Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties

Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs3Zn6B9O21 and AZn2BO3X2 (A = Na, K, Rb, NH4; X = Cl, Br) series with KBe2BO3F2-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB3O6 and Ba4Na2Zn4(B3O6)2(B12O24) with novel edge-sharing [BO4]5− tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme conditions such as high pressure are not essential to obtain edge-sharing [BO4]5−-containing borates; Ba4K2Zn5(B3O6)3(B9O19) and Ba2KZn3(B3O6)(B6O13) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi2ZnOB2O6 and BaZnBO3F are promising non-linear optical (NLO) materials; Zn4B6O13 and KZnB3O6 possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.


Statistical Analysis of Structural Configurations
In order to better understand the structural diversity of zincoborates, systematic analysis of the Zn-B-O system was carried out by taking the Inorganic Crystal Structure Database (ICSD-4.2.0, the latest release of ICSD-2019/1) as the source of data. The connection modes of zinc cations with oxygen or halogen anions as well as the influence of the zinc cations to the structures are investigated. The thresholds of bond lengths were applied for the Zn-O and B-O bonds at the maximum in the continuity distribution of the Zn-O distances (2.6 Å ) and the B-O distances (1.65 Å ), respectively. All the available anhydrous and disorder-free zinc-containing borates (64 cases) are summarized. Structural comparisons were carried out and summarized as follows: (1) The Zn-O, Zn-X (X = F, Cl, Br) bond lengths of the summarized compounds range from 1.814-2.479 Å , 2.023-2.516 Å , respectively, which is in agreement with reasonable values. As shown in Figure 1a, most of the  [85] and Pb8Zn(BO3)6 [86], respectively.

Zincoborates Possessing Special Structural Features
It is expected that the introduction of zinc cations into borates can enrich the structural diversity and be beneficial to obtain new zincoborates with special structural features. In this section, several compounds with distinctive crystal structure characteristics are given.

Zincoborates Possessing Special Structural Features
It is expected that the introduction of zinc cations into borates can enrich the structural diversity and be beneficial to obtain new zincoborates with special structural features. In this section, several compounds with distinctive crystal structure characteristics are given.

Zincoborates with Novel Edge-Sharing [BO4] 5− Tetrahedra
On the basis of the borate structures discovered, the [BO4] 5− units usually connect to each other via corner-sharing (cs-) rather than edge-sharing (es-) or face-sharing [103][104][105]. In terms of Pauling's 3rd and 4th rules and the orbital interpretation rules, the connection mode of es-polyhedra for highvalence and low coordinated small cations is scarcely seen except under extreme conditions such as high pressure (HP), for the reason that the repulsion interactions between the adjacent cations and anions may be increased when two anion-based polyhedra adopt edge-sharing connection mode [76,77]. Thus, the formation of es-[BO4] 5− tetrahedra is extremely unfavored, and, the es-[BO4] 5− units can only be observed in very few borates.

