Molten Boric Acid Research Articles

Stable heteroepitaxial interface of Li-rich layered oxide cathodes with enhanced lithium storage

Lithium and oxygen activities can have substantial influences on the kinetics of ion and electron transport and the structural integrity of Li-rich layered oxide (LLO) cathodes, since reversible oxygen redox is ascribed to the extra capacity beyond the theoretical capacity from transition metal redox at high voltages. Herein, we demonstrate a liquid-solid interfacial reaction to generate a heteroepitaxial interface with tunable Li/O activities on LLOs using molten boric acid. The experimental and theoretical analyses indicate that the atomic scale interface is comprised of a disordered rock salt structure containing substantial Li/O vacancies along the layered structure, associated with a segregation tendency of Ni and Co. The formation of this heteroepitaxial interface with Li/O vacancies improves the ionic/electronic conduction and electrochemical/structural stability, leading to a high discharge capacity of 283 mA h g-1 with initial Coulombic efficiency of 91.7% (0.1 C, 2.0–4.7 V vs. Li+/Li), excellent rate performance (246 and 159.7 mAh g-1 at 1 C and 10 C, respectively) and enhanced cyclic performance with a capacity retention of 92% after 100 cycles. The findings highlight the importance of a well-engineered interface for the design of high performance layered cathode materials for Li storage.

Chirality and polarity in the f-block borates M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, and Cf).

The reactions of trivalent lanthanides and actinides with molten boric acid in high chloride concentrations result in the formation of M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, Cf). This cubic structure type is remarkably complex and displays both chirality and polarity. The polymeric borate network forms helical features that are linked via two different types of nine-coordinate f-element environments. The f-f transitions are unusually intense and result in dark coloration of these compounds with actinides.

Synthesis of Divalent Europium Borate via in Situ Reductive Techniques

A new divalent europium borate, Eu[B8O11(OH)4], was synthesized by two different in situ reductive methodologies starting with a trivalent europium starting material in a molten boric acid flux. The two in situ reductive techniques employed were the use of HI as a source of H2 gas and the use of a Zn amalgam as a reductive, reactive surface. While both of these are known reductive techniques, the title compound was synthesized in both air and water which demonstrates that strict anaerobic conditions need not be employed in conjunction with these reductive methodologies. Herein, we report on the structure, spectroscopy, and synthetic methodologies relevant to Eu[B8O11(OH)4]. We also report on a europium doping study of the isostructural compound Sr[B8O11(OH)4] where the amount of doped Eu(2+) ranges from 2.5 to 11%.

Alkali Transition Metal Phosphites as Potential Cathode Materials Grown from Molten Boric Acid

AbstractThis is the first study on developing six alkali transition metal phosphites as potential candidates of cathode materials grown from molten boric acid, a simple, low energy costing method compared with conventional solid‐state reactions. Li2[Mn(HPO3)2] (1) and Li[V(HPO3)2] (2) are featured with high theoretical capacitance values up to 117 and 123 mAh/g, respectively. With the replacement of Li+ by Na+ or K+, four new compounds with strongly related layer topology Na2[Co4(HPO3)5] (3), K2[Co3(HPO3)4] (4), K2[Mn2(HPO3)3] (5), and K[V(HPO3)2] (6) were prepared. The synthesis mediated by molten boric acid automatically provides reductive atmosphere and helps maintain low valence in metal centers.

Recent advances in trivalent f-element borate chemistry

The reactions of LnCl3 · 6 H2O with molten boric acid result in the formation of three different products depending on the identity of the lanthanide metal. The analogous reactions using AnCl3 · 6 H2O (An = Pu(III), Am(III), and Cm(III) with molten boric acid under the same conditions do not yield the same results as those of the 4f series or of the other 5f elements. For the most part, the actinide products obtained have no lanthanide analogues making them unique compounds. Furthermore, the reactions of LnX3 · yH2O (X = Br, I) (Ln = La-Nd) with molten boric acid results in the formation of two products. The analagous set of reactions was performed with PuX3 · yH2O (X = Br, I) which, yet again, gave rise to completely novel compounds. All of these compounds are three-dimensional framework structures comprised of sheets. These sheets are made up of both BO3 and BO4 units which are arranged in such a maner that creates triangular holes in which the lanthanide or actinide sites reside. The borate units of the triangular holes provide six oxygen atoms that coordinate in the equatorial region resulting in unique nine- and/or ten-coordinate geometries. The similarities and differences between the 4f and 5f borate compounds are discussed with particular emphasis on the periodic trends of this system.

ChemInform Abstract: Differentiating Between Trivalent Lanthanides and Actinides.

AbstractThe reaction products of LnCl3 (Ln: Y, La—Lu) and AnCl3 (An: Pu, Am, Cm) with molten boric acid are characterized by single crystal XRD, UV/VIS/NIR spectroscopy, and DFT quantum chemical calculations.

