Recent Progress in Crystalline Borates with Edge-Sharing BO₄ Tetrahedra

Abstract

Crystalline borates have received great attention due to their various structures and wide applications. For a long time, the corner-sharing B–O unit is considered a basic rule in borate structural chemistry. The Dy₄B₆O₁₅ synthesized under high-pressure is the first oxoborate with edge-sharing [BO₄] tetrahedra, while the KZnB₃O₆ is the first ambient pressure borate with the edge-sharing [BO₄] tetrahedra. The edge-sharing connection modes greatly enrich the structural chemistry of borates and are expected to expand new applications in the future. In this review, we summarize the recent progress in crystalline borates with edge-sharing [BO₄] tetrahedra. We discuss the synthesis, fundamental building blocks, structural features, and possible applications of these edge-sharing borates. Finally, we also discuss the future perspectives in this field.

1. Introduction

Borates show rich structural chemistry and have broad applications as birefringent materials and nonlinear optical (NLO) materials [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]. The famous KBe₂BO₃F₂ (KBBF), LiB₃O₅ (LBO), and β-BaB₂O₄ (β-BBO) crystals are used to generate ultraviolet (UV) or deep-UV lasers through cascaded frequency conversion in practical application [32,33,34]. α-BaB₂O₄ (α-BBO) is an excellent UV birefringent crystal with a wide transparency window from 190 nm to 3500 nm and a large birefringence of 0.15 at 266 nm [35]. To date, the number of synthetic borates and borate minerals are over 3900 in the documented literature [1]. Three types of B–O units of linear [BO₂], triangular [BO₃], and tetrahedral [BO₄] are observed in these borates in which linear [BO₂] with sp hybridized chemical bonds are extremely rare; only 0.1% of borates contain the linear [BO₂] configuration. M₅Ba₂(B₁₀O₁₇)₂(BO₂) (M = K, Rb) and NaRb₆(B₄O₅(OH)₄)₃(BO₂) are three typical examples; the former two compounds contain unusual [BO₂] with the traditional [BO₃] and [BO₄] units and exhibit suitable birefringence (Δn = 0.06) and transparency windows down to the deep-UV region (<190 nm) [36,37]. Theoretical analyses reveal that the [BO₃] and [BO₄] units have the smaller polarizability anisotropy compared with linear [BO₂]. While the latter one is the first noncentrosymmetric and chiral structure with the linear [BO₂] unit and displays a weak second-harmonic generation response (SHG) (0.1 × SiO₂) and wide transparency of about 21.2% at 200 nm [38].

In 2021, Pan and coworkers summarized the synthesis, fundamental building blocks (FBBs), symmetries, structure features, and functional properties of the reported anhydrous borates [1]. The FBBs of polynuclear borates are generally formed by corner-/edge-sharing [BO₃] and [BO₄] units. Cs₃B₇O₁₂ contains a large FBB with 63 boron atoms in which 35 (or 37) BO₃ triangles and 28 (or 26) BO₄ tetrahedra are linked to form thick anionic sheets stacked along the c direction [39]. Mg₇@[B₆₉O₁₀₈(OH)₁₈] contains 42 [BO₃] triangles and 27 [BO₄] tetrahedra; it exhibits a supramolecular framework with hexagonal snowflake-like channels; unique triple-helical ribbons are found in {B₆₉} FBBs [40]. This huge [B₆₉O₁₀₈(OH)₁₈] cluster represents the largest FBB in borates. The FBBs can further polymerize into 1D chains, 2D layers, and 3D networks [41,42,43,44,45,46,47,48,49]. For example, we obtained three alkali and alkaline earth-metal borates, namely Ba₂B₁₀O₁₆(OH)₂·(H₃BO₃)(H₂O), Na₂B₁₀O₁₇·H₂en, and Ca₂[B₅O₉]·(OH)·H₂O [41,42,43], in which pentaborates are used to construct a single-layered structure, 2D microporous layers, and a 3D network, respectively. Ca₂[B₅O₉]·(OH)·H₂O is impressive with a dense net consisting of pcu B−O net and dia Ca−O net and exhibits a short UV cutoff edge below 200 nm and a strong SHG response of ~three times that of KH₂PO₄ (KDP) [43].

