AB₁₁O₁₆(OH)₂ (A = K and Cs): interpenetrating 2D layers with large birefringence

Abstract

Two new isostructural alkali metal hydroxyborates, AB₁₁O₁₆(OH)₂ (A = K and Cs), with two-dimensional (2D) interpenetrating [B₁₁O₁₈(OH)₂]_∞ layers have been successfully obtained. The calculated birefringences of KB₁₁O₁₆(OH)₂ and CsB₁₁O₁₆(OH)₂ are 0.104 and 0.100 at 546 nm, respectively. Moreover, KB₁₁O₁₆(OH)₂ exhibits a short DUV cutoff edge (195 nm).

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Birefringent materials have the capability to modulate polarized light, so they are expected to be used as polarization devices, which are essential in photoelectric modulators, photoelectric switches, optical isolators and coherent systems.^1–4 In addition, birefringence is also one of the pivotal optical parameters for nonlinear optical (NLO) crystals. Furthermore, moderate birefringence enables the crystal to achieve phase matching at a specific wavelength.^5–7 Borates, in particular, can be regarded as an excellent candidate for exploring birefringent materials due to various coordination patterns of B (BO₃ or BO₄), which can further be integrated via sharing oxygen atoms to form isolated groups, one-dimensional (1D) chains, 2D layers or 3D networks.^8,9 As is known to all, borates with coplanar arrays of isolated [BO₃]³⁻, [B₂O₅]⁴⁻, or [B₃O₆]³⁻ groups might produce large birefringence,^10–14 such as α-BaB₂O₄ (α-BBO, 0.122 at 546 nm),² Li₂Na₂B₂O₅ (0.095 at 532 nm),¹⁵ and Ba₂Ca(B₃O₆)₂ (0.125 at 546 nm).¹⁶ Particularly, a 1D chain structure polymerized by coplanar [BO₃]³⁻ is expected to have a larger average anisotropic polarizability, generating a large birefringence, for example, Ca(BO₂)₂ (0.136 at 532 nm).¹⁷ In addition, the stereochemical activity lone pair (SCALP) cations also make a remarkable contribution to their huge birefringence except for the [BO₃]³⁻ anion group,^18–22 such as α-BiB₃O₆,²³ SbB₃O₆,²⁴ and Sn₂B₅O₉Cl.²⁵ However, applications of the SCALP-containing borates in the ultraviolet (UV) region are limited due to the red shift of the cutoff edge.

How to design UV birefringent materials? In crystal engineering, it has been proved to be an effective way to obtain new crystals based on a well-known structural template for ion substitution or co-substitution.^26–28 Through the chemical substitution strategy, a new crystal Sn₂B₅O₉Cl with a large birefringence gain (16 times that of its isostructural Ba₂B₅O₉Cl) has been synthesized after the substitution of Ba²⁺ with Sn²⁺;^25,29 Na₃Ba₂(B₃O₆)₂F (NBBF) can be structurally considered as a derivative from co-substituting the Ba²⁺ atoms in α-BBO with 3Na⁺ and F⁻ atoms in NBBF;³⁰ K₇M^IIRE₂(B₅O₁₀)₃ (M^II = Ca, Sr, Ba, K/RE_0.5; RE = Y, Lu, Gd)³¹ are derived from simultaneously replacing the Ba²⁺ groups in β-BBO with multiple rare-earth, alkali, and alkaline-earth metals. Recently, research has shown that introducing a fluorine atom into a BO₄ tetrahedron forms BOF groups (BO₃F, BO₂F₂, and BOF₃),³² which are beneficial to improve the birefringence of borates due to their large polarizability anisotropy. In addition, hydroxyl (OH⁻) and fluorine (F⁻) have similar electronic compositions; therefore, hydroxyborates may show similar optical properties to fluorooxoborates if the arrangement of the functional active units is reasonable.³³ Prior to this work, we obtained NH₄B₁₁O₁₆(OH)₂,³⁴ which shows a short cutoff edge and large birefringence. In this work, guided by the above design strategy, we used alkali metals (K and Cs) to replace NH₄, and finally two new crystals KB₁₁O₁₆(OH)₂ (KBOH) and CsB₁₁O₁₆(OH)₂ (CBOH) exhibiting relatively large birefringence and short UV cutoff edges were acquired in a closed system. Herein, characterization results on the optical and thermal properties, as well as theoretical calculation, were also presented.

The polycrystalline sample of KBOH was prepared through a solid-state reaction in a sealed system (Experimental section in the ESI†). The purity of the polycrystalline sample was confirmed by powder X-ray diffraction, and the experimental pattern agrees well with the calculated result, except for an impurity peak (Fig. S1 in the ESI†).

