Galloborates as ultraviolet nonlinear optical crystals: advances and perspectives

Abstract

Metal borates are excellent source materials for exploring short-wavelength nonlinear optical (NLO) crystals. Galloborates show rich structural chemistry with various coordination configurations of Ga cation and B–O anionic units and are suitable candidates as ultraviolet NLO crystals. Up to now, the shortest cut-off edge of galloborates was reported to be down to 190 nm in KCs₂Ga(B₅O₁₀)(OH), while the largest second harmonic generation (SHG) effect of galloborates was reported to be up to 4.6 times that of KH₂PO₄ (KDP) in Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃. Herein, we give a detailed summary of the recent progress in NLO inorganic galloborates, where these galloborates are grouped into two types in terms of their compositions: (1) alkali/alkaline earth metal galloborates and (2) alkali/alkaline earth metal galloborate halides. We discuss their structural features, band gaps, and SHG intensities. Finally, we give future perspectives in this field.

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1. Introduction

Nonlinear optical (NLO) materials are widely used to convert a specific wavelength of light to half its original.^1–17 The discovery of LiB₃O₅ (LBO), β-BaB₂O₄ (β-BBO), and KBe₂BO₃F₂ (KBBF) in borate systems greatly accelerated the development of ultraviolet (UV) and deep-UV NLO crystals.^18–20 These crystals are used to obtain coherent UV and deep-UV light via the cascaded frequency conversion of Nd:YAG lasers (1064 nm). According to the anionic group theory, [B₃O₇], [B₃O₆], and [BO₃] are the NLO-active functional units in LBO, β-BBO, and KBBF, while Li, Ba, and K cations contribute less. To date, a large number of alkali and alkaline earth metal borate-based NLO crystals have been found.^21–27 One very recent example is Ba₄B₁₄O₂₅, which shows a highly polymeric three-dimensional geometry with a closed-loop anionic framework constructed by the fundamental building blocks (FBBs) [B₁₄O₃₃], and it is a deep-UV transparent NLO crystal with strong second harmonic generation (SHG) (3.0 × KH₂PO₄, KDP).²⁸ Generally, a perfect UV or deep-UV NLO crystal should possess certain characteristics including a wide optical transparency window, large NLO coefficient, and moderate birefringence, and it should be easy to grow large single crystals.

The modification of the borate anionic unit is an effective method to find new NLO crystals.²⁹ Fluorooxoborates are considered the best candidates for the next generation of deep-UV NLO materials.³⁰ Fluorinated [BO_xF_4−x] (x = 1–3) units show larger polarizability anisotropy than that of [BO₄], and the higher electronegativity of fluorine ions usually leads to blue-shifted cut-off edges in fluorooxoborates. To date, more than 80 examples of fluorooxoborates have been found, among which more than one-third are crystallized in noncentrosymmetric space groups.³¹ NH₄B₄O₆F displays a wave-like layer constructed from a corner-sharing anionic structural motif of [B₄O₈F], and it is an excellent deep-UV NLO crystal with a very short cut-off edge (156 nm) and large NLO effect (3.0 × KDP).³² In addition, the newly discovered sp hybridized linear [BO₂] in synthetic borates shows a larger polarizability anisotropy than that of [BO₃] and [BO₄] units according to the theoretical analyses.³³ This functional unit may provide more opportunities to discover other NLO and birefringent materials.

The substitution or partial substitution of alkali and alkaline earth metal cations by other metal cations in borates lead to a new family of borates with excellent properties.^34,35 A larger band gap may be achieved by partially eliminating the dangling bond of the terminal O atoms, as evidenced by Zn₂BO₃(OH),³⁶ Cs₃Zn₆B₉O₂₁,³⁷ β-Rb₂Al₂B₂O₇,³⁸ and CsAlB₃O₆F.³⁹ Zn₂BO₃(OH) shows a KBBF-like structure by replacing [BeO₃F] with [ZnO₃(OH)] and exhibits a short UV cut-off edge (204 nm) and a SHG response of 1.5 × KDP.³⁶ Galloborates are suitable candidates for UV NLO crystals due to their rich structures, wide transmittance, and excellent optical properties. To date, 12 NLO inorganic galloborates have been reported, and the shortest cut-off edge of galloborates was reported to be down to 190 nm in KCs₂Ga(B₅O₁₀)(OH),⁴⁰ while the largest SHG effect of galloborates was reported to be up to 4.6 × KDP in Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃.⁴¹ In this frontier article, we review the recent progress in NLO galloborates. These galloborates are divided into two types in terms of their chemical compositions: (1) alkali/alkaline earth metal galloborates, and (2) alkali/alkaline earth metal galloborate halides. The chemical formulas, space groups, SHG intensities, and band gaps of these galloborates are summarized in Table 1. We also give the further perspectives and challenges in this field.