In 2002, Huppertz and coworkers reported the high-pressure synthesis of Dy₄B₆O_15; it is the first oxoborate with an edge-sharing BO₄ tetrahedra [50]. The edge-sharing [BO₄] tetrahedra in Dy₄B₆O₁₅ changes the rule of corner-sharing [BO₃]/[BO₄] units in borate structural chemistry. In addition, it is considered that the extreme synthetic conditions, such as high pressure, is necessary for edge-sharing borates. In 2010, the discovery of KZnB₃O₆ changed this view; KZnB₃O₆ represents the first ambient pressure edge-sharing [BO₄]-containing borate [51]. To date, edge-sharing [BO₄]-containing borates are still rare; less than 1% of borates contain edge-sharing BO₄ tetrahedra. Over the past decade, the synthesis, crystal structures, and properties of KBBF-like borates [52], fluorooxoborates [53,54], high-temperature borates [55], high-pressure borates [56], f-element borates [57], zincoborates [58,59], aluminoborates [60,61], borogermanates [62], hybrid d- or p-block metal borates [63], and hydrated borates with non-metal or transition-metal complex cations have been well reviewed [64]. Herein, we give a detailed summary of the recent progress in crystalline borates with edge-sharing BO₄ tetrahedra. These edge-sharing borates can be grouped into two types in terms of their synthetic method: (i) high pressure synthesis of borates with edge-sharing [BO₄] tetrahedra and (ii) ambient pressure synthesis of borates with edge-sharing [BO₄] tetrahedra. We discuss the synthesis, FBBs, structural features, potential applications, and future perspectives of edge-sharing borates.

2. High Pressure Synthesis of Borates with Edge-Sharing [BO₄] Tetrahedra

The existence of uncommon edge-sharing [BO₄] tetrahedra disobeys Pauling’s third rule. The borates containing the so-called edge-sharing [B₂O₆] dimer were initially believed to be obtained only under extreme conditions, such as high temperature and high pressure. Since the first case of this species was discovered, multi-anvil high-pressure synthesis is the dominant route to obtain the new edge-sharing [BO₄] tetrahedra-containing borates. Up to now, there are 26 high-pressure edge-sharing borates within the scope of discussion. Boron atoms tend to coordinate with four O atoms to form [BO₄] tetrahedra under a high-pressure environment, as evidenced by most of these high-pressure compounds constructed merely from [BO₄] tetrahedra. Even in [BO₃]-containing borates, such as high-pressure AB₃O₅, the proportion of the [BO₃] triangle is only 1/3.

2.1. Rare Earth Borates

RE₄B₆O₁₅ (RE = Dy and Ho). Dy₄B₆O₁₅ is the first reported metal borate with edge-sharing [BO₄] tetrahedra; it was obtained under high-temperature (1273 K) and high-pressure (8 Gpa) conditions by Huppertz et al. in 2002 [50]. Shortly after, isostructural Ho-analogues was prepared under the same extreme high-pressure condition in 2003 [65]. The RE₄B₆O₁₅ series crystallize in the monoclinic crystal system with the space group of C2/c (no. 15); their structures exhibit corrugated ²[B₆O₁₅]_∞ layers formed by the linkage of the adjacent [B₁₂O₃₅] clusters (Figure 1b). The large [B₁₂O₃₅] cluster, incorporating edge-sharing and corner-sharing [BO₄] tetrahedra with the ratio of 8:4, can be considered as the FBB of RE₄B₆O₁₅ (Figure 1a). Furthermore, the interlayer rare earth ions connect these corrugated layers to form the final 3D structures (Figure 1c). The multi-anvil techniques, which can offer external pressures, accelerate the discovery of borates with unusual edge-sharing [BO₄] tetrahedra and initiate the era of exploring such borates under multi-anvil high-pressure conditions.

α-RE₂B₄O₉ (RE = Sm, Eu, Gd, Tb, Dy, Ho and Y). α-RE₂B₄O₉ borates (RE = Sm, Eu, Gd, Tb, Dy, Ho and Y) are another rare earth borate series with edge-sharing [BO₄] tetrahedra reported in the period of 2002 to 2017 [66,67,68,69]. Similar to the RE₄B₆O₁₅ series, the α-RE₂B₄O₉ series crystallize in the same space group (C2/c, no. 15) in which all the incorporating boron atoms are four-coordinated. In these structures, the complex [B₂₀O₅₅] FBB is comprised with edge- and corner-sharing [BO₄] tetrahedra according to the ratio of 18:2 (Figure 2a and blue blanket in Figure 2b). With respect to the whole covalent B–O framework of RE₄B₆O₁₅, the ³[B₆O₁₅]_∞ network is formed by the linkage of [B₂₀O₅₅] FBBs, the rare earth cations located in the channels (Figure 2b).

