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

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 P 2 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.