Effect of pH and Reaction Time on the Structures of Early Lanthanide(III) Borate Perchlorates

Reactions of LnCl(3)·6H(2)O (Ln = La-Nd, Sm, Eu), concentrated (11 M) perchloric acid, and molten boric acid result in the formation of four different compounds. These compounds are Ln[B(8)O(10)(OH)(6)(H(2)O)(ClO(4))]·0.5H(2)O (Ln = La-Nd, Sm), Pr[B(8)O(11)(OH)(4)(H(2)O)(ClO(4))], Ln[B(7)O(11)(OH)(H(2)O)(2)(ClO(4))] (Ln = Pr, Nd, Sm, and Eu), and Ce[B(8)O(11)(OH)(4)(H(2)O)(ClO(4))]. All Ln(III) cations are ten-coordinate with a capped triangular cupola geometry and contain an inner-sphere, monodentate perchlorate moiety. This geometry is obtained because of the coordination of the oxygen donors within the polyborate sheet which create triangular holes and provide residence for the lanthanide metal centers. Aside from Ln[B(8)O(10)(OH)(6)(H(2)O)(ClO(4))]·0.5H(2)O (Ln = La-Nd, Sm), which are two-dimensional sheet structures, all other compounds are three-dimensional frameworks in which the layers are tethered together by BO(3) units found roughly perpendicular to the sheets. Furthermore, a change in product is observed depending on the reaction duration while holding all other synthetic variables constant. This report also demonstrates that lanthanide borates can be prepared in extreme acidic conditions.

Effects of Large Halides on the Structures of Lanthanide(III) and Plutonium(III) Borates

Reactions of LnBr(3) or LnOI with molten boric acid result in formation of Ln[B(5)O(8)(OH)(H(2)O)(2)Br] (Ln = La-Pr), Nd(4)[B(18)O(25)(OH)(13)Br(3)], or Ln[B(5)O(8)(OH)(H(2)O)(2)I] (Ln = La-Nd). Reaction of PuOI with molten boric acid yields Pu[B(7)O(11)(OH)(H(2)O)(2)I]. The Ln(III) and Pu(III) centers in these compounds are found as nine-coordinate hula-hoop or 10-coordinate capped triangular cupola geometries where there are six approximately coplanar oxygen donors provided by triangular holes in the polyborate sheets. The borate sheets are connected into three-dimensional networks by additional BO(3) triangles and/or BO(4) tetrahedra that are roughly perpendicular to the layers. The room-temperature absorption spectrum of single crystals of Pu[B(7)O(11)(OH)(H(2)O)(2)I] shows characteristic f-f transitions for Pu(III) that are essentially indistinguishable from Pu(III) in other compounds with alternative ligands and different coordination environments.

Differentiating between Trivalent Lanthanides and Actinides

The reactions of LnCl(3) with molten boric acid result in the formation of Ln[B(4)O(6)(OH)(2)Cl] (Ln = La-Nd), Ln(4)[B(18)O(25)(OH)(13)Cl(3)] (Ln = Sm, Eu), or Ln[B(6)O(9)(OH)(3)] (Ln = Y, Eu-Lu). The reactions of AnCl(3) (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B(4)O(6)(OH)(2)Cl] and Pu(2)[B(13)O(19)(OH)(5)Cl(2)(H(2)O)(3)], Am[B(9)O(13)(OH)(4)]·H(2)O, or Cm(2)[B(14)O(20)(OH)(7)(H(2)O)(2)Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO(3) and/or BO(4) groups. There is a shift from 10-coordinate Ln(3+) and An(3+) cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm(3+) is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am(III) and Cm(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce(III) borate is similar to the Pu(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals.

ChemInform Abstract: Curium(III) Borate Shows Coordination Environments of Both Plutonium(III) and Americium(III) Borates.

AbstractCrystals of Cm2 [B14O20(OH)7(H2O)2Cl] are prepared by reaction of CmCl3 with molten boric acid (autoclave, 240 °C, 7 d).

Curium(III) Borate Shows Coordination Environments of Both Plutonium(III) and Americium(III) Borates

Crystals of Cm2 [B14O20(OH)7(H2O)2Cl] are prepared by reaction of CmCl3 with molten boric acid (autoclave, 240 °C, 7 d).

ChemInform Abstract: Recent Progress in Actinide Borate Chemistry

AbstractReview: 85 refs.

Interstitial Incorporation of Plutonium into a Low-Dimensional Potassium Borate

The molten boric acid flux reaction of PuBr(3) with KBO(2) at 200 °C results in the formation of large light-yellow crystals of K[B(5)O(7)(OH)(2)]·H(2)O:Pu(4+). Single-crystal X-ray diffraction experiments on the Pu-doped K[B(5)O(7)(OH)(2)]·H(2)O demonstrate two features: (1) K[B(5)O(7)(OH)(2)]·H(2)O:Pu(4+) adopts a one-dimensional borate chain structure with void spaces between the chains. (2) The doping plutonium atoms do not reside on the potassium sites. The latter are not fully occupied. Both laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and energy-dispersive spectrometry analyses indicate that plutonium atoms are uniformly distributed in crystals of K[B(5)O(7)(OH)(2)]·H(2)O with an atomic K:Pu ratio of approximately 65:1 measured by LA-ICP-MS. UV-vis-NIR spectra taken from both freshly made and one day old crystals show that the plutonium present within the crystals is predominantly characterized by Pu(IV). A small amount of Pu(III) is also present initially, but slowly oxidized to Pu(IV) via interaction with oxygen in the air. X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopic measurements confirm that plutonium is mainly present as a form similar to that of a PuO(2) cluster. The combined results suggest that the clusters containing Pu(IV) ions are uniformly distributed in the void spaces between the borate chains.

