Fe-Mg-Si (Iron-Magnesium-Silicon)

Metallurgical Synthesis of Mg2FexSi1–x Hydride: Destabilization of Mg2FeH6 Nanostructured in Templated Mg2Si

“…Mg is immiscible with iron (Fe) and miscible with silicon (Si). 22,23 While no stable Mg−Fe alloy phase is formed, the hydride phase of Mg 2 FeH 6 , which consists of Mg 2+ and [FeH 6 ] 4− ions, is well-known to form. 24−26 Mg 2 FeH 6 has a K 2 PtCl 6 -type cubic structure (Fm3̅ m), and the enthalpy for hydride formation is −98 kJ mol −1 H 2 , 26 which is more stable than that of MgH 2 (−75 kJ mol −1 H 2 ).…”

“…34 However, the Mg 2 Fe x Si 1−x ternary phase cannot be formed under the equilibrium conditions because of the immiscible Mg−Fe system. 22,23 In the present work, we tried to synthesize the hydride of Mg 2 Fe x Si 1−x in which Si is substituted by FeH 6 in a Mg cubic sublattice and vice versa. Trinh et al have synthesized Mg 2 Fe x Si 1−x -H by hydrogenation of Mg 2 Fe x Si 1−x thin films, and, indeed, conversion electron Mossbauer spectroscopy has indicated the formation of Mg 2 FeH 6 domains in the range of x ≥ 0.5.…”

“…Here we present nanostructured Mg-based transition-metal hydrides that exhibit tunable thermodynamics in the Mg–Fe–Si–H system. Mg is immiscible with iron (Fe) and miscible with silicon (Si). , While no stable Mg–Fe alloy phase is formed, the hydride phase of Mg 2 FeH 6 , which consists of Mg 2+ and [FeH 6 ] 4– ions, is well-known to form. − Mg 2 FeH 6 has a K 2 PtCl 6 -type cubic structure ( Fm 3̅ m ), and the enthalpy for hydride formation is −98 kJ mol –1 H 2 , which is more stable than that of MgH 2 (−75 kJ mol –1 H 2 ). − On the other hand, it has been reported that Mg 2 Si reacts with H, accompanying the phase segregation into MgH 2 and Si. − The enthalpy change for this reaction is −36.4 kJ mol –1 H 2 , but the required reaction temperature is still high, which is due to a kinetic limitation associated with phase segregation. Mg 2 Si has a CaF 2 -type cubic crystal structure ( Fm 3̅ m ).…”

“…Hence, Mg 2 FeH 6 and Mg 2 Si have the same space group, while the difference in the unit cell volume V is only 4.2% ( a = 6.442 Å and V = 267.3 Å 3 for Mg 2 FeH 6 and a = 6.351 Å and V = 256.2 Å 3 for Mg 2 Si) . However, the Mg 2 Fe x Si 1– x ternary phase cannot be formed under the equilibrium conditions because of the immiscible Mg–Fe system. , …”