Table 1 Noncentrosymmetric galloborates and their properties

Compounds	Space groups	SHG intensities	Band gaps	Ref.
Rb2Ga(B5O10)(H2O)4	C2221	1.0 × KDP	3.54 eV	43
KCs2Ga(B5O10)(OH)	I4̄2d	1.4 × KDP	6.30 eV	40
Ba4Ga[B10O18(OH)5](H2O)	Cc	0.2 × KDP	4.12 eV	46
Na4Ga3B4O12(OH)	F4̄3c	0.1 × KDP	4.90 eV	47
Na5Ga[B7O12(OH)]2·2B(OH)3	C2	4.6 × KDP	3.90 eV	41
Ba3Ga2[B3O6(OH)]2[B4O7(OH)2]	Fdd2	3.0 × KDP	5.0 eV	48
K2Ba4Li2Ga4B6O21	P6̄2c	0.5 × KDP	5.49 eV	54
Ba2GaB4O9Cl	P42nm	1.0 × KDP	N/A	50
NaBa4(GaB4O9)2Cl3	P42nm	1.5 × KDP	3.76 eV	49 and 53
NaBa4(GaB4O9)2Br3	P42nm	1.1 × KDP	3.71 eV	53
K3Ba3Li2Ga4B6O20F	P6̄2c	0.7 × KDP	>6.20 eV	55
Rb3Ba3Li2Ga4B6O20F	P6̄2c	0.5 × KDP	>6.20 eV	55

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2. Alkali/alkaline earth metal galloborates

K₂Ga(B₅O₁₀)(H₂O)₄ was synthesized by Liu et al. via a mild solvothermal method in 2007. It was the first reported member of borates featuring a three-dimensional zeolite-like [GaB₅O₁₀]_∞ framework.⁴² Liu et al. reported the synthesis of K₂Ga(B₅O₁₀)(H₂O)₄ and discussed its structural features; however, they did not further study its physical properties. The [GaB₅O₁₀]_∞ anionic skeletons, comprised of four-fold coordinated [GaO₄] tetrahedra and [B₅O₁₀] pseudo tetrahedra, are able to encapsulate guest molecules and counter cations ranging from inorganic components to organic amines. Isostructural galloborate Rb₂Ga(B₅O₁₀)(H₂O)₄ was discovered later.⁴³ The similar hybrid member (H₂EDAP)Ga(B₅O₁₀)(H₂O) exhibited an SHG response 0.5 times that of KDP.⁴⁴ The flexibilities of the zeolite-like [GaB₅O₁₀]_∞ frameworks are reflected in the tuneable sizes of their skeletons for guest encapsulation, with their changeable topologies transforming from dia to und if [GaO₄] tetrahedra and [B₅O₁₀] units act as 4-connected nodes.⁴⁰ Rb₂Ga(B₅O₁₀)(H₂O)₄ and KCs₂Ga(B₅O₁₀)(OH) are two galloborates in this category, and they show non-isostructural anionic frameworks with different topologies. Their optical properties have been preliminarily studied.

