Communication pubs.acs.org/IC
Two KBBF-Type Beryllium Borates MBe2B2O6 (M = Sr, Ba) with a Three-Dimensional (Be2B2O6)∞ Network Zhi Fang,†,‡ Fei Liang,†,‡ Mingjun Xia,† Lijuan Liu,*,† Qian Huang,†,‡ Shu Guo,†,‡ Xiaoyang Wang,† Zheshuai Lin,† and Chuangtian Chen† †
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Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
about nine structures in this system have been reported) and the example of SBBO, we continue our exploration in this system. As a result, in this work, we found two new structures, BaBe2B2O6 (TRB; CCDC 1561042) and SrBe2B2O6 (SBB; CCDC 1561041), and both structures are characterized with a special 3D multilayer network, (Be2B2O6)∞. Different from the previously reported SBB17 and TRB,18 which are not built up by KBBF-type 6-membered rings, these two new crystals are highly consistent with our proposal. Single crystals of this new SBB crystal were grown by the spontaneous nucleation method from a SrF2−B2O3−NaF flux system. Considering the high toxicity of the BeO powder, the following operations are done in a ventilation environment. A mixture of SrCO3, BeO, SrF2, B2O3, and NaF at a molar ratio of 2−4:3−7:3:3−6:2−7 was served into a Φ40 platinum crucible and then melted at 980 °C. After 15 days of slow cooling to 680 °C and another 4 days of cooling to room temperature, colorless SBB single crystals were obtained. A similar procedure was employed in the growth of this new TRB with a PbO−PbF2− B2O3 flux system. To synthesize the polycrystalline SBB, raw materials such as SrCO3, BeO, and H3BO3 with a molar ratio of 1:2:2 were placed in a platinum crucible after uniform mixing and then held at 950 °C for 4 days. The purities of these assynthesized samples were checked by powder X-ray diffraction (PXRD; see Figure S1). As shown in Figure S2, the differential scanning calorimetry (DSC) curve of SBB shows one sharp endothermic peak at around 1109 °C in the heating process and one exothermic peak at about 885 °C in the cooling process. PXRD of the residuals determined the remaining phases as SrB2O4 and SBB. Therefore, SBB melts incongruently and should be grown with flux. Both SBB and TRB crystallize in the monoclinic space group C2/c. Because the two compounds are isostructural, the structure of SBB is described for representation. There are three chemically independent O atoms in SBB, while B and Be atoms all occupy one chemically independent environment. As shown in Figure 1, the SBB crystal is featured with a “stairlike” 3D network, and B and Be atoms are all coordinated with three and four O atoms, generating BO3 triangles and BeO4 tetrahedra, respectively. The bond lengths of the B−O bonds are in the region of 1.358(5)−1.391(5) Å, while the Be−O bonds differ
ABSTRACT: Two new polyborates, BaBe2B2O6 and SrBe2B2O6, in a three-dimensional (Be2B2O6)∞ network featuring KBBF-type two-dimensional planes are synthesized. Compared with KBBF, both compounds possess comparable optical birefringence and deep-ultraviolet (deep-UV) cutoff edges and exhibit better bulk growth habits owing to their three-dimensional networks, which make them applicable deep-UV optical materials.
B
orates have attracted continuous interest for their rich structural chemistry and excellent performances in various fields especially in nonlinear/birefringence optics1−4 and photocatalytic chemistry.5 Commonly, B atoms can be either three- or four-coordinated to form a BO3 triangle or a BO4 tetrahedron, respectively, which can further link together to form numerous structures. Also, they can be connected with other atoms like Be/ Al/Zn to expand their structural diversity.6−9 In particular, beryllium borates have played an important role in deepultraviolet (deep-UV; λ < 200 nm) nonlinear-optical (NLO) crystals, namely, KBe2BO3F2-type (KBBF-type) crystals like KBBF,10,11 NaBe2BO3F2,10,12 RbBe2BO3F2 (RBBF)10,13 and BaBe2BO3F314 and Sr2Be2B2O7-type (SBBO-type) crystals like SBBO15 and Ba2Be2B2O7.16 Among them, only KBBF and RBBF crystals are applicable for deep-UV generation by direct secondharmonic generation. The excellent NLO performance originated from its featured two-dimensional (2D) (Be2BO3F2)∞ layer with a 6-membered-ring building block. The adjacent (Be2BO3F2)∞ layers are only connected by a K−F electric force, so the crystal has a serious layering growth habit. SBBO15 was developed to solve the layering growth habit of KBBF with a three-dimensional (3D) (Be3B3O6)∞ network. SBBO did exhibit a less platelike growth habit. Unfortunately, SBBO has structural polymorphism, and its structure has not been well solved until now. The loose multilayer (Be3B3O6)∞ network of SBBO [each single layer is featured by 12-membered loops, while each single layer within the (Be3B3O6)∞ network is connected by Be−O−Be bridging connections] is easily rotated and leads to structural polymorphism. Hence, searching for new models to solve the layering growth habit of KBBF is still active and meaningful. Inspired by the structure−property relationships of KBBF and SBBO, we propose that a new 3D network with a KBBF-type 2D plane is promising to solve the layering problem. Considering the less explored AO−BeO−B2O3 (A = Mg, Ca, Sr, Ba) system (only © 2017 American Chemical Society
Received: July 13, 2017 Published: September 25, 2017 12090
DOI: 10.1021/acs.inorgchem.7b01743 Inorg. Chem. 2017, 56, 12090−12093
Communication
Inorganic Chemistry
planes. From Figure 2, the (Be2BO3F2)∞ layer can be divided into numerous small “A”-type planes, which are exactly the same as the A-type (Be2B2O6)∞ planes, while the B-type (Be2B2O6)∞ planes can be viewed as inserted planes to connect the adjacent A-type (Be2B2O6)∞ planes along the out-of-plane direction. Therefore, the (Be2B2O6)3D∞ network is of less layer habit and much denser BO3 groups compared with the 2D (Be2BO3F2)∞ layer. The density of the BO3 groups in SBB (0.0182 Å−3) is almost doubled compared with that of KBBF (0.0094 Å−3); also it is about 1.32 times that of SBBO (0.0138 Å−3). Such dense BO3 groups are very scarce in the currently known beryllium borates. From Figure 2, adjacent (Be2B2O6)3D∞ networks are connected by SBBO-type O−Sr−O bridging connections; hence, the internetwork connections of SBB are also supposed to be beneficial for less layering growth habit. With enhancement of the less platelike (Be2B2O6)3D∞ network and the SBBO-type internetwork connections, the SBB crystal is supposed to exhibit a better bulk growth habit than KBBF. To test the validity of this hypothesis, we conducted a series of growth experiments. Finally, in Figure 3a, a lump SBB crystal with the size of 2.9 × 2.8 × 1.4 mm3 has been obtained by a spontaneous nucleation method, which shows no serious layering growth habit.