Zincoborates with Novel Edge-Sharing [BO 4 ] 5− Tetrahedra
On the basis of the borate structures discovered, the [BO 4 ] 5− units usually connect to each other via corner-sharing (cs-) rather than edge-sharing (es-) or face-sharing [103][104][105]. In terms of Pauling's 3rd and 4th rules and the orbital interpretation rules, the connection mode of es-polyhedra for high-valence and low coordinated small cations is scarcely seen except under extreme conditions such as high pressure (HP), for the reason that the repulsion interactions between the adjacent cations and anions may be increased when two anion-based polyhedra adopt edge-sharing connection mode [76,77] [113,114]. Very recently, β-CsB 9 O 14 , the first triple-layered borate with es-[BO 4 ] 5− tetrahedra, was obtained under the vacuum sealed condition [115].
KZnB 3 O 6 crystallizes in the space group of P1 (No. 2) [69,70]. As shown in Figure 6, its structure contains a remarkable [B 6   KZnB3O6 was synthesized using a conventional solid-state reaction under ambient pressure. In detail, a single-phase white powder of KZnB3O6 was prepared by grinding a stoichiometric mixture of K2CO3, ZnO, and H3BO3, which was heated to 500 o C to decompose the salt and annealed at 750 °C for 24 h. Single crystals of KZnB3O6 were obtained by spontaneous nucleation by melting the obtained pure phase powder at 820 °C, then slowly cooling the melt to 600 °C at a rate of 1 °C h −1 . Although the synthesis condition of KZnB3O6 is different from that of previously reported HP borates, further examination of the edge-sharing geometry reveals that the B due to the like-charges repulsion, which will push higher valence ions apart in the es-polyhedra to minimize the electrostatic potential.
Theoretical insight into the structural stability of KZnB3O6 was carried out by Yang and coworkers [116]. They investigated the molecular dynamics, lattice dynamics and electronic properties of es-KZnB3O6 and cs-KZnB3O6 (hypothetical one, constructed based on isostructural KCdB3O6) via density functional theory. Molecular dynamics simulations show that, es-KZnB3O6 is stable from 100 to 1000 K while cs-KZnB3O6 deforms with bond stretching. Analysis of lattice dynamics shows that, a soft-mode reflecting the dynamic instability exists in the cs-KZnB3O6, which probably comes from an overlong Zn-O bond in the [ZnO5] 8- KZnB 3 O 6 was synthesized using a conventional solid-state reaction under ambient pressure. In detail, a single-phase white powder of KZnB 3 O 6 was prepared by grinding a stoichiometric mixture of K 2 CO 3 , ZnO, and H 3 BO 3 , which was heated to 500 • C to decompose the salt and annealed at 750 • C for 24 h. Single crystals of KZnB 3 O 6 were obtained by spontaneous nucleation by melting the obtained pure phase powder at 820 • C, then slowly cooling the melt to 600 • C at a rate of 1 • C h −1 . Although the synthesis condition of KZnB 3 O 6 is different from that of previously reported HP borates, further examination of the edge-sharing geometry reveals that the B-O bond lengths and O-B-O angles in KZnB 3 O 6 are consistent with those of HP borates. As a common feature, the O-B-O angles within the [B 2 O 2 ] ring of es-[BO 4 ] 5− are remarkably reduced and the B-O bonds within the ring are elongated due to the like-charges repulsion, which will push higher valence ions apart in the es-polyhedra to minimize the electrostatic potential.
Theoretical insight into the structural stability of KZnB 3 O 6 was carried out by Yang and coworkers [116]. They investigated the molecular dynamics, lattice dynamics and electronic properties of es-KZnB 3 O 6 and cs-KZnB 3 O 6 (hypothetical one, constructed based on isostructural KCdB 3 O 6 ) via density functional theory. Molecular dynamics simulations show that, es-KZnB 3 O 6 is stable from 100 to 1000 K while cs-KZnB 3 O 6 deforms with bond stretching. Analysis of lattice dynamics shows that, a soft-mode reflecting the dynamic instability exists in the cs-KZnB 3     The above findings prove that the borate structure is very flexible and confirm the feasibility of incorporating the es-[BO4] 5− configuration into traditional borate chemistry to enrich the borate structure.  The above findings prove that the borate structure is very flexible and confirm the feasibility of incorporating the es-[BO 4 ] 5− configuration into traditional borate chemistry to enrich the borate structure. The above findings prove that the borate structure is very flexible and confirm the feasibility of incorporating the es-[BO4] 5− configuration into traditional borate chemistry to enrich the borate structure.

Zincoborates with Two Kinds of Isolated Anion Groups
According to the Pauling's fifth rule [76,77], the number of essentially different kinds of constituents in a crystal tends to be small, which means that the number of components of various types in a crystal tends to be small. For most borates, there is only one kind of isolated B-O group in the structure [117,118]

Zincoborates with Excellent Properties
Based on the previous reports, it should be emphasized that Zn-O/F polyhedra, especially the [ZnO 4 ] 6− and [ZnO 3 F] 5− tetrahedra, have impacts on both crystal structures and properties. In this section, a series of zincoborates with UV/DUV cutoff edges, second-order NLO properties and anomalous thermal expansion properties are briefly reviewed.

Zincoborates with Short Ultraviolet (UV) Cutoff Edges
With the rapid development of UV technology, NLO and birefringent materials with high transparency in the UV regions are generally required from both an academic and technological standpoint [119][120][121]. Since the d-d or f -f electronic transitions will have a negative influence on the large energy band gap, thus, in consideration of the absorption edge, it is a common strategy to introduce cations without d-d or f -f transitions (such as alkali and alkaline-earth metals) to blue shift the cutoff edge to the UV regions [122,123]. Besides, cations with fully occupied d or half-occupied f electronic shells, such as Zn 2+ , Gd 3+ , and Y 3+ , can also be used in UV materials since their electronic shells can effectively inhibit unfavorable electronic transitions [124][125][126][127]. Insofar as we know, there are a number of zinc-containing borates reported in the UV/DUV regions. For instance, Ba 3 (ZnB 5 O 10 )PO 4 (~180 nm) [60], Cs 3 Zn 6 B 9 O 21 (~200 nm) [45,46], AZn 2 BO 3 X 2 (A = Na, K, Rb, NH 4 ; X = Cl, Br) series (~190-209nm) [58,59], K 7 ZnSc 2 B 15 O 30 (~200 nm) [128], K 3 ZnB 5 O 10 (~190 nm) [129], Cs 12 Zn 4 (B 5 O 10 ) 4 (below 185 nm) [130], etc. Hence, the Zn-containing borate system is also a candidate for exploring promising UV even DUV materials.