Rb₂Ga(B₅O₁₀)(H₂O)₄ synthesized by Mao et al. in 2012 crystallizes in the chiral space group of C222₁ (no. 20).⁴³ In the structure of Rb₂Ga(B₅O₁₀)(H₂O)₄, the centred Ga cations are four-coordinated with neighbouring [B₅O₁₀] FBBs (Fig. 1a), while the [B₅O₁₀] FBBs share bridging μ₂-O atoms with the epitaxial Ga cations. The alternate connectivity between the [GaO₄] tetrahedra and [B₅O₁₀] FBBs gives rise to the three-dimensional [GaB₅O₁₀]_∞ framework with the dia topology (Fig. 1b and c). Rb cations and guest water molecules with the ratio of 1 : 2 occupy the vacancies to ensure the charge balance. The band gap of Rb₂Ga(B₅O₁₀)(H₂O)₄ is 3.54 eV, which is slightly smaller than that of isostructural (NH₄)₂Al(B₅O₁₀)(H₂O)₄ (3.96 eV) and K₂Al(B₅O₁₀)(H₂O)₄ (4.45 eV). Powder SHG measurements based on Kurtz–Perry rules revealed that Rb₂Ga(B₅O₁₀)(H₂O)₄ shows a moderate SHG intensity comparable to that of KDP, while the SHG responses of (NH₄)₂Al(B₅O₁₀)(H₂O)₄ and K₂Al(B₅O₁₀)(H₂O)₄ are two times that of KDP.⁴⁵ Given that [B₅O₁₀] FBBs have two roughly perpendicular [B₂O₅] dimers, the [BO₃] triangles are naturally unable to align. The upper limit of the SHG response is restricted by the geometric configuration of [B₅O₁₀] FBBs.

Fig. 1 (a) [GaB₅O₁₀] fragment in the structure of Rb₂Ga(B₅O₁₀)(H₂O)₄. (b) Structure of Rb₂Ga(B₅O₁₀)(H₂O)₄. (c) The dia topology of the [GaB₅O₁₀]_∞ anionic skeleton of Rb₂Ga(B₅O₁₀)(H₂O)₄. (d) [GaB₅O₁₀] fragment in the structure of KCs₂Ga(B₅O₁₀)(OH). (e) Structure of KCs₂Ga(B₅O₁₀)(OH) incorporating three kinds of channels. (f) The und topology of the [GaB₅O₁₀]_∞ anionic skeleton of Rb₂Ga(B₅O₁₀)(H₂O)₄. Rb, Ga, B, O, and H atoms are shown in rose, orange, olive, red, and white, respectively.

In 2023, Yang et al. used gallium isopropoxide as a gallium source and synthesized a mixed alkali metal hydrated galloborate KCs₂Ga(B₅O₁₀)(OH), and then immediately uncovered its potential as an NLO crystal with a deep-UV transparency window.⁴⁰ It should be noted that large amounts of galloborates and aluminoborates have been obtained under mild hydro/solvothermal condition since Yang et al. first proposed the use of metal isopropoxides as new sources for replacing the traditional inorganic metal oxides in 2009.⁴⁵ KCs₂Ga(B₅O₁₀)(OH) crystallizes in the tetragonal space group I4̄2d (no. 122), and its zeolitic framework [GaB₅O₁₀]_∞ exhibits a totally different topology to that of Rb₂Ga(B₅O₁₀)(H₂O)₄ (Fig. 1c and f). The asymmetric unit of KCs₂Ga(B₅O₁₀)(OH) contains half a Ga atom, half a [B₅O₁₀] cluster, and split alkali metal cations and their disordered coordinated hydroxyl groups. Although the [GaO₄] tetrahedra in KCs₂Ga(B₅O₁₀)(OH) are also coordinated with neighbouring four [B₅O₁₀] FBBs, the geometric configuration of the [GaB₅O₁₀] fragment in KCs₂Ga(B₅O₁₀)(OH) is different from that in Rb₂Ga(B₅O₁₀)(H₂O)₄ (Fig. 1a and d). As depicted in Fig. 1e, there are two types of open tunnels (the green and yellow channels) along the [001] direction, while there is another type of open tunnel along the [100] direction (the purple channels). Split alkali metal cations and disordered hydroxyl groups fill these channels. The experimental bandgap of KCs₂Ga(B₅O₁₀)(OH) converted from diffuse reflection data using the Kubelka–Munk equation is as large as 6.30 eV, indicating that KCs₂Ga(B₅O₁₀)(OH) may have a wide transparency window in the short-wavelength UV spectral region. Powder SHG measurements of KCs₂Ga(B₅O₁₀)(OH) demonstrated that it exhibits an enhanced SHG response of about 1.4 times that of KDP.

