Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Edge-Sharing BO4 Tetrahedra in the Structure of Hydrothermally Synthesized Barium Borate: α‑Ba3[B10O17(OH)2] I-Hsuan Jen,† Yi-Chang Lee,† Cheng-En Tsai,† and Kwang-Hwa Lii*,†,‡ †
Department of Chemistry, National Central University, Zhongli, Taiwan 320, Republic of China Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China
‡
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2002.5 For example, Dy4B6O18 was synthesized in a multianvil apparatus under 8 GPa and at 1000 °C. In 2010, Jin et al. and Wu et al. both reported the synthesis of a new borate, KZnB3O6, by solid state reaction at high temperature under ambient pressure.6,7 It is the first compound with es-BO4 synthesized without the use of high pressure. More recently, a quaternary zinc borate with a 2D layer structure containing es-BO4 was also reported.8 It is evident that high pressure is not a necessary condition for the formation of es-BO4. Up to the present, all borates with es-BO4 have been synthesized via solid state reaction at high temperature under high pressure or ambient pressure. In this study, we describe the first borate with es-BO4 tetrahedra, α-Ba3[B10O17(OH)2], which was synthesized in aqueous solution under hydrothermal conditions. Its polymorph, β-Ba3[B10O 17(OH) 2], was also obtained and structurally characterized. α- and β-Ba3[B10O17(OH)2] were synthesized through hightemperature, high-pressure hydrothermal reactions in sealed gold ampouls and characterized by single-crystal and powder X-ray diffraction (Figure S1).9,10 All atoms in the structure of α-Ba3[B10O17(OH)2] are located at general positions. The results of bond-valence sum calculation for Ba and B atoms agree well with their valences. O(2) and O(7) have valence sums of 1.11 and 1.27, respectively, and the values for all other O atoms are close to 2.12 The values for O(2) and O(7) reveal clearly that they are hydroxyl groups. The structure consists of three Ba atoms, four BO3 triangles, and six BO4 tetrahedra. The average B−O bond lengths for BO3 and BO4 groups are 1.369−1.377 Å and 1.469−1.485 Å, respectively, which are in good agreement with the grand mean [3]B−O and [4]B−O distances.2a The 3D borate framework of 1 is built of two fundamental building blocks (FBBs). As shown in Figure 1a, FBB1 is observed for the first time and consists of unusual edge-sharing BO4 tetrahedra, which has a center of inversion at the midpoint of the shared edge. Each tetrahedron of the edgesharing unit shares two corners with two additional tetraheda to form a ring. One of the two tetrahedra is linked by sharing corners with two BO3 triangles to form a ring. The descriptor for FBB1 can be written as 4Δ6□:[⧄⧅]− −|−−| according to Hawthorne et al. and Huppertz, where the [ ] delimiters denote the central linking unit, and the clusters that are connected to the central unit are separated by the symbol |.2a,13
ABSTRACT: Two polymorphs of a new barium borate, α- and β-Ba3[B10O17(OH)2], have been synthesized through hydrothermal reactions at 500 °C and 1000 bar and their structures determined by single-crystal X-ray diffraction. The 3D framework structure of the α-form is formed of two different fundamental building blocks (FBB) with the descriptors 4Δ6□:[⧄⧅]− −|−−| and 2Δ3□:−. The former FBB is unique and contains unusual edge-sharing BO4 tetrahedra. The βform has a double layer structure which is formed of two stereoisomers of a pentaborate polyanion with the descriptor 2Δ3□:−. Within a double layer, one sheet composed of FBBs in l configuration is connected by sharing tetrahedral vertices to the other sheet of FBBs in d configuration. The α-form is the first compound with edge-sharing BO4 tetrahedra synthesized in aqueous solution under hydrothermal conditions.
