KNa₂La₂(BO₃)₃: a shortite-type lanthanide borate exhibiting strong nonlinear optical activity induced by isolated [BO₃] triangles and distorted [LaO₉] polyhedra

Abstract

The targeted synthesis of ultraviolet (UV) nonlinear optical (NLO) materials with strong second-harmonic generation (SHG) activity and wide transparency window remains a formidable challenge. Herein, a prospective rare-earth borate UV NLO crystal, KNa₂La₂(BO₃)₃, was extracted by spontaneous crystallization using a flux method. The title compound crystallizes into the acentric orthorhombic space group Amm2 (no. 38), analogous to those of shortite, and affords a three-dimensional framework built from planar [BO₃] triangles and distorted [LaO₉] motifs. The optical spectroscopy analyses demonstrate that KNa₂La₂(BO₃)₃ displays a short cutoff edge of 212 nm, accompanied by a wide band gap of 5.3 eV. Notably, it achieves an enhanced phase-matching SHG efficiency of 2.6 × KDP, comparable with those of the other NLO rare earth borate oxide crystals. Besides, theoretical analyses revealed that the optimized optical polarizability of KNa₂La₂(BO₃)₃ is predominantly ascribed to the cooperative effects of the isolated [BO₃] and distortive [LaO₉] structural units. This finding showcases a promising UV NLO crystal and sheds light on further exploration of rare earth metal-based NLO materials with fascinating performances.

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Introduction

Second-order nonlinear optical (NLO) crystals have been widely used in laser science and technology, as they possess the capability of frequency conversion that is able to expand the wavelength range of common laser sources to the spectral region.^1,2 In particular, those used in the ultraviolet (UV) range (λ < 400 nm) play a pivotal role in the fields of optoelectronics, such as laser micromanufacturing, information storage, biological tissue imaging, and photoemission spectroscopy.^3–6 Consequently, considerable research has been devoted to designing and fabricating potential UV NLO materials.

The primary requirement for targeting a NLO material is that it must be crystallographically acentric. The efficient approach to predesigning acentric structures is to introduce NLO-active chromophores as fundamental building blocks,⁷ such as the second-order Jahn–Teller (SOJT) distortive cations with local asymmetric coordination (e.g., Ti⁴⁺, Nb⁵⁺, Mo⁶⁺),⁸ stereo-chemically active lone-pair cations (e.g., Bi³⁺, Te⁴⁺, Sn²⁺, etc.),⁹ or π-conjugated oxyanions (e.g., [BO₃]³⁻, [CO₃]²⁻, [NO₃]⁻).¹⁰ Although, the compounds containing the SOJT distorted cations are widely utilized to increase the possibility of generating favorable microscopic polarizability when arranged in an optimal manner, nevertheless, they can also cause the UV absorption cutoff edge of the material to be adversely redshifted, thus hardly maintaining adequate transmittance over the UV region.¹¹ Compared with the situation above, borates have attracted considerable research interest and are consistently viewed as competitive candidates for UV NLO crystals with intriguing performances because the optically active π-conjugated B–O units adopt coplanar configurations that promote a wide transparency window down to the UV region and large microscopic second-order hyperpolarizabilities.^12,13 Accordingly, this served as inspiration for the discovery of a wide variety of UV NLO materials with [BO₃] triangles, including KBe₂BO₃F₂,¹⁴ Li₄Sr(BO₃)₂,¹⁵ K₃B₆O₁₀Cl,¹⁶ Ba₄B₁₁O₂₀F,¹⁷ and Ba₃Mg₃(BO₃)₃F₃.¹⁸ Although the development of laser technology has been facilitated by these crystals, exploiting new NLO materials that have simultaneously enhanced second harmonic generation (SHG) efficiency and high UV transparency remains an enormous challenge.

