posted on 2020-11-03, 18:07authored byKristina Spektor, Wilson A. Crichton, Stanislav Filippov, Sergei I. Simak, Andreas Fischer, Ulrich Häussermann
The
formation of ternary hydrogen-rich hydrides involving the first-row
transition metals TM = Fe and Co in high oxidation states is demonstrated
from in situ synchrotron diffraction studies of reaction mixtures
NaH–TM–H2 at p ≈ 10
GPa. Na3FeH7 and Na3CoH6 feature pentagonal bipyramidal FeH73– and octahedral CoH63– 18-electron complexes,
respectively. At high pressure, high temperature (300 < T ≤ 470 °C) conditions, metal atoms are arranged
as in the face-centered cubic Heusler structure, and ab initio molecular
dynamics simulations suggest that the complexes undergo reorientational
dynamics. Upon cooling, subtle changes in the diffraction patterns
evidence reversible and rapid phase transitions associated with ordering
of the complexes. During decompression, Na3FeH7 and Na3CoH6 transform to tetragonal and orthorhombic
low pressure forms, respectively, which can be retained at ambient
pressure. The discovery of Na3FeH7 and Na3CoH6 establishes a consecutive series of homoleptic
hydrogen-rich complexes for first-row transition metals from Cr to
Ni.