T
he synthesis of crystalline borate materials has been an extensive research area in past decades. A main purpose of the research is to synthesize new nonlinear optical materials for applications in the UV and deep-UV regions.1 The crystal chemistry of metal borates is very complex, and their structures cover discrete triangular BO33− and tetrahedral BO45− units, polynuclear clusters, 1D chains, 2D sheets, and 3D frameworks.2 The diversity of the structures of borates perhaps exceeds those of silicates and phosphates. Pauling proposed five rules to predict and rationalize ionic crystal structures, namely, the radius ratio rule; the electrostatic valence rule; sharing of polyhedron corners, edges, and faces; crystals containing different cations; and the rule of parsimony.3 According to the third and fourth rules, edge-sharing of the MO4 (M = B, Si, P) tetrahedra was considered unable to exist except under extremely high pressure. No phosphate is known to have edge-sharing PO4 tetrahedra. For silicates, the only phase reported to have edge-sharing SiO4 tetrahedra is the fibrous polymorph of silica described by Weiss and Weiss.4 The oxidation numbers of P, Si, and B in PO43−, SiO44−, and BO45− are +5, +4, and +3, respectively. Therefore, the electrostatic repulsion between BO45− tetrahedra is expected to be less than those for PO43− and SiO44−. Indeed, exceptions are only met in borates, and a good number of metal borates which contain edge-sharing (es-) BO4 tetrahedra have been synthesized under extreme conditions by Huppertz et al. since © XXXX American Chemical Society
Received: February 5, 2019
A
DOI: 10.1021/acs.inorgchem.9b00345 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 1. (a) FBB1 of the α-form containing edge-sharing BO4 tetrahedra. (b) FBB2 of the α-form. Key: green tetrahedra, BO4; olive triangles, BO3; small open circles, H atoms. Figure 3. Structure of the α-form along the a axis. The FBB1 and FBB2 are indicated. Key: green tetrahedra, BO4; olive triangles, BO3; blue circles, Ba atoms; small open circles, H atoms.
The FBB2 is presented in Figure 1b and can be described as 2Δ3□:−, which contains two BO3 triangles and three BO4 tetrahedra that share vertices to form two rings that fuse through a common tetrahedron. This FBB has been observed in the borate minerals probertite, NaCa[B5O7(OH)4]·3H2O,14 and hilgardite, Ca2[B5O9]Cl· H2O,15 and the synthetic borates Ba2[B5O9]Cl·0.5H2O,16,17 M2[B5O9]Cl (M = Ca, Sr, Ba, Pb),18,19 La2CaB10O19,20 and Na2Ba2[B10O17(OH)2].21 There are two possible stereoisomeric configurations of this polyanion, based on placement of the borate triangle with respect to the noncentral borate tetrahedron in the second ring. These two configurations are enantiomorphous with each other and are called left-handed (l) and right-handed (d).22 The FBB2 in the α-form is dB5O12. The B−O bond lengths within the planar B2O2 ring of the edge-sharing tetrahedra are longer (1.507 Å, 1.559 Å) than the average B−O bond lengths for BO4 tetrahedra, whereas the two B−O bonds outside the ring are shorter (1.420 Å, 1.454 Å; Figure 2). The bridging atom, O(3), is also bonded to a third