According to previous studies, the enriched structural chemistry of lanthanide metal ions enables them to produce local asymmetric coordination environments, similar to the second-order SOJT distortions.¹⁹ In particular, some rare-earth metal ions with closed-shell electron systems, such as Y³⁺, Sc³⁺, La³⁺, and Lu³⁺, inhibit the intrinsic d–d or f–f electronic transitions and thus give sufficient optical transparency in the UV window, as seen in compounds like La₂CaB₁₀O₁₉,²⁰ K₅Mg₂La₃(BO₃)₆,²¹ and K₇CdRE₂B₁₅O₃₀ (RE = Sc, Y, Gd, Lu).²²

Motivated by these, we focused special attention on the rare-earth metal–borate system in order to fabricate novel UV NLO crystals with improved optical properties. Our design ideas are mainly considered as follows: (i) La³⁺ has a large ionic radius among the rare earth metals and exhibits flexible coordination environments,²³ which is preferable to produce localized structural distortion, and it is chosen for this work as the rare-earth metal ion; and (ii) the violet-shifting of the UV absorption edge and widening of the band gap can be successfully induced by the introduction of light alkali metal ions without d–d or f–f electronic transitions. For example, the lanthanide borate NLO crystal Na₃La₂(BO₃)₃ features an enlarged SHG intensity (2 × KDP) and a short UV absorption edge of 213 nm.²⁴ Consider that isomorphism can occur between sodium and potassium.²⁵ Therefore, by introducing the light alkali metal ions K⁺ and Na⁺, the first effective fabrication of a new rare-earth borate NLO crystal, KNa₂La₂(BO₃)₃ (KNLBO), was experimentally accomplished using the [BO₃] triangle system in combination with the flexible coordinated La³⁺ rare earth metal ion. KNLBO crystallizes into the non-centrosymmetric Amm2 (no. 38) space group. Its arrangement mode of [BO₃] planar triangles is analogous to that of [CO₃] in carbonates, like Na₂Ca₂(CO₃)₃, which affords a typical shortite structure.²⁶ As expected, the title compound fully achieved the coexistence of strong phase-matching SHG activity (2.6 × KDP) and short UV cut-off edge (212 nm), along with a broad band gap (5.3 eV), which indicates that KNLBO is a potential UV NLO material. Herein, we present its synthesis, crystal structure, thermal behavior, optical properties, and theoretical analyses.

Experimental

Polycrystalline synthesis

Polycrystalline KNLBO was synthesized using the starting materials K₂CO₃ (Sinopharm, 99.9%), Na₂CO₃ (Sinopharm, 99.9%), La₂O₃ (Sinopharm, 99.9%), and B₂O₃ (Sinopharm, 99.9%) with stoichiometric ratios via a standard solid-state reaction approach. Initially, the mixture contained within the crucible was inserted in a muffle furnace and was preheated at 450 °C for 24 h. Afterward, the products were sintered at no less than 700 °C for 72 h with several intermittent regrindings. Then, the target polycrystalline compound KNLBO was experimentally prepared, and its purity was further supported by powder X-ray diffraction analyses.

Powder X-ray diffraction (PXRD)

PXRD analyses of the polycrystalline KNLBO were carried out through a conventional Rigaku SmartLab 9 kW diffractometer with monochromatized Cu Kα radiation (λ = 1.5418 Å) recorded ranging from 10 to 70° at a step size of 0.01° with a step time of 0.3 s at room temperature.

Second-harmonic generation (SHG) characterization

On the basis of the Kurtz–Perry approach,²⁷ the polycrystalline powder SHG measurements of KNLBO were implemented under an incident laser beam (λ = 1064 nm) emitted from a Q-switched Nd:YAG laser at ambient temperatures. Because the powder SHG intensity depends on the particle size range of powder, the pure polycrystalline sample of KNLBO was screened and graded into the following several different particle ranges: 21–51, 51–74, 74–105, 105–125, 125–177 and 177–210 μm. Meanwhile, the standard polycrystalline samples KDP (KH₂PO₄) and LBO (LiB₃O₅) were also sieved into the corresponding size ranges to conduct pertinent comparisons.

Thermal properties

Differential scanning calorimetry (DSC) combined with thermogravimetric analyses (TGA) for the target compounds was conducted on a NETZSCH STA 449F5 TG/DSC thermal analyzer. The polycrystalline KNLBO was simultaneously heated up to 1200 °C at a rate of 10 °C min⁻¹ and then slowly cooled down to ambient temperature under a N₂ atmosphere.