Zincoborates with Large Second-Order Non-Linear Optical (NLO) Response
The increasing need for high-power all-solid-state UV light sources promotes the development of NLO borate crystals, especially those with large second-order NLO responses and short UV-transmission cutoff edges [131,132]. After continuous efforts in the past few decades, a series of borate-based NLO materials were developed and have been widely used in many optoelectronic devices, such as KBBF [12,13], β-BBO [16], LBO [17], CsLiB 6 O 10 (CLBO) [133,134], etc. Up to now, many NLO zincoborates with good performance in the UV/DUV regions have been discovered. In this section, we focus on recent studies of zincoborate crystals with good second-order NLO properties and the representative ones are included in Table 1.  [45,46]. The absorption edge of Cs 3 Zn 6 B 9 O 21 is below 200 nm in the UV region and its powder SHG efficiency is approximately 3.3 times that of KDP, which implies that Cs 3 Zn 6 B 9 O 21 has potential application prospects as an UV NLO material. Remarkably, Cs 3 Zn 6 B 9 O 21 has a small density of the [BO 3 ] 3− triangles but exhibits a large SHG response in the KBBF family. Based on the calculation of the dipole moments, the inversion symmetry lifting atomic distortions (Figure 9), electronic structure and atom-cutting analysis, the enhanced SHG response originates from the cooperative effect of coparallel [  Crystals of KZn2BO3Cl2, RbZn2BO3Cl2, KZn2BO3Br2, and RbZn2BO3Br2 can be obtained by high temperature solution method as well as solvothermal techniques, while crystals of NH4Zn2BO3Cl2 were grown only by solvothermal techniques due to the decomposition of ammonium compounds at high temperature. The series of borates are isostructural with KBBF and preserve the NLOfavorable structural features [58,59]. Remarkably, this series of materials exhibits strong SHG responses of approximately more than 2 times that of benchmark KBBF, and the compounds are phase-matchable in the visible and UV regions and possess UV-transmission cutoff edges (~200 nm), indicating that this series of crystals may have potential application in the short-wave NLO field. Theoretical calculations reveal that the SHG enhancement mainly originates from the distorted    Cl 2 were grown only by solvothermal techniques due to the decomposition of ammonium compounds at high temperature. The series of borates are isostructural with KBBF and preserve the NLO-favorable structural features [58,59]. Remarkably, this series of materials exhibits strong SHG responses of approximately more than 2 times that of benchmark KBBF, and the compounds are phase-matchable in the visible and UV regions and possess UV-transmission cutoff edges (~200 nm), indicating that this series of crystals may have potential application in the short-wave NLO field. Theoretical calculations reveal that the SHG enhancement mainly originates from the distorted [ZnO 3 X] 5 apical F atoms of [ZnO3F2] bipyramids to form a 3D framework (Figure 10). Within a single 2 [ZnBO3F] layer, the [ZnO3F2] 6− bipyramid facilitates its three neighboring [BO3] 3− units to arrange into a perfect coplanar alignment in the plane through three basal or equatorial bonds of [ZnO3F2] 6− bipyramid. Meanwhile, among different layers, the [BO3] 3− units are also governed by the [BaO6F3] 13− polyhedra and arranged parallel to each other in neighboring layers. The perfectly coplanar manner of the [BO3] 3− groups produces a cooperative effect and gives maximum contribution to the NLO response. As a result, a large NLO effective coefficient, 2.8 × deff (KDP), is observed.

Bi2ZnOB2O6
Bi2ZnOB2O6 was first reported by J. Barbier et al. in 2005 and its structure was determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data [81]. Two years later, the crystal of Bi2ZnOB2O6 with a size of 0.4  0.4 × 0.5 mm 3 was prepared by the conventional solid-state reaction method [82]. Until 2009, Pan group firstly obtained the high quality sizable single crystal by the top-seeded growth method [83]. The structure of Bi2ZnOB2O6 consists of 2 ∞[ZnB2O7] layers alternating with six-coordinated Bi 3+ cations ( Figure 12

Other Zinc-Containing Compounds with NLO Properties
Bi 2 ZnOB 2 O 6 Bi 2 ZnOB 2 O 6 was first reported by J. Barbier et al. in 2005 and its structure was determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data [81]. Two years later, the crystal of Bi 2 ZnOB 2 O 6 with a size of 0.4 × 0.4 × 0.5 mm 3 was prepared by the conventional solid-state reaction method [82]. Until 2009, Pan group firstly obtained the high quality sizable single crystal by the top-seeded growth method [83].

