Ba₃Al₂B₁₂O₂₄: a beryllium-free member of the Sr₂Be₂B₂O₇ family with a ²_∞[B₁₂Al₂O₂₈] double-layered structure

Abstract

Nonlinear optical (NLO) crystals capable of expanding the spectral region of solid-state are of great importance for many high-tech applications, yet their rational structure design remains a great challenge because of the conflicting property requirements among second harmonic generation (SHG) response, ultraviolet (UV) cut-off edge, and birefringence. Herein, based on the chemical disubstitution of the classic NLO crystal Sr₂Be₂B₂O₇ (SBBO), i.e., substituting [BO₃] triangles with larger π-conjugated [B₃O₇] groups and substituting high-toxic [BeO₄] tetrahedra with environment-friendly [AlO₄] tetrahedra, a new high-performance aluminoborate NLO crystal, Ba₃Al₂B₁₂O₂₄, has been successfully designed and synthesized. The theoretical calculations and optical property measurements indicate that Ba₃Al₂B₁₂O₂₄ exhibits not only the largest SHG response among the reported aluminoborates (2.7 × KH₂PO₄), but also a short UV absorption edge (<190 nm) and moderate birefringence. More importantly, Ba₃Al₂B₁₂O₂₄ melts congruently, and a single crystal with a size of 9 × 5 × 2 mm³ has been grown by the top-seeded-solution-growth method. These demonstrate that Ba₃Al₂B₁₂O₂₄ is a promising NLO crystal and the chemical disubstitution is a feasible strategy for the exploration of functional materials.

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Introduction

Nonlinear optical (NLO) materials, as the core components of all-solid-state lasers, play a crucial role in current scientific and technological applications.^1–8 Based on these NLO crystals, ultraviolet (UV) and deep-ultraviolet (DUV; λ < 200 nm) coherent radiation can be directly obtained through cascading second harmonic generation (SHG), which has a variety of applications in laser micromachining, semiconductor manufacturing, photolithography, and high-resolution spectroscopy.^9–15

During the past few decades, borates have been considered a major part of the exploration of UV or DUV NLO crystals because of their excellent structural units, i.e., [BO₃] or [BO₄], and their combinations subsequently result in the formation of a wide variety of structural topologies,^16–18 such as [BO₃] in KBe₂BO₃F₂ (KBBF),¹⁹ [B₃O₆] in β-BaB₂O₄ (β-BBO),²⁰ [B₃O₇] in LiB₃O₅ (LBO),²¹ and [B₃O₇] in CsLiB₆O₁₀ (CLBO).²² These functional units make borate possess not only a short UV cutoff edge, but also a large SHG response in non-centrosymmetric (NCS) structures through their excellent combination and arrangement. Among them, in KBBF, [BO₃] groups with a coplanar configuration and an aligned arrangement are linked by [BeO₃F] to build unique ²_∞[Be₂BO₃F₂] layers, which result in the balance of its short cutoff edge, suitable SHG response and birefringence. These superior optical properties make KBBF the sole NLO crystal to generate DUV coherent radiation light by SHG. However, its strong layering tendency due to the weak interlayer bonding and the high toxicity of the contained beryllium limits the wide applications of KBBF in the DUV region.¹⁹ To overcome the layer habit of KBBF, an effective strategy is to develop borates with reinforced interlayer bonding by introducing stronger B–O/Be–F bonds or developing single layers into double layers.²³ The successful examples include NaSr₃Be₃B₃O₉F₄ (NSBBF),²⁴ Na₂CsBe₆B₅O₁₅ (NCBBO),²⁵ Sr₂Be₂B₂O₇ (SBBO),²⁶ etc. However, the toxicity of beryllium still hinders their applications.²⁷

Aluminoborates have been considered attractive candidates for replacing Be-borates or Be-borate fluorides in UV NLO materials since the tetrahedral coordination environment of the Al³⁺ cation is similar to that of the Be²⁺ cation, and aluminum has lower toxicity than beryllium.²⁸ Based on this, a series of aluminoborates have been synthesized, such as K₂Al₂B₂O₇,^29,30 β-Rb₂Al₂B₂O₇,²⁸ Cs₂Al₂B₆O₁₃,³¹ K₃Ba₃Li₂Al₄B₆O₂₀F,³² Rb₃Ba₃Li₂Al₄B₆O₂₀F,³³ etc. Among them, K₂Al₂B₂O₇ has a transparent window into the DUV region (180–3600 nm) and an appropriate birefringence of Δn = 0.07@589 nm. The shortest Type I phase-matching (PM) wavelength is evaluated to be 232.5 nm, which can be used for the fourth harmonic generation (FHG) of the Nd:YAG laser (1064 nm).³⁴ The results demonstrate the UV NLO application potential of K₂Al₂B₂O₇. As such, we will continue to explore new UV or DUV NLO crystals in aluminoborates.