Figure 1. Structural properties of the SBB crystal.
from 1.612(5) to 1.638(5) Å. BO3 triangles and BeO4 tetrahedra are alternatively arranged in the ratio of 1:1, forming two types of 2D (Be2B2O6)∞ planes, the A and B planes, to serve as the “facets” of the “stairs”. The infinite (Be2B2O6)∞ “facets” are stacked in the sequence of “···ABAB···” along the c direction, giving rise to the 3D (Be2B2O6)3D∞ “stairs” architecture, while the A and B (Be2B2O6)∞ planes all share the same KBBF-type plane building blocks despite their totally opposite alignment of the apical Be−O bonds. When the (Be2B2O6)3D∞ network is projected to the (Be2B2O6)∞ plane, the A and B planes all overlap in the region of two lines of BO3 triangles, which are all connected by an infinite b-oriented zigzag line of concurrent BeO4 tetrahedra. As a result, every BeO4 group within the A/B plane is directly connected to one BeO4 tetrahedron and two BO3 triangles by point-sharing to form the KBBF-type 6membered ring, while the out-of-plane Be−O bonds are used for connection of the two adjacent B/A planes. As seen from the b direction, the A and B planes all share the same finite width at about 6.263(5) Å. As for the A-site cation, Sr atoms occupy one independent crystallography environment and are all eightcoordinated to O atoms between adjacent (Be2B2O6)3D∞ “stairs”, with the lengths of the Sr−O bonds ranging from 2.628(5) to 2.811(5) Å. As illustrated in Figure 1, the (Be2B2O6)3D∞ network is built up by the infinite stacking of 2D A- and B-type (Be2B2O6)∞
Figure 3. (a) Photograph of the as-grown SBB crystals (1 blue grid stands for 10 × 10 mm2). (b) UV−vis spectroscopy of SBB.
Because SBBO-type and titled SBB-type structures are the only two observed models featured with the (Be2BO3F2)∞-layerresembled 3D networks in the AO−BeO−B2O3 (A = Mg, Ca, Sr, Ba) system up to now, the structural stability of the SBB model also arouses our concern. Structural refinements of SBB identified it as having no obvious structural problems (R = 0.0274; wR2 = 0.0541; the highest peak in the final difference
Figure 2. Structural revolution from KBBF to the SBB crystal and structural comparisons of SBBO and SBB. 12091
DOI: 10.1021/acs.inorgchem.7b01743 Inorg. Chem. 2017, 56, 12090−12093
Communication
Inorganic Chemistry map is 0.46 e Å3−) compared with SBBO (R = 0.043; wR2 = 0.063; the highest peak in the final difference map is 5.6 e Å3−). This priority could be ascribed to its much more stable (Be2B2O6)3D∞ network structure. The (Be3B3O6)∞ planes in SBBO are connected by the long O−Be−O and O−Sr−O bridging connections to form the (Be3B3O6)3D∞ network; meanwhile, the (Be3B3O6)∞ planes are all featured with loose 12-membered loops. As discussed by Meng et al.,18 structural polymorphism of SBBO is mainly caused by the rotation of BeO4 groups along the c direction, which could be further ascribed to the “discomfort” brought by its large-size Sr atoms within the (Be3B3O6)3D∞ network and the “tolerance” included in its loose 12-membered-loop building blocks of the (Be3B3O6)3D∞ plane. By contrast, the (Be2B2O6)3D∞ network is built up by the stacking of (Be2B2O6)∞ planes, while adjacent (Be2B2O6)∞ planes are directly connected with short Be−O bonds by excluding the large-size Sr atoms from the interplane region. Also, the (Be2B2O6)∞ planes are all featured with dense KBBFtype 6-membered rings, as a result, there are no SBBO-type “discomfort” between the adjacent (Be2B2O6)∞ planes and a much smaller “tolerance” in the (Be2B2O6)∞ plane; hence, rotation of the BeO4 groups between the adjacent planes has been weakened to a much smaller level. All of these advances guarantee that SBB is much more stable. Furthermore, UV−vis spectroscopy of SBB was collected by a Cary 5000 UV−vis−near-IR spectrophotometer with the asgrown SBB crystal in Figure 3. The results indicate that the cutoff wavelength of SBB is below 185 nm. This value is comparable to those of alkaline/alkaline-earth beryllium borates, such as NaSr3Be3B3O9F4 (170 nm)19 and BaBe2BO3F3 (
