Polymorphism of Calcium Decahydrido-closo-decaborate and Characterization of Its Hydrates

Abstract

Metal closo-borates and their derivatives have shown promise in several fields of application from cancer therapy to solidstate electrolytes partly owing to their stability in aqueous solutions and high thermal stability. We report the synthesis and structural analysis of αand β-CaB 10 H 10 , which are structurally and energetically similar, both showing a tetrahedral coordination of Ca 2+ to four closo-borate cages. The main distinctions between the αand βpolymorph are found in the crystal system (monoclinic or orthorhombic), topology (wurtzite or cag), and the degree of displacement of Ca 2+ from the center of the coordination tetrahedron. Neutron vibrational spectroscopy measurements further revealed distinct perturbations in the cation−anion interactions arising from the different crystal structures. We also synthesized and structurally investigated five stoichiometric hydrates, CaB 10 H 10 • xH 2 O, x = 1, 4, 5, 6, and 7, and discovered an order−disorder polymorphic transition, αto β-CaB 10 H 10 •6H 2 O. The hydrates reveal a rich structural diversity with ordered structures, CaB 10 H 10 •xH 2 O, x = 1, 4, 5, 6, and 7, as well as disordered structures, x = 6 and 8. The latter allow for a continuum of compositions within 7−8 molecules of crystal water. The DFT-optimized experimental crystal structures reveal complex networks of three types of hydrogen interactions: dihydrogen bonds, B−H δ− ••• +δ H−O; hydrogen− hydrogen interactions, B−H•••H−B; and hydrogen bonds, O−H δ+ ••• −δ O−H. A rather short B−H•••H−B (2.14 Å) interaction is observed for CaB 10 H 10 •5H 2 O, which is locally stabilized by four hydrogen bonds.

Thermal Polymorphism in CsCB11H12

“…The labeling as αand α -polymorphs is not unambiguous because they appear simultaneously at the same temperature, but the α -polymorph exists only in a limited temperature range. They may correspond to two polymorphs with very close free energies, as has been recently observed in CaB 10 H 10 [12]. In the rt diffraction data of a CsCB 11 H 12 sample containing 7% Rb alkali-metal substitution, we have observed, besides the dominant peaks of γ-CsCB 11 H 12 , two other sets of diffraction peaks.…”

Metallic and complex hydride-based electrochemical storage of energy

“…Metal closo-borates have been useful as new monovalent cationic conductors, however, appear significantly less successful in generating new divalent cationic electrolytes. A series of metal decahydro-closo-decaborates, MB 10 H 10 (M = Ca, Sr, and Mn) all have rather low ionic conductivity [170,171].…”

Hydrogen storage in complex hydrides: past activities and new trends

“…bonded to boron with a single covalent bond (2c, 2e bond) and are therefore rather stable. Metal closo-borates are often synthesised by cation exchange in aqueous solutions, this includes calcium decahydrido-closo-decaborate, α-and β-CaB 10 H 10 , where a variety of hydrates are also described, CaB 10 H 10 •xH 2 O, x = 1, 4, 5, 6, 7 [231]. Owing to their high stability, this class of material has been considered a 'hydrogen sink' for the release and uptake of hydrogen and it was expected to hamper their reversible hydrogen storage.…”

“…Derivatives with neutral molecules have also been discovered, mainly with protic hydrogens which coordinate via hydrogen bonding, but also others containing donor atoms which can coordinate to a metal in the structure, such as NH 3 , H 2 O etc observed for e.g. CaB 10 H 10 •xH 2 O, x = 1, 4, 5, 6, 7 [231].…”