Spektor Kristina, Crichton Wilson A, Filippov Stanislav, Simak Sergei I, Häussermann Ulrich
ESRF , The European Synchrotron Radiation Facility , F-38000 Grenoble , France.
Theoretical Physics Division, Department of Physics , Chemistry and Biology (IFM) Linköping University , SE-581 83 Linköping , Sweden.
Inorg Chem. 2019 Aug 19;58(16):11043-11050. doi: 10.1021/acs.inorgchem.9b01569. Epub 2019 Jul 31.
The complex transition metal hydride MgCrH has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H]. Here, we investigated the formation of MgCrH by in situ studies of reaction mixtures of 3MgH-Cr-H at 5 GPa. The formation of the known orthorhombic form (o-MgCrH) 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-MgCrH. 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-MgCrH) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-MgCrH was found to crystallize in 2/ space group ( = 5.128 Å, = 16.482 Å, = 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-MgCrH contains pentagonal bipyramidal complexes [CrH] and interstitial H. The arrangement of metal atoms and interstitial H resembles closely that of the high pressure orthorhombic form of MgMnH. 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-MgCrH being more stable than m-MgCrH by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-MgCrH is very similar to that of o-MgCrH. The stable 18-electron complex [CrH] is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.
复杂过渡金属氢化物MgCrH此前是在高压条件下合成的。它包含首个第6族纯氢化物配合物[Cr(II)H]。在此，我们通过对5 GPa下3MgH - Cr - H反应混合物的原位研究来探究MgCrH的形成。在635 °C以上的温度下，尽管速率相对较慢，但已观察到已知正交晶型（o - MgCrH）的形成。在约750 °C的温度下，一种高温相迅速形成，缓慢冷却后转变为o - MgCrH。高压下的相变在加热时约735 °C可逆发生，在缓慢冷却时约675 °C可逆发生。快速冷却时，得到一种单斜多晶型物（m - MgCrH），随后可在常压下回收并分析。发现m - MgCrH在2/空间群中结晶（ = 5.128 Å， = 16.482 Å， = 4.805 Å，β = 90.27°）。高分辨率同步加速器粉末衍射数据对其结构的解析借助了第一性原理密度泛函理论计算。与正交多晶型物一样，m - MgCrH包含五角双锥配合物[CrH]和间隙H。金属原子和间隙H的排列与MgMnH的高压正交晶型极为相似。这表明在Mg - Cr - H和Mg - Mn - H体系中，高压和高温条件下氢化物配合物的形成和稳定原理相似。计算得出的焓与压力关系预测，在常压下o - MgCrH比m - MgCrH稳定6.5 kJ/mol，在5 GPa下稳定13 kJ/mol。m - MgCrH的电子结构与o - MgCrH非常相似。稳定的18电子配合物[CrH]反映在占据态中，计算得出的带隙约为1.5 eV。
