency even after exposure to air for several months. Thermal analysis demonstrated excellent thermal stability, with the crystal remaining stable up to 210 °C (Fig. S5†), comparable to most reported sulfamic NLO crystals.^27–29

The crystal structure of KCd(NH₂SO₃)₃, as depicted in Fig. 1(a) and (b), belongs to the non-centrosymmetric hexagonal space group P6₃ (no. 173). In this structure, a significantly polar [CdN₃O₃] octahedron is formed, where three N atoms and O atoms are located on opposite sides, leading to differences in bond lengths. Adjacent octahedra are interconnected by sharing SO₃NH₂⁻ with dipole moments of all octahedra uniformly aligned along the c-axis, thus forming a non-centrosymmetric structural framework. To maintain charge balance within this framework, K⁺ ions fill the gaps, resulting in the three-dimensional crystal structure of KCd(NH₂SO₃)₃.

Fig. 1 (a) The dipole moment of KCd(NH₂SO₃)₃ along the c-axis; and (b) the 3D crystal structure of KCd(NH₂SO₃)₃ in the a–b plane.

The transformation from the classic cubic perovskite structure ABX₃ to KCd(NH₂SO₃)₃ is depicted in Fig. 2 and can be understood through a strategic three-step structural design approach. (1) Inducing octahedral distortion and bandgap expansion: To create a distorted octahedron while widening the bandgap, NH₂SO₃⁻ was selected as the X-site in the BX₆ unit. The presence of highly electronegative N and O atoms provides varied coordination environments for the B-site cation, facilitating distortion. Additionally, the large HOMO–LUMO gap of NH₂SO₃⁻ (approximately 8.17 eV)²⁷ contributes to an extensive ultraviolet transparency window for the material. (2) Stabilizing the octahedral model and avoiding red shift: The selection of the B-site ion is crucial for forming a stable B(NH₂SO₃)₆ octahedral configuration with NH₂SO₃⁻ while preventing the cutoff edge red shift associated with the second-order Jahn–Teller effect in d⁰ metals. Using the octahedral factor calculation, Cd²⁺ (μ = 0.48) was chosen from d¹⁰ metals as the B-site cation. Both N and O atoms in NH₂SO₃⁻ can form covalent bonds with Cd²⁺, resulting in a distorted [CdN₃O₃] octahedral unit with Cd–O and Cd–N bond lengths of 2.303(4) Å and 2.371(5) Å, respectively. As each atom originates from the tetrahedral NH₂SO₃⁻, the eventual BX₆ unit formed is Cd(NH₂SO₃)₆. (3) Ensuring structural stability and crystal system modification: To ensure the stability of the ACd(NH₂SO₃)₃ structure and avoid crystallization in a non-birefringent cubic crystal system, K (t = 0.88) was selected as the A-site cation. This choice was guided by electroneutrality principles and tolerance factor calculations (Fig. 3). The reduction in the tolerance factor facilitates the transition of the cubic perovskite of ABX₃ into a monoclinic phase, thus enabling changes in the crystal system. Ultimately, KCd(NH₂SO₃)₃ was realized on the perovskite framework, maintaining the quintessential ABX₃ structure.

Fig. 2 (a) The BX₆ ball-and-stick model; (b) BX6 octahedron; (c) the perovskite ABX₃ cubic framework; (d) the CdN₃O₃ ball-and-stick model; (e) Cd(NH₂SO₃)₆ as a combination of octahedral and tetrahedral structures in the BX₆ unit; and (f) the KCd(NH₂SO₃)₃ monoclinic prismatic distorted perovskite framework.

Fig. 3 The tolerance factor for each alkali metal (Li, Na, K, Rb, and Cs) and the maintained reasonable range for phase transition and structural stability.

The ultraviolet transmission spectrum of KCd(NH₂SO₃)₃ reveals an exceptionally short UV cutoff edge below 190 nm (Fig. 4a), corresponding to a large bandgap of 6.5 eV, which is in excellent agreement with the HSE06-calculated value of 6.22 eV (Fig. S6†). According to the anionic group theory, the formation of a large bandgap is primarily attributed to the HOMO–LUMO gap of the group. Our calculations on NH₂SO₃⁻ reveal a HOMO–LUMO gap of approximately 8.17 eV, significantly larger than 6.5 eV, indicating that sulfamate salts possess a distinct advantage of wide ultraviolet transmittance. This is further corroborated by a series of examples of sulfamate salts.

Fig. 4 (a) The UV/vis diffuse-reflectance spectrum of KCd(NH₂SO₃)₃; the inset shows the wafers used for testing. (b) Powder SHG measurement at 1064 nm. (c) Calculated SHG coefficients of KCd(NH₂SO₃)₃. (d) PDOS analysis for KCd(NH₂SO₃)₃.

