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

Inorganic Chemistry 2018 Februrary 6
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 Mg3 MnH7 were established. Previously known hexagonal Mg3 MnH7 (h-Mg3 MnH7 ) formed at pressures 1.5-2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg3 MnH7 ) 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-Mg3 MnH7 , respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg3 MnH7 (m-Mg3 MnH7 ). The structure elucidation of o- and m-Mg3 MnH7 was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg3 MnH7 to be more stable than h-Mg3 MnH7 above 4.3 GPa. Phonon calculations revealed o-Mg3 MnH7 to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg3 MnH7 on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg3 MnH7 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.

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