Unraveling Hidden Mg-Mn-H Phase Relations at High Pressures and Temperatures by in Situ Synchrotron Diffraction

The Mg-Mn-H system was investigated by in situ high pressure studies of reaction mixtures MgH₂-Mn-H₂. The formation conditions of two complex hydrides with composition Mg₃MnH₇ were established. Previously known hexagonal Mg₃MnH₇ (h-Mg₃MnH₇) formed at pressures 1.5-2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg₃MnH₇) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn(I)H₆]^5- complexes and interstitial H^-. Interstitial H^- is located in trigonal bipyramidal and square pyramidal interstices formed by Mg²⁺ ions in h- and o-Mg₃MnH₇, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg₃MnH₇ (m-Mg₃MnH₇). The structure elucidation of o- and m-Mg₃MnH₇ was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg₃MnH₇ to be more stable than h-Mg₃MnH₇ above 4.3 GPa. Phonon calculations revealed o-Mg₃MnH₇ to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg₃MnH₇ on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg₃MnH₇ is very similar. The stable 18-electron complex [MnH₆]^5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of in situ investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides.