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

Abstract

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.

Exploring the Mg–Cr–H System at High Pressure and Temperature via in Situ Synchrotron Diffraction

“…The sample preparation procedure followed closely the one described for the investigation of the Mg–Mn–H system . The samples were prepared in a glovebox under argon atmosphere using powdered MgH 2 (Sigma-Aldrich, 97%) and powdered Cr metal (Chempur, 99.99%).…”

“…Little is known about the actual conditions and processes of Mg 3 CrH 8 formation, possibly involving intermediate phases and/or polymorphism. For Mg 3 MnH 7 rather complex phase relations at high pressure and temperature conditions were recently revealed from in situ X-ray diffraction studies . Generally, this method provides insight into pressure–temperature phase diagrams and reaction pathways and allows the determination of optimum synthesis conditions and recovery strategies for high pressure materials.…”

Formation of Lightweight Ternary Polyhydrides and Their Hydrogen Storage Mechanism

“…Discovery of metal polyhydride groups under high pressure has widely attracted reader’s attention because it has novel features and a wide range of applications. − The high-pressure effect on a multicomposition material provides exceptional conditions for structural formation of new compositions without the Octet rule. − Attractive properties of metal polyhydrides are mainly related to their high-temperature superconductivity − and application as a hydrogen storage material for mobile application. − For alkali-hydride compounds, monohydride was well-known as the MH form for all alkali metals (M) in the I-A group. Later, polyhydrides of alkali metals were also observed and proposed such as the polyhydrides of Na and Li. − Struzhkin et al synthesized Na-polyhydrides under high-pressure conditions, and they have reported the first observation of the polyhydrides of Na (NaH 3 and NaH 7 ) above 40GPa .…”

Na3FeH7 and Na3CoH6: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis

“… 19 In situ studies of hydrogenations at pressures up to 12 GPa have been recently facilitated at the beamline ID06-LVP, ESRF. 20 , 21 …”

“…Ammonia borane, BH 3 NH 3 , which decomposes neatly into inert BN and H 2 at comparatively low temperatures (200–300 °C), has been proven an ideal internal H source . In situ studies of hydrogenations at pressures up to 12 GPa have been recently facilitated at the beamline ID06-LVP, ESRF. , …”