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

Abstract

High-pressure behavior of α-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 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 high-energy 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 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 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.

First-principles investigation of LaMg2Ni and its hydrides

“…For the full hydride LaMg 2 NiH 7 , the band gap is predicted to be 0.821 eV using GGA method. This value is close to the literature finding of 0.9 eV (GGA value) 16 , but is expected to be underestimated due to the strong on-site Coulomb interactions at d and f electronic states 41 – 43 . In general, a good agreement for band gap between theory and experiment can be obtained by adjusting the Hubbard U using GGA + U calculations 41 – 43 .…”

Inclusion of Neon into an Yttrium Borohydride Structure at Elevated Pressure – An Experimental and Theoretical Study

“…At first sight the value of bulk modulus (B 0 ) of α‐Y(BH 4 ) 3 is not unusual, as it is similar to those typically observed for borohydrides – contained within the limits from 10.9(4) GPa for α‐Mg(BH 4 ) 2 to 33.8(10) GPa for δ‐Mn(BH 4 ) 2 . [ 20,35 ] However, it is clear from Figure 4 that above ca. 2 GPa the calculated molecular volume of α‐Y(BH 4 ) 3 is markedly smaller than the experimental one, with the difference much beyond the typical error of ca.…”

“…[ 30–34 ] Interestingly, two borohydrides, Mg(BH 4 ) 2 and Mn(BH 4 ) 2 , reveal phase transition around 1 GPa (pressure range easily achievable in a hydraulic press, like laboratory pelletizer, or even during high‐energy milling) to the denser δ phases, which are metastable under ambient conditions. [ 20,35 ]…”

Complex hydrides for energy storage

“…g-and d-Mg(BH 4 ) 2 with, respectively, material densities of 0.55 and 0.99 g/mL and volumetric hydrogen densities of 82 and 147 g H 2 /L [21]. Manganese borohydride shows similar building principles and framework structures to Mg(BH 4 ) 2 , having slightly different stability ranges for porous and dense phases [23] as well as quite different accessibility of synthetic routes [24,25].…”