LaAeAl₃S₇ (Ae = Ca, Sr): Cairo pentagonal layered thioaluminates achieving a good balance between a strong second harmonic generation response and a wide bandgap

Abstract

Breaking through the incompatibility between a strong second harmonic generation (SHG) response and a wide bandgap in an infrared nonlinear optical (IR NLO) crystal is still a huge challenge. With this in mind, we have proposed a feasible design strategy involving rational combination of highly electropositive rare-earth (Re³⁺) and alkaline-earth metals (Ae²⁺) as cations and a strongly covalent AlS₄ anionic group as the “NLO-active unit” into the crystal structure, which affords the successful synthesis of two new quaternary IR NLO thioaluminates: LaAeAl₃S₇ (Ae = Ca, Sr). Note that the unprecedented Cairo pentagonal (AlS₄)_n layers in LaAeAl₃S₇ can be viewed as the first discovery among the structures of all reported thioaluminates and this layered structure benefits from the strong optical anisotropy that further achieves the imperative phase matchability in LaAeAl₃S₇. Both of them possess the widest optical bandgaps (Ca: 3.76 and Sr: 3.78 eV) in known rare-earth NLO chalcogenides. Remarkably, LaAeAl₃S₇ were also proven to be the first cases concurrently exhibiting wide bandgaps (>3.5 eV) and strong SHG effects (>0.5 × AgGaS₂) among known rare-earth NLO chalcogenides. Theoretical analysis verifies that their excellent NLO properties originate from the synergistic effect between AlS₄ and (La/Ae)S₈ anionic groups. This work will inspire exploration into new IR NLO candidates in rare-earth thioaluminate systems to achieve a superior property balance.

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Introduction

Nonlinear optical (NLO) crystals have shown extensive applications in tunable laser systems.^1–15 As for an excellent infrared (IR) NLO crystal, it should satisfy the following performance conditions: a wide IR transmission region, strong second harmonic generation (SHG) response, large optical bandgap, high laser damage threshold (LDT), and reliable chemical stability.^16–18 Unfortunately, there is still the undesirable incompatibility between the strong SHG response and wide bandgap in IR NLO crystals because of their inherent inverse relationship,^19,20 which is also reflected in several commercial crystals such as AgGaQ₂ (Q = S, Se)²¹ and ZnGeP₂.²² They exhibit a good SHG response but relatively narrow energy bandgaps and low LDTs, which further limit their application. Based on this, a good balance between a strong SHG response (d_ij > 0.5 × AgGaS₂) and wide bandgap (E_g > 3.5 eV) has become an urgent challenge for the discovery of new excellent IR NLO crystals and many researchers have proposed several effective design strategies and research systems.^23–29 For example, mixed alkali/alkaline-earth metal sulfides possess wide bandgaps but relatively small SHG effects; introduction of halogen ions into the structures of typical chalcogenides means the mixed-anion chalcohalides can achieve a suitable property balance but the halides will corrode and destroy the silica tubes and further enhance the difficulty of the material synthesis. Considering the above status, we have focused on other research systems and thioaluminates exhibit native advantages to obtain a wide bandgap because their strongly covalent Al–S bond has a minor influence on optical absorption. However, up to now, thioaluminates have rarely been studied in NLO and only a few NLO thioaluminates have been reported for their NLO performances such as Al_0.50Dy₃(Si_0.50Al_0.50)S₇ (2.22 eV and 2 × KTiOPO₄ (KTP)), Al_0.38Dy₃(Si_0.85Al_0.15)S₇ ^30,31 (2.03 eV and 1 × KTP), BaAl₄S₇ ³² (3.95 eV and 0.5 × AgGaS₂) and LiAlS₂ ³³ (5.13 eV and 0.2 × AgGaS₂). Besides, the inherent relationship between the AlS₄ anionic group and the SHG origin has not been clearly determined so far. Recent study indicates that rare-earth centered polyhedral ReS_n units make a great contribution to the origin of the NLO response, therefore, rare-earth based thioaluminates appear to have a high potentiality to break the incompatibility between the critical performances (E_g and d_ij) based on the synergistic effect between ReS_n and AlS₄ units. With this in mind, we have proposed one design strategy involving rational combination of rare-earth (La³⁺) and alkaline-earth metals (Ae²⁺) as cations into the structures of thioaluminates to achieve the birth of two new rare-earth thioaluminates: LaAeAl₃S₇ (Ae = Ca, Sr). Both of them exhibit the first examples of Cairo pentagon layers composed of AlS₄ tetrahedra in reported thioaluminates after a detailed survey of the Inorganic Crystal Structure Database (ICSD) (Fig. 1a) (Table S1†). Their performances were systematically measured and the results show that LaAeAl₃S₇ were proven to be promising IR NLO materials because of the successful breakthrough in the incompatibility between the large E_g (>3.5 eV) and strong d_ij (>0.5 × AgGaS₂). First-principles calculation analysis demonstrates that their SHG responses originate from the synergistic effect between AlS₄ and (La/Ae)S₈ units. Moreover, we have also summarized the critical properties (E_g and d_ij) in all known rare-earth chalcogenides (Fig. 1b) (Table S2†) and the survey results show that LaAeAl₃S₇ could be regarded as the first cases achieving the breakthrough of the “3.5 eV wall” among all reported rare-earth NLO chalcogenides.^31,34–48