triangular and two tetrahedral vertices with FBB1s in adjacent sheets. There are two kinds of narrow channels parallel to the a axis where the Ba2+ cations are located. The smaller channel is surrounded by one FBB1 and two FBB2s, whereas the larger one is surrounded by two FBB 1s and two FBB2s. The hydroxyl groups are directed toward the open space in the larger channel. The structure of the α-form is very interesting with respect to the existence of edge-sharing BO4 tetrahedra. It is one of the few compounds with es-BO4. All of the other compounds were synthesized through solid state reactions at high temperature under high pressure or ambient pressure. For the first time we show that an oxoborate with es-BO4 can be synthesized under hydrothermal conditions. The solution chemistry of borates is very complex. In aqueous solutions, borates exist in many forms, and polymeric anions containing structural OH units can be formed. Hydrothermal crystal growth is a dynamic equilibrium process between precipitation and dissolution reactions. The polyborate anions in the solution could be predominantly the same as found in the crystals. Although the synthesis of α-Ba3[B10O17(OH)2] under hydrothermal conditions implies that polyborate anions containing es-BO4 tetrahedra could exist in aqueous solutions, the experimental evidence for the existence of es-BO4 tetrahedra in solution has not been reported. The pentaborate monoanion [B5O6(OH)4]− is observed in solution and occurs in synthetic and mineral borates. 2 3 The pentaborate trianion [B5O6(OH)6]3−, which has the same structure as the FBB2, is found in the mineral ulexite, NaCa[B5O6(OH)6].24 The trianion might also exist in solution and polymerize with other polyborate species to form extended networks. The structure of the β-form consists of Ba2+ cations and two different FBBs with the same formula of B5O11(OH). All atoms are situated at general positions. The bond-valence sums for Ba, B, and all O atoms except O(1) and O(19) agree well with their valences. The values for O(1) and O(19) denote hydroxyl groups. The pentaborate groups are formed of two three-membered rings of one BO3 triangle and two BO4 tetrahedra which are connected by a common tetrahedron. The descriptor for this FBB is 2Δ3□:−, which is the same as that for the FBB2 in the α-form. As shown in Figure 4a the structure of the β-form contains both stereoisomers, namely, l-B5O11(OH) and d-B5O11(OH). Each l-B5O11(OH) group shares corners with four neighboring l-
Figure 2. Bond lengths in Å and angles in deg in the edge-sharing BO4 tetrahedra of the α-form.
tetrahedral boron atom. The O−B−O bond angle inside the B2O2 ring (89.96°, 90.04°) is considerably smaller than those outside the ring (113.15−115.21°) in order to reduce the electrostatic repulsion between two B3+ cations. The B···B distance (2.169 Å) is slightly longer than the corresponding distances in those borates containing edge-sharing BO4 tetrahedra. As shown in Figure 3, the 3D framework of the α-form consists of alternating sheets of FBB1 and FBB2 in the ac plane. The FBB1s are arranged as discrete clusters within a sheet but are linked into a 3D array by sharing two triangular vertices with two tetrahedral vertices of neighboring FBB2s and two tetrahedral vertices with two triangular vertices of neighboring FBB2s in adjacent sheets. The FBB2s link into a sheet by sharing one triangular and three tetrahedral vertices with neighboring FBB2s and are linked by sharing one B
DOI: 10.1021/acs.inorgchem.9b00345 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
X-ray powder patterns and ball-and-stick presentation of the fundamental building blocks in structure of αBa3[B10O17(OH)2] (PDF) Accession Codes
CCDC 1893895 and 1894329 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: + 44 1223 336033
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Figure 4. (a) Two stereoisomers of the pentaborate B5O12 in the βform. (b) Structure of the β-form approximately along the [11̅0] direction. The sheets formed by the pentaborate l-B5O12 or d-B5O12 are indicated. Key: green tetrahedra, BO4; olive triangles, BO3; blue circles, Ba atoms. H atoms are not shown.
AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Kwang-Hwa Lii: 0000-0003-3150-1361
B5O11(OH) groups to form a sheet in the ab plane, which links into a double sheet by sharing tetrahedral vertices with an adjacent sheet formed by d-B5O11(OH) groups (Figure 4b). The sheet sequence along the c axis is ...d−l l−d d−l..., where l and d represent sheets formed of l-B5O11(OH) and dB5O11(OH) groups, respectively, and the horizontal line between l and d indicates the two sheets are connected to form a double layer. Ba(1)2+ and Ba(2)2+ cations are located in the channels within an intralayer region. The double layers are linked through Ba(3)2+ cations located in the interlayer region. The hydroxyl anions O(1) and O(19) are placed on the B triangles which occur on both sides of the double layer. The structure of the β-form is similar to those of La2CaB10O19 and Na2Ba2[B10O17(OH)2].20,21 The latter two compounds also adopt the space group C2 and contain double layers formed of B5O12 double-ring pentaborate groups. They have similar unit cell dimensions along the a and b axes, but the lengths of their c axes are half of that for the β-form. This is because the two compounds consist of only l-B5O11(OH) and the sheet sequence along the c axis is ...l−l l−l... In conclusion, two polymorphs of Ba3[B10O17(OH)2] have been synthesized through high-temperature, high-pressure hydrothermal reactions and their structures determined by single-crystal X-ray diffraction. The α-form has a 3D framework structure with the Ba2+ cations being located at sites in the structural channels. The framework is built up from two different FBBs The FBB1 is observed for the first time and contains an unusual es-BO4 tetrahedra. The α-form is not only one of the few compounds with es-BO4 but also the first one obtained from hydrothermal reaction. In contrast to the αform, the β-form has a 2D layer structure with the Ba2+ cations being situated in the intra- and interlayer regions. The layers are formed of two stereoisomers of an FBB with the same descriptor as the FBB2 in the α-form. Each layer consists of double sheets with the two stereoisomers being segregated in each sheet. Numerous barium borates have been synthesized and characterized. Our exploratory synthetic work discovered two new examples in this enormous phase space. The existence of an es-BO4 tetrahedra in the structure of the α-form may have important implications for the chemistry of aqueous borate solutions.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for financial support and Prof. B.-C. Chang for SHG measurements.
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REFERENCES
(1) (a) Chen, C.; Liu, L.; Wang, X. Fluorine-Containing Beryllium Borates as Nonlinear Optical Crystals for Deep-Ultraviolet Laser Generation; Pan, S.; Wang, Y.; Poeppelmeier, K. R. FrequencyDoubling Oxide Flurides, Borate Fluorides, and Fluoroxoborates. In Photonic and Electronic Properties of Fluoride Materials; Tressaud, A., Poeppelmeier, K., Eds.; Elsevier: Amsterdam, 2016; Ch. 6 and Ch. 15. (b) Tran, T. T.; Yu, H.; Rondinelli, J. M.; Poeppelmeier, K. R.; Halasyamani, P. S. Deep Ultraviolet Nonlinear Optical Materials. Chem. Mater. 2016, 28, 5238−5258 and references therein . (c) Halasyamani, P. S.; Zhang, W. Viewpoint: Inorganic Materials for UV and Deep-UV Nonlinear-Optical Applications. Inorg. Chem. 2017, 56, 12077−12085. (2) (a) Hawthorne, F. C.; Burns, P. C.; Grice, J. D. The Crystal Chemistry of Boron. In Boron: Mineralogy, Petrology and Geochemistry, Reviews in Mineralogy; Grew, E. S., Anovitz, L. M., Eds.; Mineralogical Society of America: Washington, DC, 1996; Vol. 33, pp 41− 115. (b) Leonyuk, N. I. Structural Aspects in Crystal Growth of Anhydrous Borates. J. Cryst. Growth 1997, 174, 301−307. (c) Keszler, D. A. Borates: Solid State Chemistry. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons: Chichester, 1994, Vol. 1, pp 318−327. (3) (a) Pauling, L. The Principles Determining the Structure of Complex Ionic Crystals. J. Am. Chem. Soc. 1929, 51, 1010−1026. (b) Pauling, L. The Nature of the Chemical Bond and the Structure of Molecules and Crystals; An Introduction to Modern Structural Chemistry, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; pp 543−562. (4) Weiss, A.; Weiss, A. Zur Kenntnis der faserigen SilicumdioxydModifikation. Z. Anorg. Allg. Chem. 1954, 276, 95−112. (5) (a) Huppertz, H.; von der Eltz, B. Multianvil High-Pressure Synthesis of Dy4B6O12: The First Oxoborate with Edge-Sharing BO4 Tetrahedra. J. Am. Chem. Soc. 2002, 124, 9376−9377. (b) Knyrim, J. S.; Roeβner, F.; Jakob, S.; Johrendt, D.; Kinski, I.; Glaum, R.; Huppertz, H. Formation of Edge-Sharing BO4 Tetrahedra in the High-Pressure Borate HP-NiB2O4. Angew. Chem., Int. Ed. 2007, 46, 9097−9100. (c) Sohr, G.; Perfler, L.; Huppertz, H. The HighPressure Thallium Triborate HP-TlB3O5. Z. Naturforsch., B: J. Chem. Sci. 2014, 69b, 1260−1268 and references therein . (6) Jin, S.; Cai, G.; Wang, W.; He, M.; Wang, S.; Chen, X. Stable Oxoborate with Edge-Sharing BO4 Tetrahedra Synthesized under Ambient Pressure. Angew. Chem., Int. Ed. 2010, 49, 4967−4970.
