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High-pressure study of Mn(BH4)2 reveals a stable polymorph with high hydrogen density

DOI: 10.1021/acs.chemmater.5b04102 DOI Help

Authors: Nikolay A. Tumanov (Université Catholique de Louvain) , Elsa Roedern (Aarhus University) , Zbigniew Łodziana , Dorrit B. Nielsen (University of Aarhus) , Torben R. Jensen (University of Aarhus) , Alexandr V. Talyzin (Umeå University) , Radovan Cerny (University of Geneva) , Dmitry Chernyshov (ESRF) , Vladimir Dmitriev (ESRF) , Taras Palasyuk (ESRF) , Yaroslav Filinchuk (Institute of Condensed Matter and Nanosciences, Université catholique de Louvain)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry of Materials , VOL 28 (1)

State: Published (Approved)
Published: November 2015

Abstract: High-pressure behavior of Mn(BH4)2 was studied up to 29.4 GPa in diamond anvil cells, using powder X-ray diffraction combined with DFT calculations and Raman spectroscopy, and two new polymorphs were discovered. The first polymorph, δ-Mn(BH4)2, forms near 1 GPa and is isostructural to the magnesium analog, δ-Mg(BH4)2. This polymorph is stable upon decompression to ambient conditions and can also be obtained by compression of α-Mn(BH4)2 in a large-volume steel press, as well as by high-energy ball milling. It shows one of the highest volumetric densities of hydrogen of 125 g H2/L at ambient conditions. δ-Mn(BH4)2 was refined in the space group I41/acd with the cell parameters a = 7.85245(6), c = 12.1456(2) Å, V = 748.91(1) Å3 at ambient conditions, it can also be described in a stable P-4n2 superstructure. Its thermal stability was studied by in situ powder X-ray diffraction and thermal analysis coupled with mass-spectroscopy. δ-Mn(BH4)2 transforms back to α-Mn(BH4)2 upon heating in the temperature range of 67-109 °C in Ar (1 bar) or H2 (100 bar) atmosphere and a decomposition is initiated at 130 °C, with release of hydrogen and some diborane. Mn(BH4)2 undergoes a second phase transition to δ’-Mn(BH4)2 in the pressure range of 8.6-11.8 GPa. δ’-phase is not isostructural to the second high-pressure phase of Mg(BH4)2, and its structure was determined in the √2a × c supercell compared to the δ-phase, and refined in the space group Fddd with a = 9.205(17), b = 9.321(10), c = 12.638(15) Å, V = 1084(3) Å3 at 11.8 GPa. Equations of state were determined for α- and δ-Mn(BH4)2.

Subject Areas: Chemistry

Instruments: I11-High Resolution Powder Diffraction

Other Facilities: NSRRC ESRF MAX-II synchrotron

Added On: 05/12/2015 19:10

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