Zincoborates with Anomalous Thermal Expansion Properties
Most of the materials exhibit positive thermal expansion, i.e., expanding on heating and contracting on cooling in three dimensions. Interestingly, an increasing quantity of materials with anomalous thermal expansion properties, such as negative thermal expansion (NTE) (materials contract along some specific directions when heated) and zero thermal expansion (ZTE) (materials can retain a constant size in a specified temperature range), have attracted a great deal of attention in laboratories and industries [141,142].
As abundant inorganic compounds resources, borates not only have promising applications as optical materials, but also are recognized with unusual thermal expansion behavior [143,144]. The bond lengths and angles of [BO 3 ] 3− triangles or [BO 4 ] 5− tetrahedra in a borate structure remain almost constant as the ambient temperature varies. When these rigid B-O groups further construct 0D clusters, 1D chains, 2D layers, or 3D frameworks, the rotation between the rigid B-O groups combined with expansion and/or tilting of other polyhedra in borate structures will control the thermal expansion property and may result in the anomalous thermal expansion. In recent years, many borate crystals have been reported to exhibit abnormal thermal expansion behaviors. For instance, the 1D NTE behavior has been detected in LiB 3 O 5 [145] and BiB 3 O 6 [146], the area NTE behaviors were discovered in LiBeBO 3 [147] and KZnB 3 O 6 [65,66], and the isotropic area NTE effect were found in KBBF [148]. Most interestingly, the 3D ZTE effect was discovered in Zn 4 B 6 O 13 [67,68], which possess the intrinsic isotropic near-ZTE behavior as the first case. The discoveries of presented zincoborates add important members to the family of materials with anomalous thermal expansion properties.

Near-Zero Thermal Expansion Properties in Zn 4 B 6 O 13
Zn 4 B 6 O 13 crystallizes in the cubic space group of I43m (No. 217) and possesses a very rare sodalite cage structure [67,68]. As shown in Figure 14,  Most of the materials exhibit positive thermal expansion, i.e., expanding on heating and contracting on cooling in three dimensions. Interestingly, an increasing quantity of materials with anomalous thermal expansion properties, such as negative thermal expansion (NTE) (materials contract along some specific directions when heated) and zero thermal expansion (ZTE) (materials can retain a constant size in a specified temperature range), have attracted a great deal of attention in laboratories and industries [141,142].
As abundant inorganic compounds resources, borates not only have promising applications as optical materials, but also are recognized with unusual thermal expansion behavior [143,144]. The bond lengths and angles of [BO3] 3− triangles or [BO4] 5− tetrahedra in a borate structure remain almost constant as the ambient temperature varies. When these rigid B-O groups further construct 0D clusters, 1D chains, 2D layers, or 3D frameworks, the rotation between the rigid B-O groups combined with expansion and/or tilting of other polyhedra in borate structures will control the thermal expansion property and may result in the anomalous thermal expansion. In recent years, many borate crystals have been reported to exhibit abnormal thermal expansion behaviors. For instance, the 1D NTE behavior has been detected in LiB3O5 [145] and BiB3O6 [146], the area NTE behaviors were discovered in LiBeBO3 [147] and KZnB3O6 [65,66], and the isotropic area NTE effect were found in KBBF [148]. Most interestingly, the 3D ZTE effect was discovered in Zn4B6O13 [67,68], which possess the intrinsic isotropic near-ZTE behavior as the first case. The discoveries of presented zincoborates add important members to the family of materials with anomalous thermal expansion properties.