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DOI: 10.1021/acs.inorgchem.9b00345 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
(17) Zhang, F.; Zhang, F.; Jia, D.; Pan, S. Synthesis, Characterization and Nonlinear Optical Property of the Barium Borate Halide Ba2B5O9Cl·0.5H2O. Mater. Focus 2015, 4, 44−49. (18) Held, P.; Liebertz, J.; Bohaty, L. Crystal Structure of Dibarium Pentaborate Chloride, Ba2B5O9Cl. Z. Kristallogr. - New Cryst. Struct. 2002, 217, 463−464. (19) Egorova, B. V.; Olenev, A. V.; Berdonosov, P. S.; Kuznetsov, A. N.; Stefanovich, S. Yu.; Dolgikh, V. A.; Mahenthirarajah, T.; Lightfoot, P. Lead-Strontium Borate Halides with Hilgardite-Type Structure and Their SHG Properties. J. Solid State Chem. 2008, 181, 1891−1898. (20) Wu, Y.; Liu, J.; Fu, P.; Wang, J.; Zhou, H.; Wang, G.; Chen, C. A New Lanthanum and Calcium Borate La2CaB10O19. Chem. Mater. 2001, 13, 753−755. (21) Vinogradova, S. A.; Pushcharovsky, D. Yu.; Arakcheeva, A. V.; Dimitrova, O. V. Crystal Structure of New Decaborate Na2Ba2[B10O17(OH)2]. Crystallogr. Rep. 2002, 47, 24−28. (22) Ghose, S. Stereoisomerism of the Pentaborate Polyanion [B5O12]9‑, Polymorphism and Piezoelectricity in th Hilgardite Group of Minerals: A Novel Class of Polar Borate Zeolite. Am. Mineral. 1982, 67, 1265−1272. (23) Schubert, D. M. Borates in Industrial Use. In Group 13 Chemistry III Industrial Applications; Roesky, H. W., Atwood, D. A., Eds.; Springer-Verlag: Berlin, 2003; Ch. 1. (24) Ghose, S.; Wan, C.; Clark, J. R. Ulexite, NaCaB5O6(OH)6· 5H2O: structure refinement, polyanion configuration, hydrogen bonding, and fiber optics. Am. Mineral. 1978, 63, 160−171.