Fig. 1 (a) Dimensional distribution of AlS₄ units in the known thioaluminates (Table S1†); (b) a summary of the SHG response and bandgap among the title LaAeAl₃S₇ and reported rare-earth NLO chalcogenides, the well-balanced area is limited between E_g > 3.5 eV and d_ij > 0.5 × AgGaS₂. Compounds 1–21 are listed in Table S2.†

Experimental methods

Synthesis

All raw materials, including La₂S₃ powder (99.99%), Al slice (99.99%), CaS and SrS powder (99.99%), and S powder (99.99%), were purchased from Beijing Hawk Science & Technology Co., Ltd. As for air-unstable La₂S₃, SrS and CaS powder, an Ar-filled glovebox was selected to complete the whole preparation process.

Single crystals of LaAeAl₃S₇ were firstly synthesized with a non-stoichiometric ratio based on the raw materials of La₂S₃, CaS/SrS, Al, S = 0.37 : 1 : 3 : 3. However, under this ratio, the yield of LaAeAl₃S₇ was very low and many AeAl₂S₄by-products were found. Thus, we gradually adjusted the proportion of raw materials after many attempts and the maximum yield (>90%) of LaAeAl₃S₇ was obtained under the optimal nonstoichiometric ratio of La₂S₃, CaS/SrS, Al, S = 0.56 : 1 : 3 : 3. Raw materials were loaded into vacuum-sealed silica tubes and then put into a temperature-programmed furnace with the following temperature controlling curves: heated up to 1473 K within 30 h and held for 90 h, then cooled to room temperature within 150 h. The transparent and colorless LaAeAl₃S₇ single crystals were obtained.

Single crystal X-ray diffraction

Selected high-quality crystals were used for data collection on a Bruker D8 VENTURE diffractometer using Mo Kα radiation (λ = 0.71073 Å) at room temperature. A multi-scan method was used for absorption correction. The crystal structures were solved by a direct method and refined using the SHELXTL program package. After the first refinement, the formula was firstly refined to be the unbalanced “La₂Al₃S₇”. The occupancy of La and Ca atoms in one site appeared to be 0.49 : 0.51 after the first random refinement. In order to obtain the balanced formula, we defined the actual occupancy of La and Ca atoms to be 0.5 : 0.5 and the final balanced formula is LaCaAl₃S₇. Similarly, this refinement process was extended to those of LaSrAl₃S₇ in this work. Rational anisotropic thermal parameters for all atoms were obtained by the anisotropic refinement and extinction correction. Detail refinement parameters and crystal data are shown in Table S3.†

Powder X-ray diffraction

Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D2 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The 2θ range was 10–70° with a step size of 0.02° and a fixed counting time of 1 s per step. Note that the calculated XRD patterns were derived from the respective single-crystal data. We have also carefully investigated the experimental XRD patterns of the title compounds and compared the extra peaks with those of other known related compounds.