Near-Zero Thermal Expansion Properties in Zn4B6O13
Zn4B6O13 crystallizes in the cubic space group of I4 ̅ 3m (No. 217) and possesses a very rare sodalite cage structure [67,68]. As shown in Figure 14,   The thermal expansion behavior of Zn4B6O13 between 13 and to 270 K was investigated by the variable-temperature X-ray diffraction (XRD) and variation of refined cell parameters (refined by the Rietveld method). As results, in the measured temperature range, no new peaks appear in all the The thermal expansion behavior of Zn 4 B 6 O 13 between 13 and to 270 K was investigated by the variable-temperature X-ray diffraction (XRD) and variation of refined cell parameters (refined by the Rietveld method). As results, in the measured temperature range, no new peaks appear in all the XRD patterns, which indicates that the structure of Zn 4 B 6 O 13 is kept in the cubic I43m space group, and the thermal expansion is completely 3D isotropic. The cell parameter of Zn 4 B 6 O 13 increases by just 0.03%, this thermal-expansion behavior is very low and consistent with the observation of positions of the XRD peaks, as shown in the insert in Figure 15, the (004) peaks remain nearly constant in the varying temperature environment. Further, the fitted average thermal expansion coefficient (by PASCal software) in the whole temperature range is 1.00(14)/MK. Particularly, from 13 to 110 K, the thermal expansion coefficient in Zn 4 B 6 O 13 is even much smaller (0.28(06)/MK), which can be accurately cataloged to ZTE. XRD patterns, which indicates that the structure of Zn4B6O13 is kept in the cubic I4 ̅ 3m space group, and the thermal expansion is completely 3D isotropic. The cell parameter of Zn4B6O13 increases by just 0.03%, this thermal-expansion behavior is very low and consistent with the observation of positions of the XRD peaks, as shown in the insert in Figure 15, the (004) peaks remain nearly constant in the varying temperature environment. Further, the fitted average thermal expansion coefficient (by PASCal software) in the whole temperature range is 1.00(14)/MK. Particularly, from 13 to 110 K, the thermal expansion coefficient in Zn4B6O13 is even much smaller (0.28(06)/MK), which can be accurately cataloged to ZTE. First principles calculations were carried out for further investigation and demonstrate that the intrinsic isotropic near-ZTE behavior of Zn4B6O13 mainly originates from the invariability of the solid [B24O48] 24− cage fixed by the [Zn4O13] 18− clusters, affirming the important impact of the relatively strong Zn-O bonds. The discovery of Zn4B6O13 with intrinsic isotropic near-ZTE behavior not only gains an important member to the family of ZTE materials, but also revives the studies on new functionalities in borates, which may eventually lead to the discovery of more exciting and exotic emerging physicochemical properties in borates.

Unidirectional Thermal Expansion in KZnB3O6
As described before, KZnB3O6 is the first borate that contains the es-[BO4] 5− tetrahedra under ambient pressure [69,70]. Lou et al. investigated the thermal expansions of KZnB3O6 from room temperature to 1013 K (Figure 16a) [65,66]. Interestingly, KZnB3O6 shows an unusual unidirectional thermal expansion along the approximate [3 ̅ 02] direction, i.e., the X3 axis direction, over the entire measured temperature (from 298 K to 1013 K). The expansions along other directions on the plane perpendicular to [3 ̅ 02] are negligibly small, i.e., the area shows zero expansion (Figure 16b). Further investigations reveal that the abnormal thermal behavior originates from the cooperative hinge rotations of [B6O12] 6− (contain es-[BO4] 5− tetrahedra) and [Zn2O6] 8− rigid groups, which are probably driven by asymmetrical elongations of K-O bonds and only leads to a quasi-unidirectional expansion upon heating. These findings will help us better understand the relationship between structure and property and might broaden the applications of borates.

Unidirectional Thermal Expansion in KZnB 3 O 6
As described before, KZnB 3 O 6 is the first borate that contains the es-[BO 4 ] 5− tetrahedra under ambient pressure [69,70]. Lou et al. investigated the thermal expansions of KZnB 3 O 6 from room temperature to 1013 K (Figure 16a) [65,66]. Interestingly, KZnB 3 O 6 shows an unusual unidirectional thermal expansion along the approximate [302] direction, i.e., the X 3 axis direction, over the entire measured temperature (from 298 K to 1013 K). The expansions along other directions on the plane perpendicular to [302] are negligibly small, i.e., the area shows zero expansion (Figure 16b). Further investigations reveal that the abnormal thermal behavior originates from the cooperative hinge rotations of [B 6 O 12 ] 6− (contain es-[BO 4 ] 5− tetrahedra) and [Zn 2 O 6 ] 8− rigid groups, which are probably driven by asymmetrical elongations of K-O bonds and only leads to a quasi-unidirectional expansion upon heating. These findings will help us better understand the relationship between structure and property and might broaden the applications of borates.

Single Crystal Growth of Zincoborates
High-quality and sizable single crystals are essential to measure fundamental properties and to accurately evaluate practical applications. Although a great deal of effort has been put into the exploration of growing sizable single crystals with high optical quality, it is still a great challenge to obtain the large-scale crystals for practical devices [149][150][151]. As for the zinc-containing system, the growth of large crystal seems more difficult since compounds containing zinc element usually have a high melting point (ZnO, 1975 °C at 5.2 MPa). Usually, the effective fluxes, such as PbO, PbF2, H3BO3, etc., are introduced to decrease the melting point and the viscosity during the growth of single crystals. Fortunately, some of them melt congruently and sizable crystals have been grown from a stoichiometric melt by the top-seeded solution growth (TSSG) and Czochralski method.