Through systematic investigations, a new beryllium-free SBBO-like aluminoborate, Ba₃Al₂B₁₂O₂₄, was successfully synthesized. In Ba₃Al₂B₁₂O₂₄, the two-dimensional (2D) ²_∞[B₃AlO₆] single layers are derived from the ²_∞[Be(BO₃)O] single layers of SBBO, in which the [BeO₄] tetrahedra are replaced by [AlO₄] tetrahedra and the [BO₃] triangles are substituted by [B₃O₇] units. The single layers are linked by the one-dimensional (1D) ¹_∞[B₆O₁₃] chains, forming ²_∞[B₁₂Al₂O₂₈] double layers with tunnels, which are filled with the Ba²⁺ cations. Meanwhile, Ba₃Al₂B₁₂O₂₄ exhibits a large SHG response (2.7 × KH₂PO₄ (KDP)), a short absorption edge (<190 nm), and suitable birefringence. Compared to other reported aluminoborates, Ba₃Al₂B₁₂O₂₄ achieves an optimal balance between SHG efficiency and DUV transparency. Notably, congruently melting thermal properties are also observed, and a single crystal with 9 × 5 × 2 mm³ has been grown by a top-seeded-solution-growth (TSSG) technique. These results demonstrate that Ba₃Al₂B₁₂O₂₄ is a promising UV NLO material. In this report, we present the synthesis, crystal structure, thermal behavior, NLO properties, structure–property relationships, and the first-principles calculations of Ba₃Al₂B₁₂O₂₄.

Results and discussion

Pure polycrystalline Ba₃Al₂B₁₂O₂₄ was synthesized through a high-temperature solid-state reaction. The experimental powder X-ray diffraction (PXRD) patterns exhibit good agreement with the calculated results derived from the structural data (Fig. S1). To investigate the thermal behavior of Ba₃Al₂B₁₂O₂₄, thermogravimetry-differential scanning calorimetry (TG-DSC) analysis was performed on the pure polycrystalline sample (Fig. S2). Clearly, there is only one endothermic peak at 840 °C on its DSC curve, and no weight loss is observed on the TG curve. To further confirm its thermal behavior, Ba₃Al₂B₁₂O₂₄ was heated to 850 °C, and the polycrystalline powder sample completely melted. Then the melt was slowly cooled to room temperature. The PXRD patterns of the sample after melting were consistent with those of the pure polycrystalline sample (Fig. S3). These results demonstrate that Ba₃Al₂B₁₂O₂₄ melts congruently. Owing to its congruent-melting behavior, a single crystal of Ba₃Al₂B₁₂O₂₄ with a size of 9 × 5 × 2 mm³ has also been successfully grown from its stoichiometric melt (Fig. S4).

The structure of Ba₃Al₂B₁₂O₂₄ was determined by single-crystal X-ray diffraction (Table S1), which shows that Ba₃Al₂B₁₂O₂₄ crystallizes in the monoclinic space group of Cc (No. 9). The asymmetric unit of Ba₃Al₂B₁₂O₂₄ contains three unique Ba, two unique Al, twelve unique B, and twenty-four unique O atoms (Table S2). All of the Al atoms are four-coordinated, forming [AlO₄] tetrahedra. The B atoms exhibit two different coordination environments: [BO₃] triangles and [BO₄] tetrahedra, which are further connected via corner-sharing to form a [B₂O₅] dimer, a [B₄O₉] group, and two types of [B₃O₇] groups with different symmetries (Fig. 1a). The [B₂O₅] dimers alternately connect with the [B₄O₉] groups to form a ¹_∞[B₆O₁₃] chain (Fig. 1b). The [B₃O₇] rings are linked with the [AlO₄] tetrahedra to create ²_∞[B₃AlO₆] layers, namely A, B, A′, and B′ (Fig. 1c). The arrangement of the ²_∞[B₃AlO₆] single layers in Ba₃Al₂B₁₂O₂₄ is ABA′B′ABA′B′…, in which the A′ (B′) layers and the A (B) layers have a mirror image relationship along the c-axis. Furthermore, the A (A′) and B (B′) layers are connected by ¹_∞[B₆O₁₃] chains to generate ²_∞[B₁₂Al₂O₂₈] double layers (Fig. 1d). These layers are separated by the Ba²⁺ cations. Meanwhile, Ba²⁺ cations also occupy the void space within the double layer. The Ba atoms are in sixfold and tenfold coordination environments, [Ba(1)O₆], [Ba(2)O₁₀], and [Ba(3)O₁₀] polyhedra. In the structure, the B–O distances fall in the range of 1.336(16) to 1.538(18) Å. The Al–O distances range from 1.728(9) to 1.780(9) Å. The Ba–O distances have a range from 2.593(9) to 3.133(9) Å (Table S3). The bond valence sum (BVS) analysis shows that the BVS values of Ba, Al, B, and O are 1.81–2.27, 3.08–3.13, 2.90–3.10, and 1.81–2.17, respectively (Table S2),³⁵ which are in agreement with their ideal oxidation states for each atom.