UV–Vis–Near-IR (NIR) diffuse-reflectance spectra

Diffuse-reflectance spectra were measured by a Shimadzu SolidSpec-3700DUV spectrophotometer in the wavelength range of 200–1100 nm at room temperature.

Raman spectra

Hand-picked crystals were firstly put on a glass slide and then a LABRAM HR Evolution spectrometer equipped with a CCD detector by a 532 nm laser was used to record the Raman spectra.

Second-harmonic generation measurement

Through the Kurtz and Perry method, powder SHG responses were investigated by a Q-switch laser (2.09 μm, 3 Hz, 50 ns) with different particle sizes, including 38–55, 55–88, 88–105, 105–150, 150–200, and 200–250 μm. The as-synthesized AgGaS₂ microcrystals were selected with the same sizes as references.

Theory calculations

In order to further investigate the relationship of structure–property, the electronic structures of the title compounds were studied by density functional theory (DFT) based on ab initio calculations. The exchange–correlation potential was calculated using the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) with the scheme. The following orbital electrons were treated as valence electrons: La: 5s² 5p⁶ 5d¹ 6s²; Ca: 3p⁶ 4s²; Sr: 4p⁶ 5s²; Al: 3s² 3p¹; S: 3s² 3p⁴. To achieve energy convergence, the plane-wave basis set energy cutoff was 660 eV within normal-conserving pseudo-potential (NCP). As key parameters for NLO crystals, the SHG coefficient and birefringence were also calculated. Owing to the discontinuity of exchange correlation energy, the experimental value is usually larger than that of the calculated band gap. Thus, scissors operators are used to make the conduction bands agree with the experimental values and the real-space atom-cutting method was used to analyse the contribution of anionic groups.

Results and discussion

In this work, single crystals of title LaAeAl₃S₇ thioaluminates were synthesized with an optimized nonstoichiometric ratio after many attempts. Submillimeter-level single-crystals were handpicked to be used for the data collection on single-crystal XRD (Table S3†). The phase-purity was verified by a powder XRD technique (Fig. S1†). The experimental PXRD patterns are basically consistent with those of the theoretical ones and a few extra tiny peaks are attributed to the AeAl₂S₄ by-products.

