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

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 <i>I</i>4₁/<i>acd</i> with the cell parameters <i>a</i> = 7.85245(6), <i>c</i> = 12.1456(2) Å, and <i>V</i> = 748.91(1) ų at ambient conditions; it can also be described in a stable <i>P-</i>4<i>n</i>2 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 √2<i>a</i> × <i>c</i> supercell compared to the δ-phase and refined in the space group <i>Fddd</i> with <i>a</i> = 9.205(17), <i>b</i> = 9.321(10), <i>c</i> = 12.638(15) Å, and <i>V</i> = 1084(3) ų at 11.8 GPa. Equations of state were determined for α- and δ-Mn­(BH₄)<sub>2