Fig. 1 (a) Basic building units (BBUs) of Ba₃Al₂B₁₂O₂₄: [B₂O₅], [B₄O₉], [AlO₄], and [B₃O₇] units; (b) ¹_∞[B₆O₁₃] chain of Ba₃Al₂B₁₂O₂₄; (c) ²_∞[B₃AlO₆] layer of Ba₃Al₂B₁₂O₂₄; (d) 2D double-layered structure of Ba₃Al₂B₁₂O₂₄.

From the above structural analysis, Ba₃Al₂B₁₂O₂₄ possesses a similar layered structure to KBBF (single-layer structure) and SBBO (double-layer structures). Therefore, we conducted their structure comparison (Fig. 2). All these crystals feature layered structural units composed of coplanar [BO₃] triangles ([B₃O₇] for Ba₃Al₂B₁₂O₂₄) and tetrahedra ([BeO₃F] for KBBF, [BeO₄] for SBBO, and [AlO₄] for Ba₃Al₂B₁₂O₂₄) (Fig. 2a–c). First, the [BeO₄] tetrahedra were used to substitute the [BeO₃F] tetrahedra in ²_∞[Be₂BO₃F] single layers of KBBF, generating ²_∞[Be(BO₃)O] single layers in SBBO. Furthermore, the [BeO₄] tetrahedra and the [BO₃] units in the ²_∞[Be(BO₃)O] single layers of SBBO were replaced by [AlO₄] tetrahedra and the [B₃O₇] units to form ²_∞[B₃AlO₆] single layers of Ba₃Al₂B₁₂O₂₄. In SBBO, the ²_∞[Be₂(BO₃)₂O] double layers are formed by the ²_∞[Be(BO₃)O] single layers through bridged O atoms. Notably, the bridged O atoms between ²_∞[Be(BO₃)O] single layers are replaced by ¹_∞[B₆O₁₃] chains in the ²_∞[B₁₂Al₂O₂₈] double layers of Ba₃Al₂B₁₂O₂₄. Meanwhile, compared with KBBF, the weak interlayer bonding (K⁺–F⁻ ionic bonds) is evidently reinforced in the other two compounds (Sr–O bonds for SBBO and Ba–O bonds for Ba₃Al₂B₁₂O₂₄). In particular, the interlayer spacing greatly decreases from 6.25 Å in KBBF to 3.43 Å in SBBO and 3.00 Å in Ba₃Al₂B₁₂O₂₄ (Fig. 2d–f).^25,36 The reduced interlayer space in Ba₃Al₂B₁₂O₂₄ is expected to reinforce the bonding force and will be conducive to overcoming the layering growth tendency. In Ba₃Al₂B₁₂O₂₄, the use of highly toxic beryllium oxide is effectively avoided by the substitution of the [BeO₄] tetrahedra with the [AlO₄] tetrahedra. Meanwhile, the structure contains planar [B₃O₇] groups with greater microscopic polarizability than the [BO₃] groups, which can be conducive to generating large SHG response and optical anisotropy.

Fig. 2 (a)–(c) Layers in KBBF, SBBO, and Ba₃Al₂B₁₂O₂₄, respectively; (d)–(f) the structural evolution from the single-layer structure of KBBF to the double-layer structures of SBBO and Ba₃Al₂B₁₂O₂₄.