Crystal preparation and structural determination

KNLBO single crystals were extracted through a spontaneous crystallization technique. By using B₂O₃–K₂CO₃ as the fluxes, the target crystals were experimentally prepared. Single-crystal XRD analyses were conducted to process the structure of KNLBO. The crystallographic data were collected by employing a Bruker D8 VENTURE CMOS X-ray source with Mo Kα radiation (λ = 0.71073 Å). Multi-scan absorption corrections were applied according to the APEX III software. Based on the SHELXTL crystallography software system, the preliminary crystal model was processed and further refined by direct methods.²⁸ The structural symmetry of KNLBO was identified by employing the PLATON program.²⁹ The detailed crystallographic data on the atomic coordinates, equivalent isotropic displacement parameters, bond distances and angles, and anisotropic displacement parameters are presented in Tables S1 and S2.†

Energy-dispersive spectroscopy

The energy-dispersive spectroscopy (EDS) analyses of the target compound were carried out with a field emission scanning electron microscope (FEI Quanta FEG 250). To obtain elemental mapping images, the field-emission scanning mode with electron microscopy was employed. Considering the limitations of the two lighter elements B and O in this test, the contents of the heavy elements La, Na and K were characterized.

Optical spectroscopy

The ultraviolet–visible (UV-vis) diffuse reflectance spectra of the powder KNLBO sample were acquired in the 200–1200 nm region using a Hitachi UH4150 spectrophotometer with BaSO₄ as a standard sample. According to the Kubelka–Munk formula,³⁰ the experimental energy gap of the title compound was further obtained. Meanwhile, the IR behavior of KNLBO was inspected over the test wavelength region of 500–4000 cm⁻¹ with a Shimadzu IR Affinity1 spectrometer at room temperature.

Computational details

To evaluate the structure–property relationship, theoretical calculations were conducted with the plane-wave pseudopotential technique implemented in the CASTEP program via density functional theory (DFT) on the actual unit cell.³¹ In this work, the density of states (DOS), band structure, and birefringence of KNLBO were systematically analyzed. In KNLBO, the electrons B 2s²2p¹, Na 2s²2p⁶3s¹, K 3s²3p⁶4s¹, La 5d¹6s² and O 2s²2p⁴ were regarded as valence electrons. The Perdew–Burke–Ernzerhof functional within the generalized gradient approximation was chosen to process the exchange–correlation effects,^32,33 which helps to accurately capture the electronic and structural properties of KNLBO. The kinetic energy cutoff for the plane-wave basis was set to be 700 eV, and a Monkhorst–Pack k-point grid of 4 × 3 × 2 was employed for sampling and numerical integration in the Brillouin zone.

Results and discussion

Preparation of KNLBO

The polycrystalline KNLBO were experimentally prepared through a high-temperature solid-state reaction method. PXRD measurements were employed to check the purity of the target sample. As illustrated in Fig. 1a, the experimentally observed PXRD patterns of KNLBO agree well with those derived from the single-crystal structure, suggesting that the prepared polycrystalline samples were of pure phase. Furthermore, single crystals of KNLBO were grown by heating the compound with the optimized B₂O₃–K₂CO₃ flux system above 800 °C in a Pt crucible, followed by cooling the solution at 1–3 °C h⁻¹ from 800 to 650 °C and then to ambient temperature by air cooling, as presented in Fig. S1.†

Fig. 1 (a) Standard and observed PXRD patterns of the as-synthesized KNLBO samples. (b) DSC and TGA curves for the KNLBO polycrystalline samples.

Thermal analyses

The TGA–DSC curves of the polycrystalline KNLBO from 25 to 1200 °C were analyzed, as displayed in Fig. 1b, in which no sharp peak is detected in the measured heating stage. Meanwhile, there is no distinct weight loss up to around 1050 °C in the corresponding TGA patterns, suggesting that the title compound exhibits good thermal stability. The varied temperature PXRD patterns of KNLBO were also analyzed (Fig. S2†).