LaAeAl₃S₇ (Ae = Ca, Sr) crystallize in the P4̄2₁m space group of the tetragonal system. In view of their similar structures, LaCaAl₃S₇ was selected as the representative to depict their structural features. La and Ca atoms are located at the one site with the occupied ratio (0.5 : 0.5). One AlS₄ unit is linked to four AlS₄ units to form a [Al₅S₁₆]¹⁷⁻ windmill cluster and these clusters further link together to compose the 2D Cairo pentagonal layers located at the ab plane. (La/Ca)S₈ polyhedra were located within the interlayers to bridge adjacent layers together to compose the overall 3D network. In this work, various link modes of AlS₄ units in structures of thioaluminates were also summarized and most of them (about 80%) possess the 0D link modes after the survey in the ICSD (Fig. 1a) (Table S1†). In addition, only three of the thioaluminates (Rb₄Al₂S₅,⁴⁹ Bi₂Al₄S₈ ⁵⁰ and Cccm-SrAl₂S₄ ⁵¹) exhibit 1D (AlS₄)_n chains but the link modes of the AlS₄ units in the 1D chains are different, for example, AlS₄ units connect with each other by edge-sharing to form similar 1D chains in Bi₂Al₄S₈ (Fig. S2a and b†) and Cccm-SrAl₂S₄ (Fig. S2e and f†), which is different to the way AlS₄ units link together by edge and corner-sharing to form a 1D chain structure in Rb₄Al₂S₅ (Fig. S2c and d†). Note that the ratio of the 3D network is only 9% and seven of them possess 3D networks formed by AlS₄ units. For instance, AlS₄ units link together by corner-sharing to form a 3D network and Ba atoms are located within the 3D tunnels to form the whole structure of BaAl₄S₇ (Fig. S3c†). In particular, Al atoms have two different coordination modes: AlS₄ and AlS₆ units in the structure of Ln₆Al_3.3S₁₄ ⁵² and AlS₆ units link together by sharing faces to form a 1D chain structure and AlS₄ units are existed in isolation (Fig. S3a and b†). Note that 2D (AlS₄)_n layers were also rarely discovered and only a few ternary thioaluminates exhibit 2D layered structures, such as FeAl₂S₄,⁵³ TlAlS₂ ⁵⁴ and AeAl₂S₄ ⁵⁵ (Ae = Ca, Sr). Although they have layered structures, the link modes of their AlS₄ units are different to those in the title LaAeAl₃S₇ (Fig. 2b and e). Therefore, the Cairo pentagonal layers in the title LaAeAl₃S₇ can be viewed as the first discovery in the known thioaluminates. For instance, 6-membered rings (MRs) exist in the structure of CaAl₂S₄ and its interlayer spacing (1.654 Å) is smaller than that of LaCaAl₃S₇ (2.393 Å) (Fig. 2a and d). Besides, AlS₄ is connected with three AlS₄ units in CaAl₂S₄ which is different to the [Al₅S₁₆]¹⁷⁻ windmill cluster in the LaCaAl₃S₇ (Fig. 2c and f). Moreover, one (La/Ca)S₈ is linked to ten AlS₄ units but the inherent link modes are different and not symmetrical, showing local asymmetry in the LaCaAl₃S₇, which is also different to that (local coordination symmetry) in the CaAl₂S₄ (Fig. 2g and h). We have also calculated the distortion degrees (Δd) of (La/Ae)S₈ in the LaAeAl₃S₇ and AeS₈ dodecahedra in AeAl₂S₄ (Table S4†) and the results show that (La/Ae)S₈ have a larger Δd (Sr: 2.763‰; Ca: 2.686‰) in LaAeAl₃S₇ than those of SrS₈ (0.030–0.092‰) and CaS₈ (0.0007–0.140‰) in AeAl₂S₄. Such a large Δd is beneficial to achieve the local coordination asymmetry and promotes a potential structural change from centrosymmetric (CS) AeAl₂S₄ to NCS LaAeAl₃S₇.

Fig. 2 (a) The crystal structure of LaCaAl₃S₇ along the b-axis; (b) the 2D layer is composed of AlS₄ units in LaCaAl₃S₇; (c) the windmill configuration [Al₅S₁₆]¹⁷⁻ cluster; (d) the crystal structure of CaAl₂S₄ along the b-axis; (e) the 2D layer is composed of AlS₄ units in CaAl₂S₄; (f) the connection mode of AlS₄ unit in CaAl₂S₄; (g) the local coordination asymmetry between (La/Ca)S₈ and AlS₄ units in LaCaAl₃S₇; and (h) the local coordination symmetry between CaS₈ and AlS₄ in CaAl₂S₄.