La₃B₆O₁₃(OH). During the synthetic process, the replacement of the anhydrous boron source with boric acid, hydrated borates, or borates containing water molecules are sometimes obtained. La₃B₆O₁₃(OH) is the first SHG-active edge-sharing [BO₄] tetrahedra-containing borate [70]. This compound was obtained by a high-pressure/high-temperature condition at 6 GPa and 1673 K and was immediately identified as an NLO crystal by Huppertz et al. in 2020. It crystallizes in the chiral space group, P2₁ (no. 4), and presents a 2D ²[B₆O₁₃(OH)]_∞ layered structure with La ions located between the layers (Figure 3). The FBB of La₃B₆O₁₃(OH) features a ‘sechser’-ring, which is constructed of one [B₂O₆], three vertex-sharing [BO₄], and one [BO₃(OH)] (Figure 3a). The [B₆O₁₆(OH)] FBBs are linked into a 2D ²[B₆O₁₃(OH)]_∞ layer along the bc plane, which further stack along [100] direction with La ions residing in the interlayer space (Figure 3b). Although La₃B₆O₁₃(OH) crystallizes in a noncentrosymmetric space group, its basic B–O units in the lattice are all the non-π-conjugated tetrahedral. La₃B₆O₁₃(OH) displays a relatively weak SHG effect. Compared to the non-π-conjugated [BO₄] tetrahedron with negligible hyperpolarization, the π-conjugated motifs represented by planar [BO₃] and [B₃O₆] in the borate system are superior NLO-active functional modules, and thus, the powder SHG response of La₃B₆O₁₃(OH) based on the Kurtz–Perry method is as weak as 2/3 times that of quartz.

2.2. Transition Metal Borates

TMB₂O₄ (TM = Ni, Fe and Co). Previous research on edge-sharing [BO₄]-containing borates mainly focus on lanthanide borates. Later, researchers achieved the combination of transition metal and edge-sharing [BO₄] tetrahedra. From 2007 to 2010, a series of high-pressure transition metal borates, TMB₂O₄ (TM = Ni, Fe and Co), were discovered by Huppertz and coworkers [71,72,73]. All boron atoms in this species are four-coordinated, and the FBB is the simplest [B₂O₆] cluster (Figure 4b). Each edge-shared [B₂O₆] dimmer is linked to four surrounding [B₂O₆] units through μ₂-O atoms, resulting in a dense 2D ²[B₂O₄]_∞ layer with six-member rings (6 MRs) (Figure 4a). The stacking of ²[B₂O₄]_∞ layer along [100] direction is further linked by interlayer TM ions, which leads to the final structure of TMB₂O₄ (Figure 4c).

γ-HfB₂O₅. In 2021, the γ-phase of HfB₂O₅, which incorporates edge-sharing [BO₄] tetrahedra, was obtained under extreme pressure (120 GPa) by Huppertz [74]. γ-HfB₂O₅ crystallizes in the centrosymmetric monoclinic space group, P2₁/c (no. 14). The tetravalent transition metal Hf⁴⁺ cation displays higher coordination numbers than divalent cations, and the FBB in γ-HfB₂O₅ is changed to [B₃O₉] with the additional one vertex-sharing [BO₄] (Figure 5a). Similar to the stuctures of TMB₂O₄ series, the structure of γ-HfB₂O₅ borate also shows layered sheets with Hf ions filling the interlayer space (Figure 5b). It is interesting to note that β-HfB₂O₅ was synthesized at 7.5 GPa in the multi-anvil press, upon further compression up to 120 GPa, a shrinkage of the cell parameters during the compression process was observed, and finally the β-phase is transformed to the γ-phase. The layer in β-HfB₂O₅ contains four MRs and eight MRs by the corner-sharing BO₄ tetrahedra, while γ-HfB₂O₅ contains ten MRs, including the edge-sharing BO₄ tetrahedra. Edge-sharing BO₄ tetrahedra in new phase γ-HfB₂O₅ shows exceptionally short B–O and B⋯B distances. The coordination number of the Hf⁴⁺ cations in γ-HfB₂O₅ increased to nine in comparison to eight in its ambient pressure counterpart.