Barium galloborate Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O) was discovered by Mao et al. in 2014.⁴⁶ Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O) crystallizes in the polar space group Cc (no. 9) and the asymmetric unit of Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O) consists of four Ba cations, one Ga cation, ten boron atoms, twenty-three oxygen atoms, and five hydrogen atoms. Its structure features a three-dimensional {Ga[B₁₀O₁₈(OH)₅]}_∞ covalent anionic skeleton composed of [B₁₀O₁₈(OH)₅] FBBs and [GaO₄] tetrahedra joined by Ga–O–B linkages (Fig. 2c). The complex FBB, [B₁₀O₁₈(OH)₅], oligomerized from seven [BO₄] tetrahedra and three [BO₃] triangles, possesses five terminal hydroxyl groups and four exocyclic oxygen atoms for further connections with Ga cations (Fig. 2a). To simplify the structural description, [B₁₀O₁₈(OH)₅] FBB is divided into two parts here (Fig. 2a): part 1 [B₃O₇(OH)] and part 2 [B₇O₁₂(OH)₄]. As depicted in Fig. 2b, each [GaO₄] tetrahedron is coordinated with three [B₃O₇(OH)] (part 2) via bridging μ₂-O atoms to form a two-dimensional {GaO[B₃O₇(OH)]}_∞ layer expanding in the (001) plane. The interlayer [B₇O₁₂(OH)₄] (part 1) linkers connect the adjacent sheets to form the three-dimensional covalent skeleton {Ga[B₁₀O₁₈(OH)₅]}_∞ with Ba cations and water molecules filling the vacancies. The diffuse-reflectance absorption spectrum of Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O) indicates that it is transparent in the UV region as its band gap is as large as 4.12 eV. The powder SHG response of Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O) is relatively weak (about 0.2 × KDP@1064 nm), which may be attributed to the low ratio of [BO₃] : {[BO₄] + [GaO₄]} and the unfavourable arrangement of [BO₃] units.

Fig. 2 (a) Complex [B₁₀O₁₈(OH)₅] FBB can be divided into two parts: part 1 [B₇O₁₂(OH)₄] and part 2 [B₃O₇(OH)]. (b) Two-dimensional {GaO[B₃O₇(OH)]}_∞ layer. (c) Structure of Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O). Ba, Ga, B, O, and H atoms are shown in light yellow, orange, olive, red, and white, respectively.

Na₄Ga₃B₄O₁₂(OH), another alkali metal galloborate incorporating [BO₃] FBBs, was synthesized under surfactant-thermal conditions by Yang et al. in 2017.⁴⁷ The structure of Na₄Ga₃B₄O₁₂(OH) crystallizes in the noncentrosymmetric cubic space group F4̄3c (no. 219) and its structure features a 3D framework composed of octahedral [Ga₆(BO₃)₄] cages. The octahedral [Ga₆(BO₃)₄] cage is constructed by four [BO₃] triangles and six Ga cations with the [BO₃] triangles capping the four faces and six Ga cations occupying the vertex-sites of octahedron (Fig. 3a). Each [Ga₆(BO₃)₄] cage further connects the neighbouring six cages, thus expanding to an unusual zeolite-topology network [Ga₃B₄O₁₂(OH)]_∞ with disordered Na cations and their coordinated hydroxyl groups filling the cavities (Fig. 3b and c). The experimental band gap (E_g = 4.9 eV) determined from its diffuse-reflectance spectrum indicates that the cut-off edge of Na₄Ga₃B₄O₁₂(OH) is in the short-wavelength UV region. The three principal refractive indices of the crystals belonging to advanced crystal family are equal (n₁ = n₂ = n₃ = n₀), even though Na₄Ga₃B₄O₁₂(OH) crystallizes in the noncentrosymmetric space group. This indicates that light waves propagate through crystals in any direction exactly like they would in an isotropic medium, without causing the birefringence phenomenon. This species is not phase-matching and the large dihedral angle between the [BO₃] triangles (70.529°) as well as the weak powder SHG response (0.1 × KDP) suggest that Na₄Ga₃B₄O₁₂(OH) may not be a satisfactory NLO crystal.

Fig. 3 (a) [Ga₆(BO₃)₄] cage. (b) Cubic cavity (blue ball) constructed from eight [Ga₆(BO₃)₄] cages (purple balls). (c) Structure of Na₄Ga₃B₄O₁₂(OH). Na, Ga, B, O, and H are shown in yellow, orange, olive, and red, respectively.