Diffuse-reflectance spectra of LaAeAl₃S₇ were measured and their optical bandgaps are 3.76 for LaCaAl₃S₇ and 3.78 eV for LaSrAl₃S₇, respectively (Fig. 3a and b), which are much larger than that of commercial AgGaS₂ (2.64 eV) and comparable to other famous NLO crystals such as BaAl₄S₇ ³² (3.95 eV), LiZnPS₄ ⁵⁶ (3.44 eV), K₃Ga₃PS₈Cl⁵⁷ (3.60 eV), BaGa₂SiS₆ ⁵⁸ (3.75 eV) and Li₂ZnSiS₄ ⁵⁹ (3.90 eV). Note that LaAeAl₃S₇ exhibit the widest optical bandgaps and they can be also viewed as the first cases to achieve the breakthrough of “3.5 eV wall” in all the reported rare-earth NLO chalcogenides. Analysis into the calculated electronic structures and density of states (DOS) shows that the title LaAeAl₃S₇ are indirect-bandgap compounds and their theoretical bandgaps are 2.505 for LaCaAl₃S₇ and 2.527 eV for LaSrAl₃S₇, respectively (Fig. 3c and d). As can be seen from their DOS diagrams, the top of the valence band (VB) and the bottom of the conduction band (CB) region are mainly occupied by the S-p and La-d with a minor contribution of Al-p orbitals and Ae-s,p orbitals producing a negligible effect on the optical bandgaps. Therefore, optical absorptions in LaAeAl₃S₇ are determined by the inherent electronic transition in La–S units (Fig. 3e and f). Note that the wide bandgap has a huge influence on improving the inherent laser damage threshold (LDT), thus, their LDTs were measured under the 1.06 μm laser with the commercial AgGaS₂ as reference. Both of them have a high laser damage resistance of about 9.0 times that of AgGaS₂, which are comparable to those of IR NLO sulfides such as KYGeS₄ ³⁹ (10 × AgGaS₂), LiGaGe₂S₆ ⁶⁰ (6 × AgGaS₂), Li₂ZnSiS₄ ⁵⁹ (10 × AgGaS₂) and Li_0.6Ag_0.4GaS₂ ⁶¹ (8.6 × AgGaS₂). The measured Raman spectra exhibit no obvious absorption peaks in the wavenumber range from 500 to 4000 cm⁻¹, indicating wide IR transmission ranges (2.5–20 μm) (Fig. 4a and b). Several Raman peaks located at 300–500 cm⁻¹ are attributed to the Al–S bond interaction, such as (334, 374, 423, 500 cm⁻¹) for LaCaAl₃S₇ and (334, 422, 498 cm⁻¹) for LaSrAl₃S₇, which are similar to those of other known thioaluminates, such as K(AlS₂)(GeS₂)⁶² (375 cm⁻¹) and Ba₂AlSbS₅.⁶³ Other peaks located at 200–300 cm⁻¹ belong to the La–S bond vibration, which are similar to those of the previously reported La₂S₃.

Fig. 3 Experimental optical bandgaps of LaCaAl₃S₇ (a) and LaSrAl₃S₇ (b); (c–f) band structures and PDOS of the title compounds.

Fig. 4 Raman spectra of LaCaAl₃S₇ (a) and LaSrAl₃S₇ (b); (c) powder SHG response versus particle size for LaAeAl₃S₇ with AgGaS₂ as reference; (d) calculated birefringences for LaAeAl₃S₇ and AeAl₂S₄; (e) calculated birefringence for Cccm-SrAl₂S₄; and (f) the 1D (AlS₂)_n chain in Cccm-SrAl₂S₄.

