Site energy distribution and defect phase diagrams of metal-hydrogen systems determined by controlled hydrogen charging and thermodynamic modelling

• Applicant: Dr. Stefan Wagner
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 585400508

Modern materials science and development have a natural focus on the microstructure, since the holism of phases and crystal defects and its interaction with solute atoms determine the fundamental properties of materials. Hence, to describe the state of materials, recently defect phases as the complexes of defects and solute atoms have been introduced into the thermodynamic description of materials. Thereby, the stability ranges of defect phases at given chemical potential are determined by the spectrum of solute-defect interaction energies, that usually have to be approximated by advanced solid state physics and quantum mechanical methods. Yet, to this day they are challenging to be addressed experimentally. Different from most alloys except of intercalating systems, metal-hydrogen systems allow to continuously modify the concentration of solute atoms, in this case the highly mobile hydrogen, by controlled hydrogen charging. This makes them ideal model alloys to tackle defect phase diagrams experimentally. When the respective chemical potential of hydrogen is measured in-situ at each global hydrogen concentration, and if reasonable assumptions for the kinds of contributing defects and for the shape of their site energy spectra are made, this enables to reconstruct the defect density, the local hydrogen concentration, and the site energy distribution of the material from the measured isotherms. From this, the defect phase diagram follows by integration of the chemical potential. In this project proposal we choose the Vanadium-Hydrogen (V-H) system as a model to experimentally determine site energy spectra and to construct defect phase diagrams, and hence to describe the thermodynamic state of the system by the combination of chemical potential measurements and thermodynamic modeling. The V-H system is an important model system for hydrogen storage materials with bcc structure and large H solubilities both in the solid solution and in the hydride phase. Thereby, proper experimental preparation of the samples allows to vary the defect structure of the Vanadium and to consider various physical constraint conditions, that alter the thermodynamic stability of defect phases. This enables us to study defect phase diagrams for different states of the V-H system. Hence, the proposed project can complement otherwise complex theoretical approaches to determine defect phase diagrams by measurements and thermodynamic reasoning, offering an experimental tool to describe the state of materials with solute atoms under varying constraint conditions.

• DFG Programme: Research Grants