High-pressure study of Mn(BH4)2 reveals a stable polymorph with high hydrogen density
(2016) Chemistry of Materials — Vol. 28, n° 1, p. 274-283 (2016)
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- High-pressure behavior of α-Mn(BH4)2 was studied up to 29.4 GPa in diamond anvil cells using powder Xray 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 analogue 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 highenergy ball milling. It shows a high volumetric density 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, and 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 X-ray powder diffraction and thermal analysis coupled with massspectroscopy. 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 the 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, and V = 1084(3) Å3 at 11.8 GPa. Equations of state were determined for α- and δ-Mn(BH4)2.
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Tumanov, N., Roedern, E., Lodziana, Z., Nielsen, D., Jensen, T. R., Talyzin, A., Cerny, R., Chernyshov, D., Dimitriev, V., Palasyuk, T., & Filinchuk, Y. (2016). High-pressure study of Mn(BH4)2 reveals a stable polymorph with high hydrogen density. Chemistry of Materials, 28(1), 274-283. https://doi.org/10.1021/acs.chemmater.5b04102 (Original work published 2016)