M₆B₂₂O₃₉·H₂O (M = Fe and Co). The first two acentric edge-sharing [BO₄] tetrahedra-containing borates M₆B₂₂O₃₉·H₂O (M = Fe and Co) were prepared under the high-pressure (6 GPa) and high-temperature (880 °C for Fe and 950 °C for Co) conditions in a Walker-type multi-anvil apparatus by Huppertz et al. in 2010 [75]. The M₆B₂₂O₃₉·H₂O series crystallize in a noncentrosymmetric orthorhombic space group, Pmn2₁ (no. 31). The unusually long B–O bond lengths as well as the short distances between the two boron cores are shown in the structure, which indicates the successful capture of intermediate states on the way to edge-sharing [BO₄] tetrahedra. The structure of M₆B₂₂O₃₉·H₂O shows a 3D [B₂₂O₃₉]_∞ anhydrous B–O framework with the Fe or Co ions and water molecules located in the structural channels (Figure 6a,c). Specifically, taking Fe₆B₂₂O₃₉·H₂O as an example, the B(11), O(2), O(15), and O(24) in the structure are not strictly in the same plane, and the B(11)-O(16) bond length (1.883(6) Å) is obviously longer than the common B–O distances (Figure 6b). The group of B(11) center and its three coordinated O(2,15,24) atoms as well as the neighboring O(16) can be regarded as a highly twisted polyhedron or the intermediate states between [BO₃] tringle and [BO₄] tetrahedron. The discovery of M₆B₂₂O₃₉·H₂O is helpful for understanding the dynamic process from the vertex-sharing [BO₃] + [BO₄] model to edge-sharing [BO₄] + [BO₄] model.

Co₇B₂₄O₄₂(OH)₂·2H₂O. Although the cobalt hydrated borate Co₇B₂₄O₄₂(OH)₂·2H₂O crystallizes in a centrosymmetric space group, Pbam (no. 55), it shares similar structural characteristics with Co₆B₂₂O₃₉·H₂O. This species was prepared under high-pressure (6 GPa) and high-temperature (1153 K) conditions by Huppertz et al. in 2012 [76]. The complex FBB of Co₇B₂₄O₄₂(OH)₂·2H₂O is comprised of twenty-two corner- and two edge-sharing [BO₄] tetrahedra with two hydroxy group locating in the mirror plane (Figure 7a). The structure of Co₇B₂₄O₄₂(OH)₂·2H₂O shows the ³[B₂₄O₄₂(OH)₂]_∞ framework with Co ions and water molecules located in the structural channels (Figure 7b).

2.3. Borates with Monovalent or Divalent A-Site Cations

AB₃O₅ [A = K, NH₄, Rb, Tl and Cs_1-x(H₃O)_x (x = 0.5–0.7)]. During the period of 2011 to 2014, the AB₃O₅ series [A = K, NH₄, Rb, Tl and Cs_1-x(H₃O)_x (x = 0.5–0.7)] were synthesized under high-pressure/high-temperature conditions by Huppertz et al. [77,78,79,80]. KB₃O₅ is the first compound with various configurations, including corner-sharing [BO₃], corner-sharing [BO₄], and edge-sharing [BO₄]. The FBB of the isostructural AB₃O₅ contains two [BO₃] triangles, four corner-sharing [BO₄] tetrahedra, and two edge-sharing [BO₄] tetrahedra (Figure 8a). It should be noted that the [B₂O₆] rings in AB₃O₅ can be regarded as six connected nodes; the two endocyclic O atoms of each [B₂O₆] ring are further connected with two corner-sharing [BO₄] tetrahedra. The total structures of the AB₃O₅ series exhibit 3D B–O anionic skeletons with monovalent cations locating the structural channels (Figure 8b). Although the boron source in the synthesis of AB₃O₅ series are boric acid, only the Cs_1-x(H₃O)_xB₃O₅ (x = 0.5–0.7) phase successfully incorporates oxonium ions into its structure.

CsB₅O₈. CsB₅O₈ is another alkali metal borate prepared under high-pressure (6 Gpa) and high-temperature (1173 K) conditions in a Walker-type multianvil apparatus [81]. Structurally, CsB₅O₈ features a similar structure to the aforementioned AB₃O₅ series. The basic B–O building blocks of CsB₅O₈ are corner-sharing [BO₃], corner-sharing [BO₄], and edge-sharing [BO₄]; these units exhibit a ratio of 2:1:2, respectively (Figure 9a). The structure of CsB₅O₈ exhibits a 3D B–O covalent framework with Cs ions locating in the structural channels (Figure 9b).