LiGa(OH)(BO₃)(H₂O) was obtained by Mao et al. in 2012, and it is the sole KBBF-like galloborate constructed from two-dimensional layers via hydrogen bonding interactions.⁴³ LiGa(OH)(BO₃)(H₂O) crystallizes in the P31c (no. 159) space group and there are three kind of basic building units in its structure: triangular [BO₃], tetrahedral hydrated [LiO₃(H₂O)], and [GaO₃(OH)] (Fig. 4a). The assembly of [BO₃], [LiO₃(H₂O)], and [GaO₃(OH)] according to the ratio of 1 : 1 : 1 in the (001) plane give rise to the formation of a two-dimensional [Li(H₂O)Ga(OH)(BO₃)]_∞ layer with coordinated water molecules and hydroxyl groups branched out from the sheet (Fig. 4b). It should be noted that the coordinated water molecules and hydroxyl groups locate in the crystallographic 3-fold axes; therefore, all the hydrogen atoms are disordered, obeying the intrinsic geometric configurations of the hydroxyl group and water molecule, and these terminal units may act as both donors and acceptors of hydrogen bonds. The interlayer O–H⋯O hydrogen bonds further induce the [Li(H₂O)Ga(OH)(BO₃)]_∞ layers to stack along the [001] directions (Fig. 4c). The functional [BO₃] units are optimally aligned in the lattice and the density of [BO₃] unit (8.22 × 10⁻³ Å⁻³) is comparable to that of KBBF (9.42 × 10⁻³ Å⁻³), which indicates that an SHG response and a birefringence comparable to that of KBBF could be expected. Unfortunately, further detailed optical properties of this crystal were not reported in the literature due to an unremovable unidentified impurity.

Fig. 4 (a) Three kinds of basic building units ([BO₃], [LiO₃(H₂O)], and [GaO₃(OH)]) of LiGa(OH)(BO₃)(H₂O). (b) Two-dimensional [Li(H₂O)Ga(OH)(BO₃)]_∞ layer. (c) Structure of LiGa(OH)(BO₃)(H₂O). Li, Ga, B, O, and H atoms are shown in purple, orange, olive, red, and white, respectively.

Borates incorporating more than one kind of B–O clusters are rare as they naturally disobey Pauling's 5th rule. The galloborates discussed above contain only one kind of FBB, but there are two cases of hydrated galloborates constructed from two kinds of FBBs.

Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃ was identified as a new NLO crystal by Yang et al. in 2022.⁴¹ Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃ crystallizes in the polar C2 (no. 5) space group and there are two kinds of FBBs in the lattice: the [B₇O₁₃(OH)] FBB and the isolated [B(OH)₃] cluster (Fig. 5a). The polymerization of [B₇O₁₃(OH)] FBBs along the b axis leads to the formation of a one-dimensional [B₇O₁₂(OH)]_∞ chain (Fig. 5b). The [GaO₄] units connect the as-formed chains to form a {Ga[B₇O₁₂(OH)]}_∞ layer expanding in the bc plane (Fig. 5c). The isolated [B(OH)₃] FBBs are not involved in the {Ga[B₇O₁₂(OH)]}_∞ layer, as they as well as the Na cations are located in the voids between adjacent layers and stabilize the whole structure by hydrogen bonds and ionic bonds (Fig. 5d). The experimental data of Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃ showed a relatively small band gap as large as 3.90 eV but a remarkable powder SHG intensity. The extremely large powder SHG response of Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃ (as large as 4.6 times that of KDP) is a record in galloborate systems. Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃ is phase-matching according to the Kurtz–Perry rules.

Fig. 5 (a) [B₇O₁₃(OH)] FBB and [B(OH)₃] FBB. (b) One-dimensional [B₇O₁₂(OH)]_∞ chain. (c) Two-dimensional {Ga[B₇O₁₂(OH)]}_∞ layer. (d) Structure of Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃. Na, Ga, B, O, and H atoms are shown in yellow, orange, olive, red, and white, respectively.