KBOH and CBOH belong to the centrosymmetric space group C2/c (no. 15) in the monoclinic system, and are isostructural to AB₁₁O₁₆(OH)₂ (A = NH₄, Rb).^34,35 Therefore, KBOH is selected as a representative to be discussed in detail. The lattice parameters of the crystal are a = 14.163(17) Å, b = 10.101(12) Å, c = 10.664(12) Å, β = 106.827(15)° and Z = 4 (Table S1 in the ESI†). In the asymmetric unit, there are one, six, nine and one crystallographically unique K, B, O, and H atoms, respectively (Table S2 in the ESI†). It exhibits a 3D framework composed of KO₈, BO₃, BO₄ and BO₂(OH) groups. The B(1)O₂(OH), B(3)O₃ and B(6)O₃ units are linked by sharing vertex O atoms to form a 3-membered ring (3-MRI), B₃O₅(OH) unit. The B(2)O₃, B(4)O₃ and B(5)O₄ units are connected with each other by sharing vertex O atoms building 3-MRII, which is further linked to another 3-MRII sharing B(5)O₄ to form a B₅O₁₀ double-loop. Then one [B₅O₁₀] double-loop and two [B₃O₅(OH)] single-rings form the fundamental building block (FBB) by sharing oxygen atoms, which is [B₁₁O₁₈(OH)₂] (Fig. 1a). Finally, the FBBs are linked together to construct two-dimensional (2D) interpenetrating [B₁₁O₁₈(OH)₂]_∞ layers, which are separated by charge balancing K⁺ cations (Fig. 1b).

Fig. 1 (a) The [B₁₁O₁₈(OH)₂] group; (b) the crystal structure of KBOH; (c) the unique single-layered anionic arrangement from other direction (brown tetrahedron, [BO₄]⁵⁻ groups; green triangle, [BO₃]³⁻ groups).

With respect to the B atoms, in the structure (Fig. 1a), the B(1), B(2), B(3), B(4) and B(6) atoms are coordinated by three O atoms, forming BO₃ triangles with B–O bond lengths falling in a reasonable range (Table S3 in the ESI†). Further, the B(5) atoms are bonded to four O atoms forming BO₄ tetrahedra with the B–O bond lengths ranging from 1.459(3) to 1.474(3) Å. All bond lengths are suitable, consistent with those observed in other borates. The K cations are bonded to eight O atoms, forming distorted KO₈ polyhedra (Fig. S2 in the ESI†), which are isolated in the structure. The K–O bond lengths are in the range of 2.701(4)–3.098(4) Å (Table S3†).

To the best of our knowledge, sixteen hydroxyl potassium borate compounds have been reported besides the title compound. Table S4 in the ESI† summarizes the formula, space group, B–O anion framework, and cutoff edge for these compounds. It can be seen that, except for K₂(B₅O₈(OH))(H₂O)₂ and the title compound, almost all hydroxyl potassium borates exhibit isolated anion groups and 1D chains, which is associated with the ratio of potassium to boron and the presence of hydroxyl groups in their structures. In general, introducing hydroxyl into borates could avoid the appearance of terminal oxygen atoms (the terminal oxygen atoms are connected to H) and reduce the dimensions of the B–O framework. In K₂(B₁₀O₁₄(OH)₄)(H₂O), KB₅O₇(OH)₂(H₂O), K₂(B₆O₉(OH)₂), and KB₅O₇(OH)₂, the FBBs contain two terminal oxygen atoms and are connected together through a common vertex to form a 1D chain. In addition, the FBBs [B₅O₁₀(OH)₂] of K₄B₁₀O₁₅(OH)₄ form a 1D chain by sharing five terminal oxygen atoms. The FBBs of K₂(B₅O₈(OH))(H₂O)₂ and the title compound are B₅O₁₀(OH) and B₁₁O₁₈(OH)₂, respectively, containing four terminal oxygen atoms, which can be bonded with other ones through sharing oxygen atoms to form a higher-dimensional anionic framework. According to our conclusion, KBOH is the first compound with two interpenetrating layers among all the hydroxyl potassium borates. We also summarized hydroxyl cesium borates in Table S5 in the ESI,† and there are six cases in addition to the compound in this paper. Cs(B(OH)₄)(H₂O)₂, Cs₂(B₄O₅(OH)₄)(H₂O)₃, Cs₂(B₁₂(OH)₁₂)(H₂O)₂, and Cs₂(B₁₂(OH)₁₂)(H₂O)₂ contain isolated B–O frameworks. In contrast, Cs_0.4(H₃O)_0.6B₃O₅, CsB₇O₁₀(OH)₂ and CBOH exhibit 2D B–O–H layers in their structures.

The IR spectrum of KBOH is presented in Fig. S3,† and the peaks are assigned by referring to ref. 34, 36, and 37. The peaks at 3315, 3228 and 1435 cm⁻¹ are attributed to the stretching vibrations of the hydroxyl group O–H. Two obvious absorption peaks located at 1321 and 1278 cm⁻¹ are caused by the asymmetric stretching in the BO₃ groups. The peak at 1136 cm⁻¹ stems from the in-plane bending of B–O–H. The absorption peak of the asymmetric stretching of the BO₃ groups is located at 1056 cm⁻¹. The absorption bands around 921, 885 and 810 cm⁻¹ arise from the symmetric stretching of BO₃. The main bands near 773 and 678 cm⁻¹ originate from the out-of-plane bending mode of the B–O bond in BO₃. Moreover, the crest below 600 cm⁻¹ characterizes the bending pattern of the BO₃ units.