NaBSi₃O₈. In 2022, Gorelova et al. studied the high-pressure modification of NaBSi₃O₈, and revealed the transformation behaviors of NaBSi₃O₈ during continuous pressure increase [82]. Unexpectedly, above 27.8 GPa the crystal structure of NaBSi₃O₈ achieves the coexistence of the rare edge-sharing [BO₄] tetrahedra and earlier unknown edge-sharing [SiO₅] square pyramids. The structures under 16.2 and 27.8 Gpa are quite different. Both the 16.2 Gpa- and 27.8 Gpa-phases crystallize in the P$\overline{1}$. The Si atoms in the 16.2 Gpa-phase are all four-coordinated, and the corner-sharing [SiO₄] tetrahedra are incorporated into the 1D [Si₃O₈]_∞ chains, while 1/3 Si atoms are five-coordinated in the 27.8 Gpa-phase. These [SiO₅] square pyramids are dimerized into [Si₂O₈] units (Figure 10a,c). SiO₄ tetrahedra undergo geometrical distortion leading to the formation of SiO₅ polyhedra due to the pressure-induced transformations. The [BO₄] tetrahedra in 16.2 Gpa-phase and the [B₂O₆] dimers in 27.8 Gpa-phase act as linkers and further stable the whole structures (Figure 10b,d).

γ-BaB₂O₄. The α- and β-phases of barium metaborate are famously commercialized birefringent and nonlinear optical materials. Relevant theoretical studies offered various predicted phase of barium metaborate. In 2022, the third phase, γ-BaB₂O₄, was synthesized experimentally by Bekker et al. under conditions of 3 GPa and 1173 K [83]. γ-BaB₂O₄ crystallizes in a centrosymmetrical space group, P2₁/n (no. 14). Its anionic B–O skeleton exhibits 1D chains, which is completely different from the isolated [B₃O₆] planar cluster in α- and β-phases. The [B₄O₁₀] FBB in γ-BaB₂O₄ is comprised of one [B₂O₆] ring and two additional [BO₃] triangles (Figure 11a); these [B₄O₁₀] FBBs further assemble into the ¹[B₂O₄]_∞ chains (Figure 11b). Finally, the [BaO₁₀] polyhedra stable the ¹[B₂O₄]_∞ chains in the lattice to form the total 3D structure of γ-BaB₂O₄ (Figure 11c). The calculated band gap is up to 7.045 eV, which implies transparency in the deep-UV region. The most intense band at a frequency of 853 cm⁻¹ in the Raman spectra corresponds to the symmetric bending mode of the B−O−B−O ring in edge-sharing tetrahedra.

α-Ba₃[B₁₀O₁₇(OH)₂]. Apart from the extreme high pressure afforded by the multi-anvil high-pressure device, the hydrothermal reactor can also provide relatively high pressure. In 2019, Lii et al. reported the structures of α-Ba₃[B₁₀O₁₇(OH)₂], which were obtained through hydrothermal reactions at 773 K and 0.1 Gpa. α-Ba₃[B₁₀O₁₇(OH)₂]. The phase containing edge-sharing [BO₄] tetrahedra crystallizes in the monoclinic space group, P2₁/n (no. 14), and presents a hydrated 3D B–O framework with Ba ions filling in the cavities (Figure 12b) [84]. In terms of its FBB, the complex [B₂₀O₄₀(OH)₄] can be divided into the double [B₅O₁₂] (the blue dotted ellipse part) and [B₁₀O₁₈(OH)₄] (the red dotted blanket) categories (Figure 12a). Unlike FBBs mentioned in other borates, the [B₂O₄(OH)₂] units act as two connected nodes in the structure as the targeted [B₂O₆] units are bounded to hydrogen atoms as terminal hydroxy groups. The aggregation of [B₅O₁₂] clusters expanding in the ac plane leads to a corrugated layer, and the hydrated [B₁₀O₁₈(OH)₄] clusters connect the adjacent antiparallel layers to form the ³[B₁₀O₁₇(OH)₂]_∞ covalent skeleton.