Ba₃Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] is another NLO-active galloborate constructed by two kinds of FBBs (Fig. 6a).⁴⁸ This species was found and identified by Yang et al. in 2013 for the first time. Ba₃Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] crystallizes in the polar space group Fdd2 (no. 43), and it features a sandwich {Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂]}_∞ layer. Each sandwich {Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂]}_∞ layer can be divided into two [GaOB₃O₆(OH)]_∞ single layer and [B₄O₇(OH)₂] linkages (Fig. 6c). As shown in Fig. 6b, only [B₃O₆(OH)] FBBs as well as the [GaO₄] tetrahedra are involved in constructing the [GaOB₃O₆(OH)]_∞ single layer with embedded eighteen-membered rings (18-MRs). A pair of [GaOB₃O₆(OH)]_∞ single layers are assembled into one sandwich layer through the bridging [B₄O₇(OH)₂] FBBs. Apart from the Ga–O ionic bonding interaction, the hydrogen bonds between two kinds of FBBs also help to stabilize the whole sandwich layer. As shown in Fig. 6d, Ba cations are located within and between the sandwiched layers to balance the charge and expand the two-dimensional layer to a three-dimensional framework. Interestingly, Ba₃Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] is isostructural with its Al counterpart. The Ba cations in the structure of Ba₃Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] are disordered (Fig. 6d) while the Ba cations in the structure of Ba₃Al₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] are ordered. The large SHG response (3.0 × KDP@1064 nm) and the band gap (E_g = 5.0 eV) are comparable to Ba₃Al₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] (3.0 × KDP@1064 nm, E_g = 5.4 eV), as can be inferred from their highly similar structures. The excellent optical properties indicate that Ba₃Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂] may be a potential NLO crystal in the UV region.

Fig. 6 (a) [B₃O₆(OH)] FBB and [B₄O₇(OH)₂] FBB. (b) Two-dimensional [GaOB₃O₆(OH)]_∞ single layer. (c) Complex {Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂]}_∞ layer. (d) Structure of Ba₃Ga₂[B₃O₆(OH)]₂[B₄O₇(OH)₂]. Ba, Ga, B, O, and H atoms are shown in yellow, orange, olive, red, and white, respectively.

3. Alkali/alkaline earth metal galloborate halides

The first galloborate halide NaBa₄(GaB₄O₉)₂Cl₃ was synthesized by Li et al. in 2006, and its structure was determined by powder X-ray diffraction data.⁴⁹ NaBa₄(GaB₄O₉)₂Cl₃ is a salt-inclusion galloborate, which shows a [GaB₄O₉] network featuring the dia topology. It is formed by the interconnection of [B₄O₉] FBBs and [GaO₄] tetrahedra with Ba cations, Cl ions, and NaCl salt filling the tunnels. Soon after the discovery of NaBa₄(GaB₄O₉)₂Cl₃, Barbier reported three isostructural borate halides, namely Ba₂TB₄O₉Cl (T = Al and Ga) and Ba₂GaB₄O₉Br, whose structures are similar with NaBa₄(GaB₄O₉)₂Cl₃.⁵⁰ Noncentrosymmetric NaBa₄(AlB₄O₉)₂X₃ (X = Cl and Br) was synthesized by Wang et al. and Pan et al. in 2012 to 2013.^51,52 In 2016, the single crystals of NaBa₄(GaB₄O₉)₂X₃ (X = Cl and Br) were successfully obtained by Pan et al.⁵³ Moreover, Pan et al. also identified [GaO₄] as an NLO-active unit for the first time. The mentioned tetraborates show the same [GaB₄O₉] anionic skeleton featuring the dia topology with salt components incorporated.

Since Ba₂GaB₄O₉Cl and Ba₂GaB₄O₉Br are isostructural, only the structure of Ba₂GaB₄O₉Cl is discussed here. Ba₂GaB₄O₉Cl crystallizes in the P4₂nm space group (no. 102). All the [GaO₄] tetrahedra in the structure are four-coordinated (Fig. 7a), and the alternative arrangement of Ga cations and B–O clusters gives rise to a zeolite-like anionic framework with the dia topology (Fig. 7c and d). There are two kinds of channels incorporated in the [GaB₄O₉]_∞ anionic skeleton for encapsulation of the salt components (see the blue and purple pipes in Fig. 7b, c and e). Ba₂GaB₄O₉Cl has a moderate SHG effect (about 1.0 × KDP@1064 nm), while the powder SHG response of Ba₂GaB₄O₉Br has not yet been reported.