Bi2ZnOB2O6
In 2009, Li et al. successfully grew the single crystal of Bi2ZnOB2O6 with high quality and dimensions of 18 mm  13 mm  6 mm through the TSSG method (Figure 17a) 83. The low melt point (no more than 700 °C), non-viscous properties, and the congruent melting performance make Bi2ZnOB2O6 capable to grow sizable single crystals. Followed by these, a sizable single crystal with sizes up to Φ 30 mm  55 mm has been obtained along the c-axis direction using the Czochralski method (Figure 17b) 152. The NLO coefficients have been determined through the Maker fringes method at 1064 nm 140. Results show that the coefficients of Bi2ZnOB2O6 relative to d36 for KDP are d31 = (2.34 ± 0.05) d36 (KDP), d32 = (7.90 ± 0.16) d36 (KDP) and d33 = (2.60 ± 0.06) d36 (KDP). The large NLO coefficients and the easy crystal growth behavior suggest that Bi2ZnOB2O6 is a promising candidate for NLO materials.

Single Crystal Growth of Zincoborates
High-quality and sizable single crystals are essential to measure fundamental properties and to accurately evaluate practical applications. Although a great deal of effort has been put into the exploration of growing sizable single crystals with high optical quality, it is still a great challenge to obtain the large-scale crystals for practical devices [149][150][151]. As for the zinc-containing system, the growth of large crystal seems more difficult since compounds containing zinc element usually have a high melting point (ZnO, 1975 • C at 5.2 MPa). Usually, the effective fluxes, such as PbO, PbF 2 , H 3 BO 3 , etc., are introduced to decrease the melting point and the viscosity during the growth of single crystals. Fortunately, some of them melt congruently and sizable crystals have been grown from a stoichiometric melt by the top-seeded solution growth (TSSG) and Czochralski method.

Bi 2 ZnOB 2 O 6
In 2009, Li et al. successfully grew the single crystal of Bi 2 ZnOB 2 O 6 with high quality and dimensions of 18 mm × 13 mm × 6 mm through the TSSG method (Figure 17a) [83]. The low melt point (no more than 700 • C), non-viscous properties, and the congruent melting performance make Bi 2 ZnOB 2 O 6 capable to grow sizable single crystals. Followed by these, a sizable single crystal with sizes up to Φ 30 mm × 55 mm has been obtained along the c-axis direction using the Czochralski method (Figure 17b) [152]. The NLO coefficients have been determined through the Maker fringes method at 1064 nm [140].

Ba3(ZnB5O10)PO4
Ba3(ZnB5O10)PO4 melts congruently, but its relatively high viscosity and melting temperature are unfavorable to obtain high-quality crystals from a stoichiometric melt. Therefore, large single crystals of Ba3(ZnB5O10)PO4 were grown through a TSSG method by using the ZnO-B2O3 self-flux system ( Figure 18

-Zn3BPO7
In 1982, Liebertz and Stahr reported the existence of Zn3BPO7 that occur in two phases with a phase transition at 602 °C 154. -Zn3BPO7 (high-temperature phase) has been characterized as a NLO crystal owning to its significant properties. However, the growth of large crystals of -Zn3BPO7 is difficult since the crystal will transfer from -to -phase. Unremitting efforts have been made to obtain sizable and high-quality -Zn3BPO7 crystals. Consequently, the phase transition of -Zn3BPO7 to -Zn3BPO7 is effectively suppressed through adopting appropriate heat treatment.

Ba3(ZnB5O10)PO4
Ba3(ZnB5O10)PO4 melts congruently, but its relatively high viscosity and melting temperature are unfavorable to obtain high-quality crystals from a stoichiometric melt. Therefore, large single crystals of Ba3(ZnB5O10)PO4 were grown through a TSSG method by using the ZnO-B2O3 self-flux system ( Figure 18

-Zn3BPO7
In 1982, Liebertz and Stahr reported the existence of Zn3BPO7 that occur in two phases with a phase transition at 602 °C 154. -Zn3BPO7 (high-temperature phase) has been characterized as a NLO crystal owning to its significant properties. However, the growth of large crystals of -Zn3BPO7 is difficult since the crystal will transfer from -to -phase. Unremitting efforts have been made to obtain sizable and high-quality -Zn3BPO7 crystals. Consequently, the phase transition of -Zn3BPO7 to -Zn3BPO7 is effectively suppressed through adopting appropriate heat treatment.

β-Zn 3 BPO 7
In 1982, Liebertz and Stahr reported the existence of Zn 3 BPO 7 that occur in two phases with a phase transition at 602 • C [154]. β-Zn 3 BPO 7 (high-temperature phase) has been characterized as a NLO crystal owning to its significant properties. However, the growth of large crystals of β-Zn 3 BPO 7 is difficult since the crystal will transfer from βto α-phase. Unremitting efforts have been made to obtain sizable and high-quality β-Zn 3 BPO 7 crystals. Consequently, the phase transition of β-Zn 3 BPO 7 to α-Zn 3 BPO 7 is effectively suppressed through adopting appropriate heat treatment. In 2000 to 2002 [136,155,156], the transparent and crack free single crystals of β-Zn 3 BPO 7 with size dimensions