3. Ambient Pressure Synthesis of Borates with Edge-Sharing [BO₄] Tetrahedra

The edge-sharing [BO₄] tetrahedra-containing borates obtained from classical high-temperature solution and cooling method make it possible to obtain this species more conveniently. More importantly, borates obtained under ambient pressure might incorporate more π-conjugated [BO₃] units. Edge-sharing borates with high [BO₃]:[BO₄] ratios, such as β-CsB₉O₁₄ (7:2) and Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) (22:2), are identified as birefringent crystals.

KZnB₃O₆. The first case of borate containing edge-sharing [BO₄] tetrahedra was synthesized under atmospheric pressure. KZnB₃O₆ was reported by Chen et al. and Wu et al. independently in 2010 [51,85]. KZnB₃O₆ crystallizes in the triclinic space group (P$\overline{1}$, no. 2) with a low symmetry. The [B₆O₁₂] FBB features isolated B–O cluster containing four [BO₃] tringles and two edge-sharing [BO₄] tetrahedra (Figure 13a). The aligned repetition of isolated [B₆O₁₂] clusters in the lattice gives a 2D [B₆O₁₂]_∞ pseudo layer (see the green dotted blankets in Figure 13b,c), the pairs of edge-sharing [ZnO₄] polyhedra connect the adjacent six [B₆O₁₂] clusters to form the ³[ZnB₃O₆]_∞ network with K cations filling the cavities. Later, KZnB₃O₆ was defined as highly thermally stable by Chen et al., and its unidirectional thermal expansion property was investigated. Its unique property is explained by the cooperative rotations of rigid groups [B₆O₁₂] and [Zn₂O₆] driven by anharmonic thermal vibrations of K ions [86,87,88]. The discovery of KZnB₃O₆ indicated that high pressure is not essential for obtaining edge-sharing [BO₄] tetrahedra-containing borates, and subsequently, ambient pressure edge-sharing [BO₄] tetrahedra-containing borates have been synthesized successfully one after another.

Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄). Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄) is another edge-sharing [BO₄] tetrahedra-containing borate obtained without an extreme pressure condition, as reported by Chen et al. in 2013 [89]. Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄) crystallizes in the triclinic space group, P$\overline{1}$ (no. 2); it features a complex sandwich-like layered structure. There are two kinds of FBBs in the structure of Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄), namely [B₁₂O₂₄] and [B₃O₆], respectively (Figure 14a,b). The aggregation of [B₁₂O₂₄] FBBs and [Zn(1)O₄] tetrahedra according to the stoichiometric ratio of 1:2 gives a layered [Zn₂(B₁₂O₂₄)]_∞ configuration expanding in the ab planes, while the second FBBs [B₃O₆] are connected to [Zn(2)O₄] to form [Zn(B₃O₆)]_∞ sheets. The assembly of two [Zn(B₃O₆)]_∞ sheets and one [Zn(B₁₂O₂₄)]_∞ layer leads to the formation of a complex [Zn₄(B₃O₆)₂(B₁₂O₂₄)]_∞ sandwiched structure. Split Na(1,2) atoms with the occupancy of 0.47:0.53 fill in the cavities of the sandwiched layers, and spherical coordinated Ba ions fill in the adjacent sandwiched layers (Figure 14c).

Li₄Na₂CsB₇O₂₀. The trimetallic borate Li₄Na₂CsB₇O₂₀ was reported by Pan et al. in 2019, and its expansion rate was investigated at the same time [90]. Li₄Na₂CsB₇O₂₀ crystallizes in a triclinic crystal system with the space group of P$\overline{1}$ (no. 2). With respect to its unique [B₁₄O₂₈] FBB, the centered [B₂O₆] ring acts as a four-connected node and further connects with one [BO₃] tringle and one [B₅O₁₁] cluster (Figure 15a). The total crystal structure of Li₄Na₂CsB₇O₂₀ displays a 3D configuration with monovalent alkali metal Li, Na, and Cs ions residing in the free spaces (Figure 15b). The temperature-dependent unit cell parameters were collected experimentally. as Additionally, the theoretical evaluation of thermal expansion along the principal axes indicate the highly anisotropic thermal expansion behavior of Li₄Na₂CsB₇O₂₀. The expansion rates for X₁, X₂, and X₃ were evaluated to be 3.51 × 10⁻⁶, 17 × 10⁻⁶, and 25.4 × 10⁻⁶ K⁻¹, respectively. This compound may be used as a thermal expansion material.