Fig. 7 (a) Coordination environment of Ga cations. (b) One-dimensional channel containing [BaCl]_∞ chains. (c) Three-dimensional [GaB₄O₉]_∞ anionic skeleton. (d) The dia topology of Ba₂GaB₄O₉Cl and NaBa₄(GaB₄O₉)₂X₃ (X = Cl, Br). (e) One-dimensional channels containing [Ba₂Cl]_∞ chains. (f) [NaCl]_∞ and [Ba₂Cl]_∞ chains filling the two kinds of channels of the [GaB₄O₉]_∞ anionic skeleton. Na, Ba, Ga, B, O, and H atoms are shown in yellow, light yellow, orange, olive, red, white, and green, respectively.

Both NaBa₄(GaB₄O₉)₂Cl₃ and NaBa₄(GaB₄O₉)₂Br₃ crystallize in the P42nm space group (no. 102), and their structures are similar to that of Ba₂GaB₄O₉Cl with different included salts. The Ba cations, Cl ions, and NaCl salt make up a ionic bonding network filling the tunnels in the [GaB₄O₉]_∞ anionic skeleton (Fig. 7c and f). Although Ga³⁺ and Al³⁺ cations have similar outer electronic configurations, the 3d electrons of Ga³⁺ cations as well as the reduced ability of dangling bond elimination narrows the band gaps of galloborates compared with isostructural aluminoborates. NaBa₄(GaB₄O₉)₂Cl₃ and NaBa₄(GaB₄O₉)₂Br₃ show narrowed band gaps (3.76 and 3.71 eV) compared with the band gaps of their aluminium counterparts (3.93 and 3.95 eV). Whereas NaBa₄(GaB₄O₉)₂Cl₃ and NaBa₄(GaB₄O₉)₂Br₃ show enhanced powder SHG responses as large as 1.5 times and 1.1 times that of KDP under 1064 nm fundamental laser radiation, the SHG responses of NaBa₄(AlB₄O₉)₂X₃ (X = Cl and Br) are 0.9 times and 0.8 times that of KDP, respectively. Theoretical calculations illustrated that the Ga–O hybrid orbitals also contribute to the optical properties and thus verify that the [GaO₄] non-π conjugated is also an NLO-active chromophore.

In 2013, Li et al. synthesized a mixed metal galloborate K₂Ba₄Ga₄Li₂B₆O₂₁ by spontaneous crystallization with K₂O–B₂O₃–LiF flux.⁵⁴ K₂Ba₄Ga₄Li₂B₆O₂₁ can be regarded as the result of chemical co-substitution from the parental K₂Al₂B₂O₇ and its formula is consistent with the triple K₂Al₂B₂O₇ (K₆Al₆B₆O₂₁). Covalent [LiO₄] and [GaO₄] tetrahedra act as 4-connected linkages, like [AlO₄] in K₂Al₂B₂O₇. The polymerization of [LiO₄], [GaO₄], and [BO₃] in the (001) plane leads to the formation of the three-dimensional [Ga₄Li₂B₆O₂₁]_∞ framework. The UV–Vis diffuse-reflectance spectrum shows that the cut-off edge of K₂Ba₄Ga₄Li₂B₆O₂₁ is about 226 nm. The structure of K₂Ba₄Ga₄Li₂B₆O₂₁ is seriously disordered (Fig. 8), and the powder SHG response of K₂Ba₄Ga₄Li₂B₆O₂₁ is as large as only half that of KDP.

Fig. 8 Structure of K₂Ba₄Li₂Ga₄B₆O₂₁. K, Ba, Li, Ga, B, and O atoms are shown in blue, light yellow, purple, orange, olive, and red, respectively.