Structural analysis of KNLBO

The crystal structure of KNLBO was experimentally solved for the first time. The single-crystal X-ray diffraction results demonstrated that KNLBO crystallizes into the acentric orthorhombic space group Amm2 with the lattice parameters a = 5.1933(6) Å, b = 11.4720(15) Å, c = 7.3074(9) Å and Z = 2, which is analogous to the shortite structure.²⁶ The detailed crystallographic data and structural refinement for KNLBO are provided in Table 1. In the crystal structure of KNLBO, the asymmetric unit consists of two types of B³⁺ cations, one unique La³⁺ cation, one independent K⁺ and two independent Na⁺ cations as well as two O²⁻ anions. As exhibited in Fig. 2a, each B atom is bonded to three O atoms and forms isolated [BO₃] triangles with B–O bond distances in the region from 1.361(16) to 1.410(11) Å. With respect to the rare-earth La atoms, they are coordinated with nine O atoms to construct a [LaO₉] basic block and the corresponding La–O bond distances vary between 2.464(2) and 2.696(3) Å. These observed values are consistent with those of other rare-earth borates.²¹ To be specific, the two kinds of planar [BO₃] units with the zero-dimensional (0D) framework are parallel on the ac and bc planes (Fig. 2b), respectively, which is similar to those of [CO₃] triangles in the carbonate Na₂Ca₂(CO₃)₃.²⁶ As presented in Fig. 2c, the adjacent [LaO₉] polyhedra are connected simultaneously via corner and face sharing to form chains. It is clearly observed in Fig. 2d that these structural motifs of the [BO₃] and [LaO₉] groups are further decorated to yield a two-dimensional (2D) configuration via corner-sharing that runs parallel to the ac plane. Then, the neighboring layers in the ac plane are further stacked along the b-axis to build a three-dimensional (3D) configuration, as depicted in Fig. 2e. The crystallographic analysis reveals that KNLBO is structurally related to the lanthanide borate Na₃La₂(BO₃)₃.²⁴ Although they have different alkali metal cations, B³⁺ and La³⁺ cations adopt a three- and nine-coordinate environment to form isolated trigonal planar units [BO₃] and distorted polyhedra [LaO₉], respectively, which construct the orthorhombic acentric 3D framework. According to structure–property relationships,³⁴ such accordant arrangements of [BO₃] triangles and distorted [LaO₉] polyhedra are beneficial for generating a significant enhancement of microscopic polarizability.

Fig. 2 Structural characterization of KNLBO. (a) The structural polyhedral representation of [B1O₃], [B2O₃] and [LaO₉]. (b and c) The spatial arrangement of the isolated [BO₃] and [LaO₉] units along the ac plane, respectively. (d) The 2D configuration composed of the [BO₃] and [LaO₉] groups via corner-sharing along the ac plane. (e) The 3D structural framework viewed along the bc plane.

Table 1 Crystal data and structural refinement of KNLBO

a R 1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Empirical formula	KNLBO
Formula weight	539.33
Temperature (K)	297 K
Crystal system	Orthorhombic
Space group	Amm2
a (Å)	5.1933(6)
b (Å)	11.4720(15)
c (Å)	7.3074(9)
Volume (Å^3)	435.36(9)
Z	2
Flack parameter	−0.04(5)
λ (Å)	0.71073
R (int.)	0.0424
GOF on (F^2)	1.246
Final R indices^a [Fo^2 > 2σ(Fo^2)]	R 1 = 0.0294, wR2 = 0.0814
R indices (all data)	R 1 = 0.0306, wR2 = 0.0849
CCDC number	2265827

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EDS analysis

To further validate the identity of Na, K, La and O elements, EDS tests for the KNLBO crystal were carried out to probe the elemental ratio and distribution. The observed results demonstrated that the molar ratio of Na/K/La//O elements was found to be around 2.09/1.10/2.66/8.43 and distributed homogeneously (Fig. S3†), which is in line with the observation of the crystal structure.

Spectroscopic properties

To characterize the optical transmittance behaviors, the UV-vis-NIR diffuse reflectance spectrum was obtained from polycrystalline KNLBO, as presented in Fig. 3a, which indicates that the title compound possesses a short UV cutoff edge at around 212 nm, corresponding to a broad optical band gap of 5.30 eV based on the Kubelka–Munk formula (inset of Fig. 3a), comparable to that of YAl₃(BO₃)₄.³⁵ Accordingly, this linear optical performance indicates its potential application in the UV region. Meanwhile, the IR spectral curves of KNLBO were also measured, as plotted in Fig. 3b. According to previous reports,^21,24 the peaks observed at around 1196 cm⁻¹ are mainly ascribed to the asymmetric stretching and symmetric stretching vibration modes of [BO₃]³⁻ units. The peaks that emerged at 744–589 cm⁻¹ predominantly correspond to the bending of the B–O groups. These observations further corroborate the existence of [BO₃]³⁻ primitives and are in accordance with those suggested by the structure of KNLBO.

Fig. 3 Optical characterization of KNLBO. (a) UV-vis-NIR diffuse reflectance spectra with a broad band gap of 5.3 eV. (b) IR transmission spectrum. (c and d) Phase-matching SHG signals for the title compound, KDP and LBO references with 1064 nm laser radiation.