BaAlBO₄. In 2019, Pan et al. reported the synthesis and experimental and theoretical studies of an edge-sharing [BO₄] tetrahedra-containing aluminum oxyborate, BaAlBO₄. BaAlBO₄ was synthesized via the high-temperature solution method under atmospheric pressure [91]. Single-crystal X-ray diffraction analysis reveals that BaAlBO₄ crystallizes in a monoclinic space group, P2₁/c (no. 14). The crystal structure of BaAlBO₄ exhibits a 3D framework, which is comprised with [AlO₄] tetrahedra, [B₄O₁₀] clusters, and A-site Ba²⁺ cations filling the structural channels. The corner-sharing [AlO₄] units in the ab plane give a 2D ²[Al₂O₅]_∞ layer with six MRs (Figure 16b). The [B₂O₆] rings connect with two [BO₃] tringles end to end to form the isolated [B₄O₁₀] cluster (Figure 16a), which can be considered as the FBB of BaAlBO₄. The combination of [B₄O₁₀] clusters and the neighboring ²[Al₂O₅]_∞ layer give the final 3D framework.

β-CsB₉O₁₄. In 2019, Pan et al. prepared the β-CsB₉O₁₄ under ambient pressure. This compound is the first case of triple-layered borate with edge-sharing [BO₄] tetrahedra [92]. Taking the [B₆O₁₂] cluster in the KZnB₃O₆ as the prototype, the sandwich-like [B₁₈O₃₄] FBB can be evolved from the combination of one [B₆O₁₂] and two anti-parallel [B₆O₁₂] double-ring units (Figure 17a). The further aggregation of [B₁₈O₃₄] FBBs in the bc plane leads to the formation of corrugated layers with A-site Ba²⁺ cations residing in the channels; the whole structure of β-CsB₉O₁₄ is formed by stacking of these triple-layered sheets along [100] direction (Figure 17b). The B–O anionic skeleton of β-CsB₉O₁₄ possesses a high [BO₃]:[BO₄] (7:2) ratio; the layered structure as well as the well-aligned [BO₃] units in the lattice lead to a large optical anisotropy. The experimental and theoretical studies indicate that β-CsB₉O₁₄ can be identified as a potential deep-ultraviolet birefringent material with a wide band gap (>6.72 eV) and large birefringence (0.115 or 0.135 at 1064 nm).

Pb_2.28Ba_1.72B₁₀O₁₉. In 2021, an edge-sharing [BO₄]-containing borate, Pb_2.28Ba_1.72B₁₀O₁₉, was obtained under ambient pressure by Pan et al. [93]. It features a 3D B–O anionic framework. Pb_2.28Ba_1.72B₁₀O₁₉ crystallizes in a monoclinic crystal system with the space group of C2/c (no. 15). Its asymmetric unit consists of one Pb atom, five B atoms, ten O atoms, and one common site of the Ba/Pb atom with the occupancy of 0.14:0.86. Unlike the [B₂O₆] basic ring in most of edge-sharing [BO₄]-containing borate with four exocyclic O atoms acting as connection nodes, the centered [B₂O₆] in [B₁₀O₂₄] FBB connects two [BO₄] tetrahedra by the two exocyclic μ₂-O atoms and two [B₃O₈] by two endocyclic μ₃-O atoms (Figure 18a). The whole [B₁₀O₁₉] anionic framework is assembled from [B₁₀O₂₄] FBBs and Pb and Ba ions located in the structural channels (Figure 18b).

K₃Sb₄BO₁₃. In 2021, Quarez et al. discovered the complete transformation of adjacent [BO₃] pairs into [B₂O₆] dimers in the α- and β-phase of K₃Sb₄BO₁₃ driven by cooling [94]. The [BO₃]-containing α-phase of K₃Sb₄BO₁₃ is obtained from the traditional high-temperature solution method, and the symmetry increasing from P$\overline{1}$ to C2/c during the cooling process, accompanied with the transformation of two close [BO₃] tringles into edge-sharing [B₂O₆] units. The structures of α- and β-K₃Sb₄BO₁₃ display complex ²[Sb₄O₁₃]_∞ layers separated by [BO₃] pairs or edge-sharing [BO₄] tetrahedra (Figure 19a,b). The anti-parallel [BO₃] pair in the α-phase displays a short B⋯B distance (3.083(6) Å) and an extremely long B⋯O secondary bond (2.623(6) Å), while the coordination spheres of corresponding B atoms in the β-phase are distorted into tetrahedra (Figure 19c,d). The low temperature brings a lattice compression, which finally leads to B₂O₆ units, which shortens the B⋯B and B⋯O distances in each pair of adjacent BO₃ triangles units. Further studies show that B K-edge electron energy loss (EELS) spectroscopes provide a characteristic signal of the B₂O₆ units; the EELS method may widely use to identify edge-sharing B₂O₆ units more convenient in the future.