Zn4B6O13
The large-sized Zn4B6O13 single crystal with dimensions of about 40 mm  40 mm  18 mm and exhibiting good optical quality was grown using the conventional TSSG method (Figure 20) 67. Optical transmittance measurements show that Zn4B6O13 possesses a wide transmission range covering a wide spectral region from the UV to the near-infrared (wavelength from 217 to 3100 nm). The UV cutoff edge of Zn4B6O13 is the shortest among the ZTE crystals, implying the potential applications of Zn4B6O13 in ultra precise optical instruments. Notably, the short UV cutoff edge of Zn4B6O13 also stems from the relatively strong Zn-O bond based on the analysis of the ab initio partial density of states. Moreover, Zn4B6O13 exhibits high thermal stability, thermal conductivity and high mechanical hardness, which are also important for practical applications. Combined with the intrinsic isotropic near-ZTE behavior, the environmentally friendly feature and easy growth habit facilitate the practical applications of Zn4B6O13.

BaZnBO3F
BaZnBO3F exhibits typical layer habit due to structural characteristics, which is familiar with the KBBF family crystals. Crystal of BaZnBO3F with the dimensions of about 20 mm  20 mm  0.5 mm has been grown by high temperature solution method from BaF2-NaF flux (Figure 21 The linear and non-linear optical properties are investigated. Results show that β-Zn 3 BPO 7 has a UV absorption edge at about 250 nm (Figure 19b) and the none-zero NLO coefficient d 11 measured by the Maker fringes method is 0.69 pm/V (1.8 times as large as that of d 36 (KDP)). The Sellmeier equations suggest that the shortest SHG wavelengths for the crystal are 399 and 605 nm for types I and II, respectively. The easy growth habit and good NLO properties make β-Zn 3 BPO 7 attractive for continued research as NLO materials.

Zn 4 B 6 O 13
The large-sized Zn 4 B 6 O 13 single crystal with dimensions of about 40 mm × 40 mm × 18 mm and exhibiting good optical quality was grown using the conventional TSSG method (Figure 20) [67]. Optical transmittance measurements show that Zn 4 B 6 O 13 possesses a wide transmission range covering a wide spectral region from the UV to the near-infrared (wavelength from 217 to 3100 nm). The UV cutoff edge of Zn 4 B 6 O 13 is the shortest among the ZTE crystals, implying the potential applications of Zn 4 B 6 O 13 in ultra precise optical instruments. Notably, the short UV cutoff edge of Zn 4 B 6 O 13 also stems from the relatively strong Zn-O bond based on the analysis of the ab initio partial density of states. Moreover, Zn 4 B 6 O 13 exhibits high thermal stability, thermal conductivity and high mechanical hardness, which are also important for practical applications. Combined with the intrinsic isotropic near-ZTE behavior, the environmentally friendly feature and easy growth habit facilitate the practical applications of Zn 4 B 6 O 13 .

Zn4B6O13
The large-sized Zn4B6O13 single crystal with dimensions of about 40 mm  40 mm  18 mm and exhibiting good optical quality was grown using the conventional TSSG method (Figure 20) 67. Optical transmittance measurements show that Zn4B6O13 possesses a wide transmission range covering a wide spectral region from the UV to the near-infrared (wavelength from 217 to 3100 nm). The UV cutoff edge of Zn4B6O13 is the shortest among the ZTE crystals, implying the potential applications of Zn4B6O13 in ultra precise optical instruments. Notably, the short UV cutoff edge of Zn4B6O13 also stems from the relatively strong Zn-O bond based on the analysis of the ab initio partial density of states. Moreover, Zn4B6O13 exhibits high thermal stability, thermal conductivity and high mechanical hardness, which are also important for practical applications. Combined with the intrinsic isotropic near-ZTE behavior, the environmentally friendly feature and easy growth habit facilitate the practical applications of Zn4B6O13.

BaZnBO3F
BaZnBO3F exhibits typical layer habit due to structural characteristics, which is familiar with the KBBF family crystals. Crystal of BaZnBO3F with the dimensions of about 20 mm  20 mm  0.5 mm has been grown by high temperature solution method from BaF2-NaF flux (Figure 21) 135. The perfect coplanar and alignment [BO3] 3-groups in the structure result in an observed large effective

BaZnBO 3 F
BaZnBO 3 F exhibits typical layer habit due to structural characteristics, which is familiar with the KBBF family crystals. Crystal of BaZnBO 3 F with the dimensions of about 20 mm × 20 mm × 0.5 mm has been grown by high temperature solution method from BaF 2 -NaF flux ( Figure 21) [135]. The perfect coplanar and alignment [BO 3 ] 3− groups in the structure result in an observed large effective NLO coefficient (2.8 × d eff KDP). BaZnBO 3 F crystal possesses chemical stability and high transmittance in the range of 300-3000 nm wavelength with the UV cut-off edge of 223 nm. In the consideration of its superior optical properties in the visible to UV range, larger-size crystals should be developed for practical applications by exploiting several methods or flux to overcome the strong anisotropic growth habit.  Reprinted from Ref. [135], Copyright (2016) with permission from Elsevier.