(7) Wu, Y.; Yao, J.-Y.; Zhang, J.-X.; Fu, P.-Z.; Wu, Y.-C. Potassium Zinc Borate, KZnB3O6. Acta Crystallogr., Sect. E: Struct. Rep. Online 2010, E66, i45. (8) Chen, X.; Chen, Y.; Sun, C.; Chang, X.; Xiao, W. Synthesis, Crystal Structure, Spectrum Properties, and Electronic Structure of a New Three-Borate Ba4Na2Zn4(B3O6)2(B12O24) with Two Isolated Types of Blocks: 3[3Δ] and 3[2Δ+1T]. J. Alloys Compd. 2013, 568, 60−67. (9) Synthesis of the two compounds: A reaction mixture of 0.0832 g of Ba(OH)2·8H2O (Merck, 98%), 0.0815 g of H3BO3 (Merck, 99.8%), and 0.32 mL of deionized water (molar ratio Ba/B = 1:5) in a 5.6-cm-long gold ampule (inside diameter = 0.48 cm), sealed completely closed, was heated in a cold-seal pressure vessel at 500 °C for 2 days. The degree of fill of the vessel by water at r.t. was 55%, and the pressure was estimated to be 1000 bar at 500 °C according to the P-T diagram for H2O. The autoclave was then slowly cooled to 350 °C at 5 °C/h and quenched in air by removing it from the tube furnace. The crystalline products were filtered, washed with water, rinsed with ethanol, and dried in a desiccator at r.t. The products contained the two polymorphs of Ba3[B10O17(OH)2] as major products in about equal amount and a small amount of Ba2B7O12(OH).11 The α-form crystallizes as blocks, whereas the βform does as plates. A pure product of each polymorph has not been obtained despite numerous attempts at the synthesis. A reaction mixture containing the same reactants in the same molar ratio as above was heated in a Teflon-lined autoclave at 200 °C for several days and slowly cooled to r.t. The polycrystalline products did not contain the two polymorphs as indicated by powder X-ray diffraction. (10) Crystal structure analysis of α-Ba3[B10O17(OH)2]: MW = 826.14 g/mol, colorless block, 0.144 × 0.091 × 0.078 mm3, monoclinic, space group P21/n, a = 6.6315(3) Å, b = 23.923(1) Å, c = 9.0173(4) Å, β = 90.878(1)°, V = 1429.8(1) Å3, Z = 4, ρcalc = 3.838 g/cm3, Mo Kα radiation (λ = 0.71073 Å), F(000) = 1488, μ = 8.291 mm−1, T = 296 K, 37 968 reflections measured, 3728 unique reflections with I > 2σ(I), Tmin/max = 0.602/0.746, 290 refined parameters, R1 = 0.0139, wR2 = 0.0339, and GOF = 1.088. βBa3[B10O17(OH)2]: colorless plate, 0.144 × 0.096 × 0.081 mm3, monoclinic, space group C2, a = 11.4957(7) Å, b = 6.7330(4) Å, c = 19.241(1) Å, β = 93.597(2)°, V = 1486.3(2) Å3, Z = 4, ρcalc = 3.692 g/cm3, μ = 7.975 mm−1, T = 296 K, 82 908 reflections measured, 3651 unique reflections with I > 2σ(I), Tmin/max = 0.50/0.56, 290 refined parameters, R1 = 0.0094, wR2 = 0.0235, and GOF = 1.080. Some colorless plate crystals of the β form were manually selected and ground into powder. The SHG responses of the powder sample were measured using a pulsed Nd:YAG laser. A definite green emission with an intensity of about half of that for a powder sample of quartz could be detected, confirming the absence of a center of symmetry. (11) Mcmillen, C. Hydrothermal Crystal Growth of Oxides for Optical Applications; Ph.D. Dissertation, Clemson University, Clemson, SC, 2007. (12) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, B41, 244−247. (13) Huppertz, H. High-Pressure Preparation, Crystal Structure, and Properties of RE4B6O15 (RE = Dy, Ho) with an Extension of the “Fundamental Building Block”-Descriptors. Z. Z. Naturforsch., B: J. Chem. Sci. 2003, 58b, 278−290. (14) Menchetti, S.; Sabelli, C.; Trosti-Ferroni, R. Probertite, CaNa[B5O7(OH)4]·3H2O: a Refinement. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, B38, 3072−3075. (15) Burns, P. C.; Hawthorne, F. C. Refinement of the Structure of Hilgardite-1A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1994, C50, 653−655. (16) Ferro, O.; Merlino, S.; Vinogradova, S. A.; Pushcharovsky, D. Y.; Dimitrova, O. V. Crystal Structures of Two New Ba Borates Pentaborate, Ba2[B5O9]Cl·0.5H2O and Ba2[B5O8(OH)2](OH). J. Alloys Compd. 2000, 305, 63−71. D
DOI: 10.1021/acs.inorgchem.9b00345 Inorg. Chem. XXXX, XXX, XXX−XXX