SHG characterization

Because this title material lacks a center of symmetry, the SHG capability of the KNLBO compound was evaluated through the modified Kurtz–Perry method under a 1064 nm pulsed laser. As depicted in Fig. 3(c and d), the strength of the SHG response for KNLBO was increased gradually with increasing particle sizes and eventually reached a maximum magnitude, displaying a phase-matching behavior. Moreover, KNLBO possesses strong SHG intensities of 2.6 × KDP and 1.4 × LBO at 1064 nm within the same particle size range of 177–210 μm. It is worth noting that its experimental SHG efficiency is comparable with those of most reported rare earth borate oxides, including La₂CaB₁₀O₁₉ (2 × KDP),²⁰ K₅Mg₂La₃(BO₃)₆ (3.1 × KDP),²¹ K₇PbY₂B₁₅O₃₀ (2.1 × KDP),²² Na₃La₂(BO₃)₃ (2 × KDP),²⁴ YAl₃(BO₃)₄ (3 × KDP),³⁵ Na₃Y₃(BO₃)₄ (3 × KDP),³⁶ and RECa₄O(BO₃)₃ (RE = Y, Gd) (3 × KDP).³⁷ This observed result is consistent with the structural features of KNLBO. Consequently, these inherent merits of the high UV transparency and a strong phase-matching SHG intensity explicitly imply that the KNLBO crystal is a good candidate for NLO applications in the UV window.

Structure–property relationship

To intuitively shed light on the structure–property relationship of KNLBO, DFT was adopted in the VASP package and theoretical calculations. The moderate calculated birefringence was determined to be 0.019 @ 1 μm, as provided in Fig. S4.† As depicted in Fig. 4a, the simulation results reveal that the valence band maximum (VBM) and the conduction band minimum (CBM) of the KNLBO crystal belong to the same position (G), implying that the title compound belongs to the direct band gap. Furthermore, the theoretical band gap of KNLBO is calculated to be approximately 4.0 eV, which is systematically smaller than the experimentally observed value. Based on the GGA approach, the represented discrepancy is normally generated by the discontinuity of the exchange–correlation function. In addition, we probed the detailed bands composed of the total and partial densities of states. It is clearly found in Fig. 4b that the electron states near the maximum of the valence bands observed ranging from −6.0 to 0.0 eV are primarily dominated by B 2p, La 5d and O 2p orbitals, with a small amount from B 2s orbitals. The electron states near the minimum of the conduction bands observed in the range of 5–10 eV primarily correspond to the La 5d, B 2p and O 2p orbitals. Nevertheless, the contribution of the alkali metal cations K⁺ and Na⁺ is almost negligible. Considering that the optical properties depend mainly upon the electronic structures near the Fermi level, these observations imply that the appreciable second-order nonlinearity of the KNLBO crystal is predominantly prompted by the cooperative effects of the coplanar [BO₃] units and distortive [LaO₉] structural primitives. Therefore, we speculate that these NLO-active [BO₃] and [LaO₉] structural motifs can be extended into the design of other rare earth metal borate UV NLO crystals.

Fig. 4 (a) Calculated band structure. (b) Partial DOS of KNLBO, in which the blue dashed line represents the Fermi level.

Conclusions

In summary, a novel acentric polar rare earth borate, KNLBO, was rationally designed and synthesized through a flux method. KNLBO features a three-dimensional framework constructed from isolated [BO₃] plane triangles and distorted [LaO₉] polyhedra, with Na⁺ and K⁺ cations residing in the cavities. The target compound possesses a short UV cutoff edge of 212 nm with an enlarged optical band gap of 5.3 eV. Furthermore, it demonstrates a strong phase-matchable SHG activity of 2.6 times that of KDP, which makes it a promising candidate for UV NLO applications. Additionally, structural and theoretical analyses suggest that the cooperative contributions of the planar [BO₃] and distorted [LaO₉] motifs are largely responsible for the significantly improved second-order nonlinearity. These findings lay a foundation for fabricating the design and exploration of novel UV NLO materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2021YFA0717800) and the Natural Science Foundation of China (Grant No. 52002273, 52002220, 61835014 and 51890864).

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Footnote

† Electronic supplementary information (ESI) available: Additional crystallographic data, crystals, varied temperature PXRD, EDS analysis, calculated birefringence of KNLBO. CCDC 2265827. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01004j

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