Abstract
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.
Highlights
Expectations for high temperature superconductivity have stimulated intensive research efforts into binary metal−H systems at high pressures, which have resulted in the discovery of unprecedented hydrogen-rich hydrides.[1−4] These hydrides, called super- or polyhydrides, exhibit stunning compositions and hydrogen structures, which include clathrate-like cages,[5−7] oligomeric chain fragments,[8] and two-dimensional layers.[9]
High temperature superconductivity approaching room temperature has been predicted for alkaline earth, La, and Y hydrides with clathrate-like structures and was recently experimentally confirmed for LaH10.10
We describe in situ Large volume presses (LVPs) hydrogenation experiments, compiled in Figure 1, during which hydride formation and phase transitions at high pressure, high temperature conditions were examined in great detail
Summary
Expectations for high temperature superconductivity have stimulated intensive research efforts into binary metal−H systems at high pressures, which have resulted in the discovery of unprecedented hydrogen-rich hydrides.[1−4] These hydrides, called super- or polyhydrides, exhibit stunning compositions and hydrogen structures, which include clathrate-like cages (as in LaH10, CaH6),[5−7] oligomeric chain fragments (as in NaHn),[8] and two-dimensional layers (as in FeH5).[9]. It has been recently shown that the application of modest pressures of around 5 GPa can already afford new hydrogenrich complex transition metal hydrides, CTMHs, in which group 4−6 metals attain unusually high H coordination numbers. These new CTMHs comprise homoleptic complex cioonnsdulciktoers.C11r−H1745Y−,et,NthbeHy9a4−re, and Ti2H146− and are semirecoverable to ambient pressure, and doping with hole carriers has been suggested to achieve superconductivity.[14] Here, we demonstrate the accessibility of hydrogen-rich CTMHs in the Na−Fe−H and Na−Co−H systems by using pressures around 10 GPa and slightly elevated temperatures up to 470 °C. In situ studies of hydrogenations at pressures up to 12 GPa have been recently facilitated at the beamline ID06LVP, ESRF.[20,21]
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