High-Pressure Study of Mn(BH4)2 Reveals a Stable Polymorph with High Hydrogen Density

Nikolay A Tumanov, Elsa Roedern, Zbigniew Łodziana, Dorrit Bolund Nielsen, Torben René Jensen, Alexandr V. Talyzin, Radovan Cerny, Dmitry Chernyshov, Vladimir Dmitriev, Taras Palasyuk, Yaroslav Filinchuk

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Abstract

High-pressure behavior of α-Mn(BH 4) 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(BH 4) 2, forms near 1 GPa and is isostructural to the magnesium analogue δ-Mg(BH 4) 2. This polymorph is stable upon decompression to ambient conditions and can also be obtained by compression of α-Mn(BH 4) 2 in a large-volume steel press as well as by high-energy ball milling. It shows a high volumetric density of hydrogen of 125 g H 2/L at ambient conditions. δ-Mn(BH 4) 2 was refined in the space group I4 1/acd with the cell parameters a = 7.85245(6), c = 12.1456(2) Å, and V = 748.91(1) Å 3 at ambient conditions; it can also be described in a stable P-4n2 superstructure. Its thermal stability was studied by in situ X-ray powder diffraction and thermal analysis coupled with mass-spectroscopy. δ-Mn(BH 4) 2 transforms back to α-Mn(BH 4) 2 upon heating in the temperature range of 67-109 °C in Ar (1 bar) or H 2 (100 bar) atmosphere, and a decomposition is initiated at 130 °C with the release of hydrogen and some diborane. Mn(BH 4) 2 undergoes a second phase transition to δ′-Mn(BH 4) 2 in the pressure range of 8.6-11.8 GPa. δ′-phase is not isostructural to the second high-pressure phase of Mg(BH 4) 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, and V = 1084(3) Å 3 at 11.8 GPa. Equations of state were determined for α- and δ-Mn(BH 4) 2.

Original languageEnglish
JournalChemistry of Materials
Volume28
Issue1
Pages (from-to)274-283
Number of pages10
ISSN0897-4756
DOIs
Publication statusPublished - 2016

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