Inspired by the discovery of K₂Ba₄Ga₄Li₂B₆O₂₁, a series of aluminoborate fluorides and galloborate fluorides (A₃Ba₃Li₂T₄B₆O₂₀F, A = K or Rb, T = Al or Ga) with similar formulas were synthesized and identified as NLO crystals.^55–57 K₃Ba₃Li₂Ga₄B₆O₂₀F and Rb₃Ba₃Li₂Ga₄B₆O₂₀F were synthesized by Xia et al. in 2018, after the discovery of K₃Ba₃Li₂Al₄B₆O₂₀F and Rb₃Ba₃Li₂Al₄B₆O₂₀F. Although galloborate fluorides A₃Ba₃Li₂Ga₄B₆O₂₀F (A = K and Rb) possess a similar formula to the aluminoborate fluorides A₃Ba₃Li₂Ga₄B₆O₂₀F (A = K and Rb), they cannot be regarded as the results of a simple chemical substitution of [AlO₄] by [GaO₄] tetrahedra. A₃Ba₃Li₂Al₄B₆O₂₀F (A = K and Rb) features a double-layered [Li₂Al₄B₆O₂₀F]_∞ sheet (Fig. 9a) while the anionic skeleton of A₃Ba₃Li₂Ga₄B₆O₂₀F (A = K and Rb) is the three-dimensional [Ga₄Li₂B₆O₂₀F]_∞ framework (Fig. 9b). The different orientations of covalent [LiO₄] and [AlO₄]/[GaO₄] are responsible for the different dimensionalities of [Li₂Al₄B₆O₂₀F]_∞ and [Ga₄Li₂B₆O₂₀F]_∞. The [LiO₄] and [AlO₄] tetrahedra marked with yellow arrows face the same positions and are coordinated with [BO₃] triangles to form the [Li₂Al₄B₆O₂₀F]_∞ double layer, whereas the [LiO₄] and [AlO₄] tetrahedra marked with yellow and blue arrows face opposite positions to expand the layered structure to a three-dimensional framework, as depicted in Fig. 9c and d. The weakened powder SHG responses of A₃Ba₃Li₂Ga₄B₆O₂₀F (A = K and Rb) were 0.7 and 0.5 times that of KDP, which are smaller than that of A₃Ba₃Li₂Al₄B₆O₂₀F (A = K and Rb) (1.5 × KDP@1064 nm). The calculated birefringence values were 0.0416 and 0.0434@800 nm, respectively.

Fig. 9 (a) Structure of KBa₃Li₂Al₄B₆O₂₀F. (b) Structure of KBa₃Li₂Ga₄B₆O₂₀F. (c) Double-layered [Li₂Al₄B₆O₂₀F]_∞ sheet. (d) Double-layered [Li₂Ga₄B₆O₂₀F]_∞ sheet. K, Ba, Li, Al, Ga, B, O, and F atoms are shown in blue, light yellow, purple, light blue, orange, olive, red, and pink, respectively.

4. Summary and perspectives

This frontier article focused on the recent progress of NLO inorganic galloborates. These galloborates are divided into two types, namely alkali/alkaline earth metal galloborates and alkali/alkaline earth metal galloborate halides, in terms of their compositions. Their chemical formulas, space groups, anionic structural features, SHG intensities, and band gaps are summarized. New raw materials and synthetic methods are used to obtain new galloborates. For example, gallium isopropoxide and the surfactant–thermal method are used in the synthetic process. In these galloborates, KCs₂Ga(B₅O₁₀)(OH) shows the shortest cut-off edge of 190 nm, while Na₅Ga[B₇O₁₂(OH)]₂·2B(OH)₃ shows the largest SHG effect of 4.6 × KDP. Although some progress has been achieved, the development of NLO galloborates is still in the primary stage, and some aspects need further study.

It is noteworthy that only the [GaO₄] configuration has been observed in these NLO galloborates. Ga cations have several other coordination geometries, such as square pyramid, trigonal bipyramidal, and octahedron. It is expected that noncentrosymmetric galloborates could be obtained by the combination of [GaO₅]/[GaO₆] with borate anionic units. The fluorination strategy has been widely used to obtain new NLO crystals. Two NLO alkali/alkaline earth metal galloborate fluorides with enlarged band gaps have been reported, where the introduction of fluoride ions greatly widens the transparency windows in the short-wavelength spectral region. In these two compounds, fluorine ions are only coordinated with alkali/alkaline earth metal cations. Neither [GaO_mF_n] (m + n = 4, 5, 6) nor [BO_xF_4−x] (x = 1, 2, 3) units are observed in these NLO galloborates. Galloborates with larger SHG effects, and shorter cut-off edges are expected if these NLO-active units can be realized in the future. It should be noted that emergent boron-based compounds with other anionic units, like [NO₃], [CO₃], [SO₄], [PO₄], [GeO₄], and [SiO₄], will accelerate the exploration of new NLO crystals.^58–63 Finally, it is important to grow large single crystals for practical applications. Compared with aluminoborates, the general raw material of Ga₂O₃ possesses a lower melting point, and the viscosity of the corresponding melts are also reduced, which are favourable to crystal growth. We believe the structural diversity of galloborates will inspire the design of more functional NLO crystals in future work.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant 21975224).

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