For Ba₃Al₂B₁₂O₂₄, a 9 × 5 × 2 mm³ crystal has been successfully obtained. The crystal can be further used to evaluate its optical properties. We measured its transparent windows from the UV to IR regions by using a polished Ba₃Al₂B₁₂O₂₄ crystal wafer with 2 mm thickness, which demonstrates that Ba₃Al₂B₁₂O₂₄ has a wide transparent spectral region from 190 to 3520 nm (Fig. 3a). This indicates that Ba₃Al₂B₁₂O₂₄ is DUV transparent, and its UV absorption edge is comparable with those of other aluminoborate materials, such as K₂Al₂B₂O₇ (180 nm),³⁰ β-Rb₂Al₂B₂O₇ (<200 nm),²⁸ Cs₂Al₂B₆O₁₃ (185 nm),³¹ K₃Ba₃Li₂Al₄B₆O₂₀F (190 nm),³² Rb₃Ba₃Li₂Al₄B₆O₂₀F (198 nm),³³ etc. As Ba₃Al₂B₁₂O₂₄ crystallizes in the NCS space group, powder SHG was measured using the Kurtz–Perry method. SHG intensity versus particle size revealed that Ba₃Al₂B₁₂O₂₄ is phase-matchable at 1064 nm and has a large SHG intensity, ∼2.7 × KDP (Fig. 3b and c). Furthermore, we investigated 11 reported NCS aluminoborate compounds containing alkali or alkaline earth metals (Table S4). When compared with other aluminoborates, Ba₃Al₂B₁₂O₂₄ exhibited the largest SHG response among these reported aluminoborates (Fig. 3d). As expected, Ba₃Al₂B₁₂O₂₄ demonstrates a good balance between SHG response and DUV transparency in aluminoborates (Fig. 3e).

Fig. 3 (a) Ultraviolet-visible and infrared (UV-Vis-IR) transmission spectra of single crystals of Ba₃Al₂B₁₂O₂₄; (b) and (c) powder SHG measurements at 1064 nm; (d) comparison of SHG responses of Ba₃Al₂B₁₂O₂₄ and reported aluminoborates; (e) scatter diagrams of SHG intensity and UV cutoff edges of Ba₃Al₂B₁₂O₂₄ and reported aluminoborates.

In order to investigate the origin of the large SHG response of Ba₃Al₂B₁₂O₂₄, the dipole moment was calculated based on the BVS method to evaluate the contributions of individual groups to the SHG response (Table S5).³⁷ We found that the dipole moments of the B–O anionic groups in Ba₃Al₂B₁₂O₂₄ are significantly larger than that of the Al–O anionic groups (Fig. 4a). This result indicates that the B–O anionic groups are important contributors to the large SHG response of Ba₃Al₂B₁₂O₂₄. Meanwhile, the [B₃O₇] units in the ²_∞[B₃AlO₆] layers maintain an almost consistent coplanar arrangement, which is conducive to producing a large SHG response (Fig. 4b).

Fig. 4 (a) The sum of the dipole moments of all corresponding units [AlO₄], [B₂O₅], [B₄O₉], and [B₃O₇] in the unit cell of Ba₃Al₂B₁₂O₂₄; (b) the arrangements of [B₃O₇] groups in Ba₃Al₂B₁₂O₂₄.

Besides these, birefringence is also important for NLO crystals to achieve a wide PM range. Therefore, the birefringence value of Ba₃Al₂B₁₂O₂₄ was measured using a cross-polarizing microscope based on the formula R = Δn × d, where R, Δn, and d are retardation, birefringence, and thickness of the crystal, respectively.³⁸ In the measurement, the crystal of Ba₃Al₂B₁₂O₂₄ with a thickness of 18.7 μm was used. And the I-order green interference color was observed using a cross-polarized light microscope (Fig. S5b). By comparing with the Michal–Levy chart, R is approximately 780 nm. Consequently, the birefringence of Ba₃Al₂B₁₂O₂₄ is about 0.042 in the visible region. The birefringence of Ba₃Al₂B₁₂O₂₄ was calculated based on the formulae ε(ω) = ε₁(ω) + iε₂(ω) and n²(ω) = ε(ω). The calculated birefringence of Ba₃Al₂B₁₂O₂₄ at 1064 nm is 0.041, which is close to the measured birefringence (Fig. S5a).