K(NpO2)3(H2O)Cl4: A Channel Structure Assembled by Two- and Three-Center Cation–Cation Interactions of Neptunyl Cations

A Np(V) compound containing three-center cation-cation interations, K(NpO(2))(3)(H(2)O)Cl(4), has been prepared by reacting Np(V) with KCl in molten boric acid. This compound forms a three-dimensional channel structure that is constructed from both two- and three-center cation-cation interactions. Three new bonding modes for cation-cation interactions are added to the summary of all known Np(V) compounds.

Surprising Coordination for Plutonium in the First Plutonium(III) Borate

The first plutonium(III) borate, Pu(2)[B(12)O(18)(OH)(4)Br(2)(H(2)O)(3)]·0.5H(2)O, has been prepared by reacting plutonium(III) with molten boric acid under strictly anaerobic conditions. This compound contains a three-dimensional polyborate network with triangular holes that house the plutonium(III) sites. The plutonium sites in this compound are 9- and 10-coordinate and display atypical geometries.

Role of Anions and Reaction Conditions in the Preparation of Uranium(VI), Neptunium(VI), and Plutonium(VI) Borates

U(VI), Np(VI), and Pu(VI) borates with the formula AnO(2)[B(8)O(11)(OH)(4)] (An = U, Np, Pu) have been prepared via the reactions of U(VI) nitrate, Np(VI) perchlorate, or Pu(IV) or Pu(VI) nitrate with molten boric acid. These compounds are all isotypic and consist of a linear actinyl(VI) cation, AnO(2)(2+), surrounded by BO(3) triangles and BO(4) tetrahedra to create an AnO(8) hexagonal bipyramidal environment. The actinyl bond lengths are consistent with actinide contraction across this series. The borate anions bridge between actinyl units to create sheets. Additional BO(3) triangles and BO(4) tetrahedra extend from the polyborate layers and connect these sheets together to form a three-dimensional chiral framework structure. UV-vis-NIR absorption and fluorescence spectroscopy confirms the hexavalent oxidation state in all three compounds. Bond-valence parameters are developed for Np(VI).

Recent progress in actinide borate chemistry

The use of molten boric acid as a reactive flux for synthesizing actinide borates has been developed in the past two years providing access to a remarkable array of exotic materials with both unusual structures and unprecedented properties. [ThB(5)O(6)(OH)(6)][BO(OH)(2)]·2.5H(2)O possesses a cationic supertetrahedral structure and displays remarkable anion exchange properties with high selectivity for TcO(4)(-). Uranyl borates form noncentrosymmetric structures with extraordinarily rich topological relationships. Neptunium borates are often mixed-valent and yield rare examples of compounds with one metal in three different oxidation states. Plutonium borates display new coordination chemistry for trivalent actinides. Finally, americium borates show a dramatic departure from plutonium borates, and there are scant examples of families of actinides compounds that extend past plutonium to examine the bonding of later actinides. There are several grand challenges that this work addresses. The foremost of these challenges is the development of structure-property relationships in transuranium materials. A deep understanding of the materials chemistry of actinides will likely lead to the development of advanced waste forms for radionuclides present in nuclear waste that prevent their transport in the environment. This work may have also uncovered the solubility-limiting phases of actinides in some repositories, and allows for measurements on the stability of these materials.

Further insights into intermediate- and mixed-valency in neptunium oxoanion compounds: structure and absorption spectroscopy of K2[(NpO2)3B10O16(OH)2(NO3)2

The reaction of Np(V) chloride with molten boric acid results in the disproportionation of Np(V) into Np(IV) and Np(VI), and the crystallization of K(2)[(NpO(2))(3)B(10)O(16)(OH)(2)(NO(3))(2)]. UV-vis-NIR spectroscopy demonstrates that in addition to the Np(VI) and Np(V) found in the crystal structure, Np(IV) is also present.

Photocatalytic Properties of Nanocrystalline Tin Oxide Synthesized by Using Molten Boric Acid as a Flux

Nanocrystalline tin oxide (S1) was prepared by using molten boric acid as a flux and was characterized by XRD, TEM, and BET analysis. Its photocatalytic activity under UV excitation was evaluated by the photodegradation of methylene blue in water and compared to that of the well-known P25 (TiO2) and SnO2 powder (S2) synthesized from hydrothermal system without the participation of H3BO3. It has been revealed that S1 exhibits similar photocatalytic activity to that of P25 powder and better than that of S2. The high photocatalytic activity of S1 is most likely to be originated from its small particle size and the boron-doping in the crystal lattice.

NH4[BGe3O8]: A New Borogermanate Framework Made of Infinite‐Chain Building Blocks.

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