Conclusions
In this review, we have examined the recent development of zincoborates, focusing on the crystal structure chemistry as well as their physicochemical properties. The introduction of the strong-bonded zinc cations into borates effectively enriches the structural diversity of borates and further results in extensive applications. Several examples were given. (1) A series of zincoborates display unique structural features in crystal chemistry of borates, for example, KZnB3O6 and Ba4Na2Zn4(B3O6)2(B12O24) with novel es-BO4 5− tetrahedra; Ba4K2Zn5(B3O6)3(B9O19), Ba2KZn3(B3O6)(O(B3O6)2) with two kinds of isolated polyborate anionic groups coexisting in one borate structure; AZn2BO3X2 (A = Na, K, Rb, NH4; X = Cl, Br) series, Cs3Zn6B9O21, BaLiZn3(BO3)3 with benign KBBF-type layered structures, etc. (2) Numerous zincoborates with brilliant physicochemical performance have emerged. For instance, Cs3Zn6B9O21, BaZnBO3F, Bi2ZnOB2O6, Ba5Zn4(BO3)6, Ba3(ZnB5O10)PO4, etc. have been suggested to be suitable for UV NLO applications; Zn4B6O13 and KZnB3O6 with anomalous thermal expansion properties have inspired the discovery of different applications for borates.
Based on the aforementioned findings, the regulation effect of introducing Zn-O/F polyhedra into borates can be emphasized. Firstly, the introduction of Zn-O/F polyhedra can effectively inhibit the polymerization of B-O anionic structures, which is beneficial to obtain isolated B-O groups. In particular, it is propitious to obtain good NLO or birefringent properties when the isolated B-O groups are induced by Zn-O/F polyhedra and exhibit a coplanar arrangement. The terminal oxygen atoms of the B-O groups are linked with zinc atoms, eliminating the dangling bonds of the B-O groups, which would further widen the transparence in the UV region. Moreover, the distorted ZnO4 6− and ZnO3F 5− tetrahedra are NLO-active structural units, which should provide an enhanced contribution to the SHG response, indicating that the zinc-containing borate system is optimal for exploring new NLO materials. However, it is not yet clearly understood which factors determine the special effect of Zn-O/F polyhedra in zincoborates. The intrinsic mechanism understanding of the special contribution of the covalent zinc cations on structural and functional regulation should be theoretically elucidated and exploited in the future, which will present an useful guide for the exploration of undiscovered NLO crystals in zincoborate system that can be practically applied for UV/DUV NLO materials.
In addition, although several zincoborates have been grown with sizable single crystals, there are still great hurdles to develop new zincoborates with excellent properties that are feasible for Reprinted from Ref. [135], Copyright (2016) with permission from Elsevier.

Conclusions
In this review, we have examined the recent development of zincoborates, focusing on the crystal structure chemistry as well as their physicochemical properties. Based on the aforementioned findings, the regulation effect of introducing Zn-O/F polyhedra into borates can be emphasized. Firstly, the introduction of Zn-O/F polyhedra can effectively inhibit the polymerization of B-O anionic structures, which is beneficial to obtain isolated B-O groups. In particular, it is propitious to obtain good NLO or birefringent properties when the isolated B-O groups are induced by Zn-O/F polyhedra and exhibit a coplanar arrangement. The terminal oxygen atoms of the B-O groups are linked with zinc atoms, eliminating the dangling bonds of the B-O groups, which would further widen the transparence in the UV region. Moreover, the distorted [ZnO 4 ] 6− and [ZnO 3 F] 5− tetrahedra are NLO-active structural units, which should provide an enhanced contribution to the SHG response, indicating that the zinc-containing borate system is optimal for exploring new NLO materials. However, it is not yet clearly understood which factors determine the special effect of Zn-O/F polyhedra in zincoborates. The intrinsic mechanism understanding of the special contribution of the covalent zinc cations on structural and functional regulation should be theoretically elucidated and exploited in the future, which will present an useful guide for the exploration of undiscovered NLO crystals in zincoborate system that can be practically applied for UV/DUV NLO materials.
In addition, although several zincoborates have been grown with sizable single crystals, there are still great hurdles to develop new zincoborates with excellent properties that are feasible for growing large single crystals. Looking into the future, continuous exploration and considerable effort should be made in growing large-size single crystals for more detailed physical measurements and practical applications.