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Title: Structure of Unsolvated Magnesium Borohydride Mg(BH4)2

Abstract

We have determined the structures of two phases of unsolvated Mg(BH{sub 4}){sub 2}, a material of interest for hydrogen storage. One or both phases can be obtained depending on the synthesis conditions. The first, a hexagonal phase with space group P6{sub 1}, is stable below 453 K. Upon heating above that temperature it transforms to an orthorhombic phase, with space group Fddd, stable to 613 K at which point it decomposes with hydrogen release. Both phases consist of complex networks of corner-sharing tetrahedra consisting of a central Mg atom and four BH{sub 4} units. The high-temperature orthorhombic phase has a strong antisite disorder in the a lattice direction, which can be understood on the basis of atomic structure.

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
929917
Report Number(s):
BNL-80506-2008-JA
TRN: US200822%%1090
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Acta Crystallographica Section B: Structural Science; Journal Volume: 63
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; ATOMS; BOROHYDRIDES; HEATING; HYDROGEN; HYDROGEN STORAGE; MAGNESIUM; SPACE GROUPS; SYNTHESIS; TEMPERATURE RANGE 0400-1000 K; national synchrotron light source

Citation Formats

Her,J., Stephens, P., Gao, Y., Soloveichik, G., Rijssenbeek, J., Andrus, M., and Zhao, J.. Structure of Unsolvated Magnesium Borohydride Mg(BH4)2. United States: N. p., 2007. Web. doi:10.1107/S0108768107022665.
Her,J., Stephens, P., Gao, Y., Soloveichik, G., Rijssenbeek, J., Andrus, M., & Zhao, J.. Structure of Unsolvated Magnesium Borohydride Mg(BH4)2. United States. doi:10.1107/S0108768107022665.
Her,J., Stephens, P., Gao, Y., Soloveichik, G., Rijssenbeek, J., Andrus, M., and Zhao, J.. Mon . "Structure of Unsolvated Magnesium Borohydride Mg(BH4)2". United States. doi:10.1107/S0108768107022665.
@article{osti_929917,
title = {Structure of Unsolvated Magnesium Borohydride Mg(BH4)2},
author = {Her,J. and Stephens, P. and Gao, Y. and Soloveichik, G. and Rijssenbeek, J. and Andrus, M. and Zhao, J.},
abstractNote = {We have determined the structures of two phases of unsolvated Mg(BH{sub 4}){sub 2}, a material of interest for hydrogen storage. One or both phases can be obtained depending on the synthesis conditions. The first, a hexagonal phase with space group P6{sub 1}, is stable below 453 K. Upon heating above that temperature it transforms to an orthorhombic phase, with space group Fddd, stable to 613 K at which point it decomposes with hydrogen release. Both phases consist of complex networks of corner-sharing tetrahedra consisting of a central Mg atom and four BH{sub 4} units. The high-temperature orthorhombic phase has a strong antisite disorder in the a lattice direction, which can be understood on the basis of atomic structure.},
doi = {10.1107/S0108768107022665},
journal = {Acta Crystallographica Section B: Structural Science},
number = ,
volume = 63,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Different methods for preparation of unsolvated magnesium borohydride, a promising material for hydrogen storage, based on exchange reaction of MgCl2 with lithium and sodium borohydride in different solvents have been evaluated. A convenient scalable method for synthesis of pure Mg(BH4)2 by ball milling a mixture of MgCl2 and NaBH4 in diethyl ether has been developed. Crystalline stable low and high temperature phases, as well as a new metastable phase of unsolvated magnesium borohydride have been prepared.
  • The decomposition of crystalline magnesium borohydride upon heating was studied using thermal desorption, calorimetry, in situ X-ray diffraction, and solid state NMR. Hydrogen release from Mg(BH4)2 occurs in at least four steps via formation of several polyborane intermediate species and includes an exothermic reaction yielding crystalline MgH2 as an intermediate. The decomposition products may be only partially recharged after the very first step and also via hydrogenation of Mg metal. The intermediate formation of amorphous MgB12H12, was confirmed by 11B NMR. A four-stage pathway for the thermal decomposition of Mg(BH4)2 is proposed.
  • The ammonia complex of magnesium borohydride Mg(BH4)2{center_dot}2NH3 (I), which contains 16.0 wt % hydrogen, is a potentially promising material for hydrogen storage. This complex was synthesized by thermal decomposition of a hexaaammine complex Mg(BH4)2{center_dot}6NH3 (II), which crystallizes in the cubic space group Fmm with unit cell parameter a = 10.82(1) Angstroms and is isostructural to Mg(NH3)6Cl2. We solved the structure of I that crystallizes in the orthorhombic space group Pcab with unit cell parameters a = 17.4872(4) Angstroms, b = 9.4132(2) Angstroms, c = 8.7304(2) Angstroms, and Z = 8. This structure is built from individual pseudotetrahedral molecules Mg(BH4)2{center_dot}2NH3 containingmore » one bidentate BH4 group and one tridentate BH4 group that pack into a layered crystal structure mediated by NH{center_dot}{center_dot}{center_dot}HB dihydrogen bonds. Complex I decomposes endothermically starting at 150 C, with a maximum hydrogen release rate at 205 C, which makes it competitive with ammonia borane BH3NH3 as a hydrogen storage material.« less
  • Aryl alkynide ligands in organometallic samarium complexes can couple to form the [PhC=C=C=CPh]{sup 2-} trienediyl ligand in [Cp*{sub 2-} Sm]{sub 2}({mu}-{eta}{sup 2}:{eta}{sup 2}-PhC=C=C=CPh). The authors studied the generality of this set of reactions with other lanthanide centers, namely, cerium, and neodymium as well as with varied charge states of the lanthanide. Not all of the reactions occur, but for those that do, the products were characterized by NMR and X-ray crystallography.
  • We have used first-principles density functional theory to relax the experimentally reported crystal structures for the low- and high-temperature phases of Mg(BH4)2, which contain 330 and 704 atoms per unit cell, respectively. The relaxed low-temperature structure was found to belong to the P6122 space group, whereas the original experimental structure has P61 symmetry. The higher symmetry identified in our calculations may be the T = 0 ground-state structure or may be the actual room-temperature structure because it is difficult to distinguish between P61 and P6122 with the available powder diffraction data. We have identified several hypothetical structures for Mg(BH4)2 thatmore » have calculated total energies that are close to the low-temperature ground-state structure, including two structures that lie within 0.2 eV per formula unit of the ground-state structure. These alternate structures are all much simpler than the experimentally observed structure. We have used Bader charge analysis to compute the charge distribution in the P6122 Mg(BH4)2 structure and have compared this with charges in the much simpler Mg(AlH4)2 structure. We find that the B-H bonds are significantly more covalent than the Al-H bonds; this difference in bond character may contribute to the very different crystal structures for these two materials. Our calculated vibrational frequencies for the P6122 structure are in good agreement with experimental Raman spectra for the low-temperature Mg(BH4)2 structure. The calculated total energy of the high-temperature structure is only about 0.1 eV per formula unit higher in energy than the low-temperature structure.« less