Since the advent in the last century, lasers have played an irreplaceable role in various frontier and industrial fields, due to their superior performances in brightness, directionality, monochromaticity and coherence [1-4]. Thereinto, all-solid-state lasers with characteristics of a high power density and a long working lifetime are important coherent light sources [5-7]. Thus, as a core component of all-solid-state lasers, nonlinear optical (NLO) crystals, especially short-wave ultraviolet (UV) NLO ones, with excellent properties are urgently needed [8-11]. As a good short-wave UV NLO crystal, it should possess a short absorption cutoff edge less than 280 nm, a stronger NLO response than that of KH₂PO₄ (KDP), and a moderate birefringence (0.050–0.100) to counteract the phase-mismatch resulting from the refractive index dispersion [12-15]. Sulfates with a wide UV transparency window always exhibit a strong NLO response, which become a promising short-wave UV NLO candidate [16-18]. However, they simultaneously exhibit an insufficient birefringence, which is hardly enough to satisfy the phase-matching condition, owing to the low polarizability anisotropy of the sulfate group [19-21]. Hence, it is necessary to develop strategies to improve sulfate's birefringence.

In recent years, a few related methods have been proposed. By introducing the strong electronegative group of [NH₂]⁻ to the [SO₄]²⁻ one, the SO₂(NH₂)₂ crystal with a moderate birefringence (0.066@589.3 nm) was explored [22]. The (NH₄)₂SO₃S crystal with [SO₃S]²⁻ heteroleptic groups could also exhibit an appropriate birefringence (0.077@546 nm) by replacing the O atom with the S atom in SO₄ [23]. After combining the distorted [SbO₄F₂]⁷⁻ octahedral group, the CsSbF₂SO₄ crystal with a large birefringence (0.112@1064 nm) was reported [24]. Because of the integration of the [BO₃]³⁻ triangular group [25,26], the (NH₄)₂B₄SO₁₀ crystal exhibits a moderate birefringence (0.053@1064 nm) [27]. In general, these methods could be summarized into two strategies, including the construction of heteroleptic tetrahedral groups to improve the configuration and the introduction of other groups with a large polarizability anisotropy.

Here, both strategies are adopted simultaneously to sharply improve the birefringence (Fig. 1). First, the dimeric heteroleptic tetrahedral group, [S₃O₆]²⁻, is designed. The dimerizing mode that uniformizes two heteroleptic groups could provide an innate uniformity, which is conducive to bringing about a large birefringence. Second, the organic triangular group, [C(NH₂)₃]⁺, is introduced. Its large polarizability anisotropy has been confirmed by a series of works, making for a large birefringence [28-31]. Thus, [C(NH₂)₃]₂S₃O₆ (G₂S₃O₆) crystal was designed and investigated.

The G₂S₃O₆ crystals were grown by an ion exchange method with raw materials of [C(NH₂)₃]₂CO₃ (aladdin, 99%) and Na₂S₃O₆ (synthesized). The Na₂S₃O₆ was obtained by a redox method with Na₂S₂O₃ (Ourchem, 98.5%) and hydrogen peroxide solutions. Single crystal X-ray diffraction data were collected by a Rigaku Mercury single crystal diffractometer. Then, the crystal structure was established and refined by OLEX2 program [32], with detailed crystal data and structure refinement as listed in Table S1 (Supporting information). Powder X-ray diffraction patterns were collected by a Rigaku Mini-flex 600 diffractometer. UV diffuse reflectance spectra were obtained by a PerkinElmer Lamda-950 spectrometer with BaSO₄ as the reference. The birefringence was studied on a ZEISS Axio Scope A1 polarizing microscope [33]. NLO responses were measured at different particle sizes by Kurtz-Perry method [34]. Based on the density functional theory, electronic structures and optical properties were calculated by CASTEP package [35]. The exchange-correlation potential was calculated by Perdew-Burke-Ernzerhof functional, based on the generalized gradient approximation [36]. By the norm-conserving pseudopotential, the interaction of the electrons with the ion cores was analyzed [37]. The NLO coefficients and second harmonic generation (SHG)-weighted electron density at zero frequency limit were calculated [38,39].