To gain insight into the mechanism underlying the formation of this large bandgap, we further calculated the projected density of states (PDOS). As shown in Fig. 4d, the conduction band minimum of KCd(NH₂SO₃)₃ is predominantly composed of Cd-s states, while the valence band maximum is jointly occupied by O-p and N-p states, suggesting that the bandgap is primarily determined by the Cd(NH₂SO₃)₆ octahedron. Notably, our previous studies have shown that the –NH₂ group exhibits a stronger electron-withdrawing effect compared to O, which may effectively confine the electron cloud around Cd, resulting in bandgap widening. This is likely the primary reason for the large bandgap in KCd(NH₂SO₃)₃.

The non-centrosymmetric structure of KCd(NH₂SO₃)₃ encouraged us to explore its SHG properties using the Kurtz–Perry method,³⁸ with KDP as a reference sample. As depicted in Fig. 4b, KCd(NH₂SO₃)₃ exhibited distinct phase-matching characteristics during frequency doubling from 1064 nm to 532 nm, accompanied by a significant SHG effect (1.1 × KDP). Theoretical calculations revealed that the largest NLO coefficient, d₃₃, is −0.47 pm V⁻¹, which is 1.2 times that of KDP (0.39 pm V⁻¹), closely corroborating the experimental results. To elucidate the mechanism underlying its NLO coefficient, we calculated the SHG density map. As shown in Fig. S7,† the SHG density is predominantly concentrated around Cd²⁺ and SO₃NH₂⁻, indicating that the SHG effect is mainly attributed to the [Cd(NH₂SO₃)₆] octahedra, whereas the K⁺ ions contribute negligibly. We have compiled most of the known UV/DUV Cd-based NLO materials, as shown in Table S6.† To the best of our knowledge, this may be the largest crystal size grown for a Cd-based DUV NLO material with large SHG effects.

To further elucidate the impact of octahedral distortion on SHG, we performed theoretical calculations on the degree of distortion, dipole moment, and hyperpolarizability of the octahedron. The standard deviation method (, where L_i is the edge length of the octahedron, and L̄ is the average value of the edge lengths) was employed to quantify the degree of distortion, as shown in Fig. S8,† when the three O–Cd–N bond angles increased from 169.007° to 180°. Also, as calculated from the data in Fig. S8† using the above formula, the degree of distortion decreased from 0.2174 to 0.0482. Concomitantly, the dipole moment decreased from 7.5713 D to 6.96 D, and the hyperpolarizability decreased from 667.01 to 512.71 (Table S5†). These findings clearly demonstrate that reducing the degree of distortion diminishes the polarity of the octahedron and corresponds to a decrease in hyperpolarizability. In contrast, an increase in the degree of distortion contributes to the enhancement of hyperpolarizability, leading to improved SHG response, which is in line with our design concept. Notably, the arrangement of units also plays a crucial role in the SHG effect of NLO crystals. In the case of KCd(NH₂SO₃)₃, the consistent alignment of [CdN₃O₃] octahedra results in a dipole moment of 17.6877 D along the c-axis, and the uniform arrangement of distorted octahedra provides favorable conditions for large SHG effects.

Conclusions

In conclusion, we have successfully designed and synthesized a novel non-centrosymmetric compound, KCd(NH₂SO₃)₃, based on a perovskite framework. Notably, this material exhibits a desirable balance of properties, featuring a deep ultraviolet (DUV) absorption edge shorter than 190 nm and a strong second-harmonic generation (SHG) response of 1.1 × KDP. These excellent properties can be attributed to the synergistic effect of distorted [CdO₃N₃] octahedra and SO₃NH₂⁻ groups. Furthermore, we have successfully grown large, high-quality crystals, achieving a transparent single crystal with dimensions of 20 × 17 × 5 mm³. The combination of excellent optical properties and good growth habits makes KCd(NH₂SO₃)₃ a promising new candidate for DUV nonlinear optical (NLO) crystals. More importantly, our design approach provides a new and feasible method for exploring DUV NLO crystals, opening up new avenues for the development of advanced optical materials.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article [and/or its ESI†].

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22471271, 22222510, 21921001, 52302008); the Natural Science Foundation of Fujian Province (2023J02026); the Self-deployment Project Research Program of Haixi Institutes, Chinese Academy of Science (CXZX-2022-JQ01, CXZX-2022-GH06), and the Youth Innovation Promotion Association CAS (Y2023082).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic and structure data (Tables S1–S4), and figures containing the measurement results and detail of KCd(NH₂SO₃)₃ (Fig. S1–S7). CCDC 2393438 for KCd(NH₂SO₃)₃. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi03043e

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