To further explore the relationship between the structure and optical properties, the electronic structure was calculated via first-principles calculations based on the density functional theory (DFT).³⁹ Ba₃Al₂B₁₂O₂₄ exhibits a direct band gap of 5.00 eV (Fig. 5a), which is smaller than the experimental result. This discrepancy can be attributed to the underestimation of the band gap (E_g) using DFT calculations.⁴⁰ Furthermore, we investigated the density of states (DOS) and partial density of states (PDOS) of Ba₃Al₂B₁₂O₂₄ (Fig. 5b). Since the optical properties of the compound are mainly determined by the states near the Fermi level,⁴¹ the top of the valence band (VB) and the bottom of the conduction band (CB) were analyzed. We found that the upper parts of the VB from −10 to 0 eV for Ba₃Al₂B₁₂O₂₄ mainly originate from the O 2p orbitals, which are mixed with a small part of B 2p orbitals and Al 3p orbitals. Meanwhile, the bottom of the CB from 0 to 10 eV primarily comprises B 2s2p orbitals with small amounts from the Ba 5d6s orbitals and Al 3s3p orbitals. From the above analysis, we can conclude that the large SHG response of Ba₃Al₂B₁₂O₂₄ is mainly attributable to the B–O anionic groups. Based on the electron localization function (ELF) maps (Fig. 5c), we can intuitively observe that the clover-shaped asymmetric electron densities are arranged around the oxygen atoms to form the NLO-active [B₃O₇] groups, which indicates that they have a positive contribution to the SHG response. Additionally, the nearby Ba cations form slightly distorted spherical asymmetric electron localized densities under the influence of the NLO-active [B₃O₇] groups, which also contributes to the SHG response (Fig. S6). Similar situations have also been found in other compounds, such as Cs₃VO(O₂)₂CO₃ and Ba₄B₁₁O₂₀F.^42,43 Therefore, the large SHG response in Ba₃Al₂B₁₂O₂₄ originates mainly from B–O groups.

Fig. 5 (a) Electronic band structure of Ba₃Al₂B₁₂O₂₄; (b) DOS and PDOS projected on the constitutional atoms of Ba₃Al₂B₁₂O₂₄; (c) ELF map of Ba₃Al₂B₁₂O₂₄.

Conclusions

In summary, a new SBBO-like aluminoborate, Ba₃Al₂B₁₂O₂₄, has been successfully synthesized by the chemical disubstitution strategy. Ba₃Al₂B₁₂O₂₄ features ²_∞[B₁₂Al₂O₂₈] double layers composed of ²_∞[B₃AlO₆] single layers that are transformed from ²_∞[Be(BO₃)O] single layers in SBBO through substituting [BeO₄] tetrahedra and [BO₃] triangles with [AlO₄] tetrahedra and [B₃O₇] units, respectively, which are further linked together via interlayer ¹_∞[B₆O₁₃] chains. This connection enables the reduced interlayer space in Ba₃Al₂B₁₂O₂₄ to reinforce the bonding force and is conducive to overcoming the layering growth tendency. More importantly, Ba₃Al₂B₁₂O₂₄ satisfies the performance balance for NLO applications in the UV region among a large SHG response (2.7 × KDP), a short UV absorption edge (<190 nm), and a suitable birefringence. Structural analyses and theoretical calculations indicate that the [B₃O₇] units with coplanar configurations and well-ordered arrangements make a significant contribution to the excellent optical properties. Meanwhile, Ba₃Al₂B₁₂O₂₄ melts congruently, which is helpful for its crystal growth. These results demonstrate that site substitution based on the classical structure is an effective strategy for developing new UV and DUV NLO crystals.

Author contributions

X. R. L. performed the experiments, data analysis, theoretical calculations, and paper writing. H. W. Y. designed and supervised the experiments. H. P. W. provided major revisions of the manuscript. Z. G. H. supervised the optical experiments. J. Y. W. and Y. C. W. helped with the analyses of the crystallization process and the data. All the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2480204 (Ba₃Al₂B₁₂O₂₄) contains the supplementary crystallographic data for this paper.⁴⁴

The data supporting this article have been included as part of the SI. Supplementary information: Experimental section and additional tables and figures. See DOI: https://doi.org/10.1039/d5sc03854e.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22575172, 52322202, 52572012 and 52172006) and Tianjin Natural Science Foundation (Grant No. 21JCJQJC00090).

Notes and references

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