The G₂S₃O₆ crystals crystallize in an asymmetric monoclinic space group of Pc (No. 7). Their purity was confirmed by the X-ray diffraction pattern (Fig. S1 in Supporting information). As shown in Figs. 2a and b, the structure of G₂S₃O₆ crystals is characterized by a wrinkled layer that is constituted by organic triangular groups of [C(NH₂)₃]⁺ and arcuate heteroleptic tetrahedral ones of [S₃O₆]²⁻ with multiple hydrogen bonds. Thereinto, three oxygen atoms bonding with the S2 atom in the [S₃O₆]²⁻ group additionally link three [C(NH₂)₃]⁺ groups by O···H—N hydrogen bonds. Analogously, in the same [S₃O₆]²⁻ group, two oxygen atoms associating with the S1 atom further connect three [C(NH₂)₃]⁺ groups in the adjacent layer. Hence, the powerful covalent bonds of S2-S3 (the S3 atom in [S₃O₆]²⁻ group is the bridged one) become the interlayer force, which could entirely eliminate the layered habit (Fig. 2c).

Theoretically, the multi-layered structure of the G₂S₃O₆ crystal could result in an improved optical performance. Hence, the linear and nonlinear optical properties of G₂S₃O₆ crystal were investigated. It exhibits a short absorption cutoff edge around 212 nm (corresponding to a band gap of 4.84 eV), as shown in the UV diffuse reflection spectrum (Fig. 3a), which satisfies the precondition for the 266 nm coherent output. Further, the birefringence was also examined, as shown in Fig. 3b and Fig. S2 (Supporting information). The sample with a thickness of 128.77 µm exhibits an optical path difference of 12.44 µm, indicating a corresponding birefringence of 0.097@546 nm, which is close to the calculated one of 0.113@546 nm. This large birefringence could manifest the strong phase-matching ability. Then, the NLO response was also investigated. As shown in Figs. 3c and d, the I-type phase-matching behaviors were verified under 1064 nm and 532 nm lasers, respectively. Simultaneously, a large NLO response of 1.4 × KDP under a 1064 nm laser was obtained, which was further confirmed by the theoretical results, as listed in Table S2 (Supporting information).

In terms of the excellent optical properties, the structure-property relationship was thoroughly explored. First, the electronic structure and density of states were investigated (Fig. S3 in Supporting information). The G₂S₃O₆ crystal is a direct band gap compound, with the top of the valence band mainly dominated by N-2p, S-3p and O-2p orbitals, and the bottom of the conduction band mainly constituted by O-2p and S-3p orbitals. It indicates the strong interactions of S-S and S-O bonds in the [S₃O₆]²⁻ groups, which are further confirmed by the electron localization function (Fig. S4 in Supporting information) with a high electron density in the vicinity of S-S and S-O bonds. Besides, the electron density of C—N bonds is also apparent, manifesting the great mutual effect. Thus, the cooperation of [C(NH₂)₃]⁺ and [S₃O₆]²⁻ groups makes for the excellent optical properties. As mentioned before, [C(NH₂)₃]⁺ and [S₃O₆]²⁻ groups with a large polarizability anisotropy are expected to bring about a sufficient birefringence. Here, their arrangements are specially discussed. For [C(NH₂)₃]⁺ groups, they mainly make up the wrinkled layers with an intralayer angle of 28.46° and a layer spacing of 2.75 Å (Fig. 4a), for [S₃O₆]²⁻ groups, their arrangement could also be taken as layers perpendicular to the guanidine ones with an intralayer angle of 60.45° and a layer spacing of 3.57 Å (Fig. 4b). This perfect configuration is in favor of a great optical anisotropy, and then a decent birefringence. At the same time, the collaboration of [C(NH₂)₃]⁺ and [S₃O₆]²⁻ groups also results in the strong NLO response. As shown in Fig. 5, the SHG-weighted electron density of the virtual electron (VE) process, the electrons concentrate on [C(NH₂)₃]⁺ and [S₃O₆]²⁻ groups in both occupied and unoccupied states, indicating their contribution to the NLO response.

In order to improve the birefringence of short-wave UV NLO sulfates, a novel semi-organic sulfate, G₂S₃O₆, was obtained by uniting the heteroleptic tetrahedral group and the triangular one. It exhibits a wrinkle-layered structure with excellent optical properties, including a short UV absorption cutoff edge, a strong NLO response, and a remarkable birefringence. These originate from the cooperation of [C(NH₂)₃]⁺ and [S₃O₆]²⁻ groups. Consequently, this work provides an effective approach for enhancing the birefringence of UV NLO sulfates. It will favor the exploitation of new short-wave UV NLO crystals.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We sincerely thank professor Shilie Pan and Zhihua Yang for theoretical calculations. This work was supported by the National Natural Science Foundation of China (Nos. 52172009 and 12275274), the Science and Technology Project of Fujian Province (Nos. 2021J01519 and 2021H0043), and the Instrument Developing Project of Chinese Academy of Sciences (No. YJKYYQ20210033).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109730.