Recent Progress in Crystalline Borates with Edge-Sharing BO4 Tetrahedra

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.


Introduction
Borates show rich structural chemistry and have broad applications as birefringent materials and nonlinear optical (NLO) materials . The famous KBe 2 BO 3 F 2 (KBBF), LiB 3 O 5 (LBO), and β-BaB 2 O 4 (β-BBO) crystals are used to generate ultraviolet (UV) or deep-UV lasers through cascaded frequency conversion in practical application [32][33][34]. α-BaB 2 O 4 (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].  4 ] units and exhibit suitable birefringence (∆n = 0.06) and transparency windows down to the deep-UV region (<190 nm) [36,37]. Theoretical analyses reveal that the [BO 3 ] and [BO 4 ] units have the smaller polarizability anisotropy compared with linear [ BO 2 ]. While the latter one is the first noncentrosymmetric and chiral structure with the linear [BO 2 ] unit and displays a weak second-harmonic generation response (SHG) (0.1 × SiO 2 ) 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 [BO 3 ] and [BO 4 ] units. Cs 3 B 7 O 12 contains a large FBB with 63 boron atoms in which 35 (or 37) BO 3 triangles and 28 (or 26) BO 4 tetrahedra are linked to form thick anionic sheets stacked along the c direction [39]. Mg 7 @[B 69 O 108 (OH) 18 ] contains 42 [BO 3 ] triangles and 27 [BO 4 ] tetrahedra; it exhibits a supramolecular framework with hexagonal snowflake-like channels; unique triple-helical ribbons are found in {B 69 } FBBs [40]. This huge [B 69 O 108 (OH) 18 ] cluster represents the largest FBB in borates. The FBBs can further   (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. La 3 B 6 O 13 (OH) is the first SHG-active edge-sharing [BO 4 ] tetrahedracontaining 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 4 ] tetrahedron with negligible hyperpolarization, the π-conjugated motifs represented by planar [BO 3 ] and [B 3 O 6 ] in the borate system are superior NLO-active functional modules, and thus, the powder SHG response of La 3 B 6 O 13 (OH) based on the Kurtz-Perry method is as weak as 2/3 times that of quartz. 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) (Figure 4c).

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 highpressure 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)  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 Hf 4+ 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- γ-HfB 2 O 5 . In 2021, the γ-phase of HfB 2 O 5 , which incorporates edge-sharing [BO 4 ] tetrahedra, was obtained under extreme pressure (120 GPa) by Huppertz [74]. γ-HfB 2 O 5 crystallizes in the centrosymmetric monoclinic space group, P2 1 /c (no. 14). The tetravalent transition metal Hf 4+ cation displays higher coordination numbers than divalent cations, and the FBB in γ-HfB 2 O 5 is changed to [B 3 O 9 ] with the additional one vertex-sharing [BO 4 ] (Figure 5a). Similar to the stuctures of TMB 2 O 4 series, the structure of γ-HfB 2 O 5 borate also shows layered sheets with Hf ions filling the interlayer space ( Figure 5b). It is interesting to note that β-HfB 2 O 5 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-HfB 2 O 5 contains four MRs and eight MRs by the corner-sharing BO 4 tetrahedra, while γ-HfB 2 O 5 contains ten MRs, including the edge-sharing BO 4 tetrahedra. Edge-sharing BO 4 tetrahedra in new phase γ-HfB 2 O 5 shows exceptionally short B-O and B· · · B distances. The coordination number of the Hf 4+ cations in γ-HfB 2 O 5 increased to nine in comparison to eight in its ambient pressure counterpart. 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 Hf 4+ cations in γ-HfB2O5 increased to nine in comparison to eight in its ambient pressure counterpart.         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 edgesharing [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).   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].    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].     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 (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

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

Ambient Pressure Synthesis of Borates with Edge-Sharing [BO 4 ] Tetrahedra
The edge-sharing [BO 4 ] tetrahedra-containing borates obtained from classical hightemperature 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 [BO 3 (Figure 15a). The total crystal structure of Li 4 Na 2 CsB 7 O 20 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 Li 4 Na 2 CsB 7 O 20 . The expansion rates for X 1 , X 2 , and X 3 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. (Figure 14a   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 P1 ̅ (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 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 Ba 2+ 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    Pb2.28Ba1.72B10O19. In 2021, an edge-sharing [BO4]-containing borate, Pb2.28Ba1.72B10O19, was obtained under ambient pressure by Pan et al. [93]. It features a 3D B-O anionic framework. Pb2.28Ba1.72B10O19 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 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).      (Figure 19a,b). The anti-parallel [BO 3 ] 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 B 2 O 6 units, which shortens the B· · · B and B· · · O distances in each pair of adjacent BO 3 (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.    (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.   [95,96]. The structure of Ba 6 Zn 6 (B 3 O 6 ) 6 (B 6 (22:20) indicate that Ba 6 Zn 6 (B 3 O 6 ) 6 (B 6 O 12 ) may have remarkable optical anisotropy. In addition, the dangling bonds of terminal in two kinds of B-O clusters are eliminated by the covalent [ZnO 4 ] tetrahedra; thus, the short-wavelength absorption cut off edge has a blue shift. The basic physical properties of Ba 6 Zn 6 (B 3 O 6 ) 6 (B 6 O 12 ) were also studied. The transmission/absorption spectra indicate that Ba 6 Zn 6 (B 3 O 6 ) 6 (B 6 O 12 ) 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 Ba 6 Zn 6 (B 3 O 6 ) 6 (B 6 O 12 ) is even larger than the commercialized α-BaB 2 O 4 . Moreover, thermal analysis demonstrates that Ba 6 Zn 6 (B 3 O 6 ) 6 (B 6 O 12 ) melts congruently. The acquirement of bulk crystals could be anticipated as is evidenced by the already grown sub-centimeter sized crystals. 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.

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

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 [BO 4 ] tetrahedra continue to develop; about 34 new edge-sharing borates containing edge-sharing B 2 O 6 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 [BO 4 ] 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 [BO 3 ] units are found under the ambient-pressure environment; the high [BO 3 ] and [BO 4 ] ratio in edge-sharing borates may be beneficial for the formation of noncentrosymmetric structures.
The signal of the B 2 O 6 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 edgesharing BO 3 /BO 4 , BO 3 /BO 3 , or even face-sharing B-O units can be realized in the future. It is also expected that the synthesis of edge-sharing [BO 3 F] 4− , [BO 2 F 2 ] 3− , and [BOF 3 ] 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 edgesharing borates, which will help us to find more applications.
Funding: This research was funded by the National Natural Science Foundation of China (Grant 21975224).
Institutional Review Board Statement: Not applicable.