Ba₆Zn₆(B₃O₆)₆(B₆O₁₂). Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) was reported by Mao et al. and Pan et al. independently in 2022 and identified as a potential birefringent crystal with a deep-ultraviolet absorption cut-off edge and strong optical anisotropy [95,96]. The structure of Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) features a 2D [ZnB₄O₈]_∞ network constructed by [ZnO₄] tetrahedra and two kinds of B–O clusters ([B₆O₁₂] and [B₃O₆]) with Ba cations located in the cavities (Figure 20). It should be noted that Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) shows extremely low symmetry (space group P-1, no. 2), and its asymmetric unit includes six Ba atoms, six Zn atoms, six planar [B₃O₆] clusters, and two [B₃O₆] fragments (half of [B₆O₁₂] cluster). To simplify the description of structure, we use B–O cluster-1 and B–O cluster-2 to represent the basic structural units (Figure 20a–c). In the sandwiched [ZnB₄O₈]_∞ layers, the top and bottom of well-aligned [B₆O₁₂] clusters are shielded by the anti-parallel ²[Zn(B₃O₆)]_∞ sheets. The …A-A’-A… stacking sequence of [ZnB₄O₈]_∞ along the [001] direction leads to the formation of the total covalent skeleton, and Ba ions act as counterions in the lattice. From the structural perspective, the uniformly arrangement of two kinds of B–O clusters and the high ratio of highly birefringence-active [BO₃] tringles and [BO₄] tetrahedra (22:20) indicate that Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) may have remarkable optical anisotropy. In addition, the dangling bonds of terminal in two kinds of B–O clusters are eliminated by the covalent [ZnO₄] tetrahedra; thus, the short-wavelength absorption cut off edge has a blue shift. The basic physical properties of Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) were also studied. The transmission/absorption spectra indicate that Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) possesses a wide transparency window from 180 nm to 3405 nm. The difference of refractive indices based on a (001) wafer at 589.3 nm is as large as 0.14, which indicates that the birefringence of Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) is even larger than the commercialized α-BaB₂O₄. Moreover, thermal analysis demonstrates that Ba₆Zn₆(B₃O₆)₆(B₆O₁₂) melts congruently. The acquirement of bulk crystals could be anticipated as is evidenced by the already grown sub-centimeter sized crystals.

4. Conclusions

The synthesis of edge-sharing borates greatly changes the rule of corner sharing B–O units in borate structures, and further work demonstrates that the extreme synthetic conditions, such as high pressure, are not necessary for edge-sharing borates. The crystalline borates with edge-sharing [BO₄] tetrahedra continue to develop; about 34 new edge-sharing borates containing edge-sharing B₂O₆ unit have been found in recent years, among which three are crystallized in noncentrosymmetric space groups, only about 10% in the whole edge-sharing borates. This ratio is much smaller than 35% for the entire borate system, which may be attributable to the [BO₄] units likely formed under the high-pressure environment [97]. Noncentrosymmetric edge-sharing borates are needed to better understand the NLO property in these types of structures. Fortunately, more π-conjugated [BO₃] units are found under the ambient-pressure environment; the high [BO₃] and [BO₄] ratio in edge-sharing borates may be beneficial for the formation of noncentrosymmetric structures.

The signal of the B₂O₆ structural motif can be unambiguously assigned in the B K-edge EELS spectrum. Some of these edge-sharing borates exhibit interesting properties, such as unusual anisotropic thermal expansion behavior. It is curious to chemists whether edge-sharing BO₃/BO₄, BO₃/BO₃, or even face-sharing B–O units can be realized in the future. It is also expected that the synthesis of edge-sharing [BO₃F]⁴⁻, [BO₂F₂]³⁻, and [BOF₃]²⁻ units in the future will greatly enrich the structural chemistry of crystalline fluorooxoborates. Finally, we should better understand the structure–property relationships of these edge-sharing borates, which will help us to find more applications.