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 MgH-Mn-H. The formation conditions of two complex hydrides with composition MgMnH were established. Previously known hexagonal MgMnH (h-MgMnH) formed at pressures 1.5-2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-MgMnH) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn(I)H] complexes and interstitial H. Interstitial H is located in trigonal bipyramidal and square pyramidal interstices formed by Mg ions in h- and o-MgMnH, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic MgMnH (m-MgMnH). The structure elucidation of o- and m-MgMnH was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-MgMnH to be more stable than h-MgMnH above 4.3 GPa. Phonon calculations revealed o-MgMnH to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-MgMnH on decompression. The electronic structure of the quenchable polymorphs h- and m-MgMnH is very similar. The stable 18-electron complex [MnH] 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.