Pressure-induced Hydrogen-hydrogen Interaction in Metal Hydrides
Bayerisches Geoinstitut, Universität Bayreuth
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Metal hydrides have become materials of great interest in light of their potential as hydrogen storage and high-temperature superconducting materials. Superconductivity in LaH10 observed at 250 K, i.e. approaching room temperature, at high pressure  has led to a gold rush on hydride exploration in high-pressure solid-state physics. However, metal hydrides remain enigmatic due to the highly variable metal-hydrogen bond as neither bulk measurements on transport properties are available at these high-pressure conditions nor is it possible to perform typical structure refinement using X-ray diffraction due to the small X-ray cross-section of hydrogen. Therefore, direct information on the role of hydrogen in these processes is so far very limited.
In combined experimental and computational work, a group at Bayerisches Geoinstitut, University of Bayreuth, has characterized the role of hydrogen in two hydride systems: iron hydride FeH  and copper hydride Cu2H . Experiments using high-pressure nuclear magnetic resonance (NMR) spectroscopy have revealed a distinct deviation from the expected ideal metallic behavior as these hydrides are compressed, suggesting pressure-induced hydrogen-hydrogen interactions. Complementary atomistic calculations reveal that significant changes in the electronic structure occur, namely, the electronic state of hydrogen in the hydrides starts contributing to their metallicity. Further detailed investigation of the three-dimensional characteristics of the electron distribution shows that this onset of hydrogen contribution to metallic electron transport is accompanied by the formation of a connected network of increased electron localization between the hydrogens in the crystalline structure (shown in Figure 1 for FeH). While such networks have been predicted previously to contribute to metallicity in general, and superconductivity in particular , the hydrogen-hydrogen distance for this to occur has always been inferred to be significantly shorter than the one observed here.
Furthermore, the NMR data provides indirect access to dynamic properties, in particular diffusion of the hydrogen cores (protons) through the lattice, a connection that is established at ambient pressure, but has not been explored previously at high-pressure conditions. Comparison of the NMR evidence with proton diffusion rates computed from molecular dynamics simulations confirms that the experimental method can be transferred to high pressure with confidence, and that hydrogen in FeH and Cu2H diffuses at a rate unusually high for solids; the high mobility of hydrogens coincides with formation of the connected hydrogen network.
The electronic structure and molecular dynamics simulations based on the density functional theory implemented in the open-source code Quantum ESPRESSO  were performed on SuperMUC-NG due to the high level of precision required in the study. For the calculation of the charge density and therefore the electron localization function displayed in the above shown in Figure 1, spatially highly resolved calculations were required. The molecular dynamics simulations in a system with large differences in atomic mass between the metal and hydrogen atoms – and therefore different dynamic scales – required very fine time steps. As the crystalline structure of the hydrides can not be uniquely identified in X-ray diffraction experiments due to the inaccessible information on hydrogen positions due the low scattering factor of hydrogen, structure prediction simulations were performed to establish the crystalline structure and chemical composition of the hydrides. This was particularly important for the Cu-H system, in which the Cu2H phase explored here had previously not been identified and characterized.
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Leonid Dubrovinsky, Thomas Meier, Gerd Steinle-Neumann (PI), Florian Trybel
Bayerisches Geoinstitut, Universität Bayreuth
Dr. G. Steinle-Neumann
D- 95440 Bayreuth (Germany)
e-mail: g.steinle-neumann [@] uni-bayreuth.de
Local project ID: pn34wi