Through the typical Kurtz–Perry method, we have investigated the powder SHG responses of LaAeAl₃S₇ with different particle sizes under 2.09 μm pulse laser and the as-synthesized AgGaS₂ crystal as reference was selected. The measured results show that SHG intensity displays a similar growing trend with the increasing particle size. At the maximum particle size (200–250 μm), the title LaAeAl₃S₇ possess large SHG responses about 0.8 times that of AgGaS₂ for LaCaAl₃S₇ and 1.1 × AgGaS₂ for LaSrAl₃S₇, respectively (Fig. 4c), which are comparable to those of wide-bandgap NLO chalcogenides such as LiZnPS₄ ⁵⁶ (0.8 × AgGaS₂), KYGeS₄ ³⁹ (1.0 × AgGaS₂), Li_0.6Ag_0.4GaS₂ ⁶¹ (1.1 × AgGaS₂), Na₂ZnGe₂S₆ ⁶⁴ (0.9 × AgGaS₂) and [Ba₄Cl₂][ZnGa₄S₁₀]⁶⁵ (1.1 × AgGaS₂). In view of the fact that the NLO coefficient (d₃₆) of AgGaS₂ is 13.0 pm V⁻¹,⁶⁶ we have also calculated the theoretical NLO coefficients (d_ij) and their maximal d_ij are 6.83 for LaCaAl₃S₇ and 7.02 pm V⁻¹ for LaSrAl₃S₇, respectively, which are basically consistent with the experimental results. The origin of the NLO effect was analyzed by the SHG-density calculation and the results show that their NLO origin was derived from the synergistic effect between AlS₄ and (La/Ae)S₈ anionic groups (Fig. 5). Note that the SHG responses (0.8–1.1 × AgGaS₂) of LaAeAl₃S₇ are larger than that (0.5 × AgGaS₂) of BaAl₄S₇, which also further verifies that incorporation of lanthanide (Ln) atoms into crystal structures provides a great strategy to enhance the SHG response. Moreover, we have also calculated the birefringence versus wavelength curves for title the LaAeAl₃S₇ and they exhibit a large optical anisotropy (Δn = 0.059 for Ca and 0.077 for Sr@2 μm), such a large Δn also further verifies the rationality of experimental phase-matching (PM) behavior. Herein, we have also calculated the Δn of CaAl₂S₄ (Fddd) and SrAl₂S₄ (Fddd and Cccm) and the results show that Fddd-AeAl₂S₄ exhibit a relatively larger Δn (0.042 and 0.043) than that of Cccm-SrAl₂S₄ (0.026) (Fig. 4d). In general, the whole birefringence has a close relationship with the microscopic anisotropic polarizability of functional groups. In the title LaAeAl₃S₇, coplanar AlS₄ units connect with each other to compose the Cairo pentagonal layers and such layered structures are beneficial to the optical anisotropy. We have also calculated the contribution of anionic groups (AlS₄ and (La/Ae)S₈) on the birefringences of title LaAeAl₃S₇ by the real-space atom-cutting method and the calculated results show that the AlS₄ unit provides the main contribution (∼76%) on the birefringence with a minor contribution (∼24%) of the (La/Ca)S₈ unit in LaCaAl₃S₇, which is similar to those in LaSrAl₃S₇ (AlS₄: 80%; (La/Sr)S₈: 20%). Besides, anisotropic distortions (Δd) of (La/Ae)S₈ units in title LaAeAl₃S₇ are much larger than those of AeAl₂S₄, which also contributes to the improvement of birefringence. Thus, the microscopic addition of AlS₄ and (La/Ae)S₈ groups makes the LaAeAl₃S₇ exhibit a relatively larger optical anisotropy than those of Fddd-AeAl₂S₄ (2D layer), Cccm-SrAl₂S₄ ((AlS₂)_n chain) (Fig. 4e and f) and BaAl₄S₇ (Δn = 0.0328, 3D network) since the contribution of alkaline-earth cations on birefringence is negligible. Therefore, combination of Cairo pentagonal layers and lanthanides into crystal structures is conducive to improving optical anisotropy, which provides a useful structure-directing design for the discovery of new potential PM NLO crystals. To sum up, considering the overall performances of title LaAeAl₃S₇, they exhibit wide bandgaps (E_g: 3.76–3.78 eV), large SHG responses (d_ij: 0.8–1.1 × AgGaS₂), high LDTs (9.0 × AgGaS₂) and good chemical stability, indicating that title LaAeAl₃S₇ could be viewed as the first cases satisfying the excellent property balance (E_g > 3.5 eV and d_ij > 0.5 × AgGaS₂) in rare-earth NLO chalcogenides.

Fig. 5 Calculated SHG-density diagrams in the occupied and unoccupied states of LaAeAl₃S₇.

Conclusions

In summary, two new LaAeAl₃S₇ thioaluminates were firstly synthesized and their measured performances verify them to be potential IR NLO candidates. Synergistic contributions between (La/Ae)S₈ and AlS₄ units afford strong NLO responses in LaAeAl₃S₇. The novel Cairo pentagonal layered structures in LaAeAl₃S₇ have the benefit of improving the optical anisotropy, which provides a structure-directing strategy for the discovery of PM crystals. This study indicates that Ln-based thioaluminates could be expected to be feasible research systems for the breakthrough in the incompatibility between a strong SHG response and a wide bandgap to achieve the imperative property balance.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51872324) and the Natural Science Foundation of Hebei Province (Grant No. E2020201005).

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Footnote

† Electronic supplementary information (ESI) available: Powder XRD, performance summary, crystal data, distortion degree. CCDC 2234185 for LaCaAl₃S₇ and 2234186 for LaSrAl₃S₇, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00048f

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