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

The complex transition metal hydride Mg₃CrH₈ has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H₇]^5-. Here, we investigated the formation of Mg₃CrH₈ by in situ studies of reaction mixtures of 3MgH₂-Cr-H₂ at 5 GPa. The formation of the known orthorhombic form (o-Mg₃CrH₈) 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-Mg₃CrH₈. 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-Mg₃CrH₈) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-Mg₃CrH₈ was found to crystallize in P2₁/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-Mg₃CrH₈ contains pentagonal bipyramidal complexes [CrH₇]^5- and interstitial H^-. The arrangement of metal atoms and interstitial H^- resembles closely that of the high pressure orthorhombic form of Mg₃MnH₇. 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-Mg₃CrH₈ being more stable than m-Mg₃CrH₈ by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-Mg₃CrH₈ is very similar to that of o-Mg₃CrH₈. The stable 18-electron complex [CrH₇]^5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.