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

Abstract

The complex transition metal hydride Mg 3 CrH 8 has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H 7 ] 5− . Here, we investigated the formation of Mg 3 CrH 8 by in situ studies of reaction mixtures of 3MgH 2 −Cr−H 2 at 5 GPa. The formation of the known orthorhombic form (o-Mg 3 CrH 8 ) was noticed at temperatures above 635 °C, albeit at a relatively slow rate. At temperatures around 750 °C a high temperature phase formed rapidly, which upon slow cooling converted into o-Mg 3 CrH 8 . The phase transition at high pressures occurred reversibly at ∼735 °C upon heating and at ∼675 °C upon slow cooling. Upon rapid cooling, a monoclinic polymorph (m-Mg 3 CrH 8 ) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-Mg 3 CrH 8 was found to crystallize in P2 1 /n space group (a = 5.128 Å, b = 16.482 Å, c = 4.805 Å, β = 90.27°). Its structure elucidation from high resolution synchrotron powder diffraction data was aided by first-principles DFT calculations. Like the orthorhombic polymorph, m-Mg 3 CrH 8 contains pentagonal bipyramidal complexes [CrH 7 ] 5− and interstitial H − . The arrangement of metal atoms and interstitial H − resembles closely that of the high pressure orthorhombic form of Mg 3 MnH 7 . This suggests similar principles of formation and stabilization of hydrido complexes at high pressure and temperature conditions in the Mg−Cr−H and Mg−Mn−H systems. Calculated enthalpy versus pressure relations predict o-Mg 3 CrH 8 being more stable than m-Mg 3 CrH 8 by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-Mg 3 CrH 8 is very similar to that of o-Mg 3 CrH 8 . The stable 18-electron complex [CrH 7 ] 5− is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.

Syntheses of Novel Hydrides Containing Light Elements under High Pressure and High Temperature

“…Novel hydrides containing light-weight elements, such as lithium and magnesium, have been synthesized via the technique. 21,[25][26][27][28][29][30][31][32][33] A number of reviews of high-pressure hydrogenation studies are also available. 10,13,34,35)…”

Hydrogen storage in complex hydrides: past activities and new trends

“…3− have been synthesized recently [322][323][324][325][326][327][328][329] using a high-temperature and high-pressure technique, as developed by Fukai et al [330].…”

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 .…”