As shown in Fig. 2, it can be found that the transmittance of KBOH is 19.1% at 195 nm, indicating that it can be used in the DUV area.

Fig. 2 UV-vis-NIR optical spectrum of KBOH.

In order to elucidate the mechanism between the optical properties and the structure of KBOH and CBOH, the first-principles calculations were performed based on density functional theory (DFT). The calculated band gaps using GGA (Fig. 3) are 6.04 and 5.85 eV with the direct band gap for KBOH (a) and CBOH (b), respectively. Simultaneously, the HSE06 method calculations show that KBOH and CBOH have large band gaps of 7.47 and 7.19 eV, respectively (Fig. S4 in the ESI†).

Fig. 3 (a) The band structure of KBOH; (b) the band structure of CBOH; (c) the birefringence calculation of KBOH; (d) the birefringence calculation of CBOH.

To understand the composition and origin of the electronic structures, the partial density of states (PDOS) for the compounds was calculated (Fig. 4). We select KBOH as a representative to be described in detail. At the top of the valence band, from −9 eV to the Fermi level, it is mainly composed of O-p orbitals slightly hybridized with B-p and H-s orbitals. The conduction bands from 6.2 eV to 14 eV are mainly contributed by B-p and B-s orbitals with slight contribution from O-p, H-s and K-p orbitals. The electron distribution near the Fermi level implies that the O-p orbitals slightly hybridized with the B-p orbital occupy the top area of the valence bands and the B-p orbital occupies the bottom region of the conduction bands. Therefore, the B–O bond in the title compounds plays a prominent role in the electronic structure and optical properties.

Fig. 4 (a) Density of states (DOS) of KBOH; (b) density of states (DOS) of CBOH.

As shown in Fig. 3c and d, the calculated birefringences of KBOH and CBOH are 0.104 and 0.100 at 546 nm by the first-principles methods. We measured the birefringence of KBOH on a polarizing microscope (ZEISS Axio Scope. A1). The maximum interference color is observed with the KBOH crystal under cross-polarized light and the thickness of the measured film is 12.50 μm (Fig. S5 in the ESI†). According to eqn (S2) in the ESI,† the retardation (R) value is 1.22 μm, and the birefringence of KBOH is 0.098 at the wavelength of 546 nm, indicating its potential application as a DUV birefringent material. Similarly, the calculated data fit the experimental ones well, so the accuracy of the calculation is also guaranteed well. Unfortunately, the measured birefringence value of CBOH is much smaller than the calculated value, and we speculate that the growth dissociates from the spindle direction.

Since the refractive index changes with the direction of light propagation in the crystal, birefringence will occur.²⁵ In order to explain the source of birefringence, this work uses the electron localization function (ELF) to investigate the relevant performance of this structure. The ELF introduced by Becke and Edgecombe is one way to check the localization;³⁸ they introduced the ELF range of [0, 1], in which a value close to 1 means that electrons are ‘well-localized’. The bonding behaviors in a crystal can be nicely illustrated by the ELF. As presented in Fig. S6,† it shows the strong hybridization between B and O atoms, which can be attributed to the covalent bond interaction between B and O atoms. Moreover, the bonding behavior interaction between the elements is inseparable from the electronic transition. Therefore, the main contributor to birefringence is directly related to the strong covalent interaction between B and O atoms.

Conclusions

In summary, two new hydroxyl borates KBOH and CBOH were successfully synthesized in a sealed system. The experimental birefringence (0.098 at 546 nm) of KBOH has been measured and agrees well with the calculated value. In addition, KBOH exhibits a short UV cutoff edge (195 nm), indicating that it can be used as an excellent material in the DUV region. The large birefringence is mainly due to the strong covalent interaction between O and B atoms. This work enriches the diversity of borates.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was completed with the help of the Shanghai Cooperation Organization Science and Technology Partnership Program (2020E01019), the Xinjiang Outstanding Young Talents in Science and Technology (2018Q004), the National Natural Science Foundation of China (U2003306, 61835014, and 51972336), and the Tianshan Innovation Team Program (2018D14001).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC numbers 2023510 and 2023511 for KB₁₁O₁₆(OH)₂ and CsB₁₁O₁₆(OH)₂, respectively; crystallographic information file (CIF) for KB₁₁O₁₆(OH)₂ and CsB₁₁O₁₆(OH)₂; experimental section, optical characterization and theoretical calculations; tables of crystallographic data, selected bond lengths and angles, and the hydroxyl borates; experimental and calculated XRD patterns of KB₁₁O₁₆(OH)₂, the coordination environments of K, and the electron localization function of KB₁₁O₁₆(OH)₂. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ce01569e

‡ These authors contributed equally to this work.

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