Vacancy diffusion in palladium hydrides.

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Physics

Background:

• Palladium self-diffusion is crucial for understanding hydrogen behavior in palladium.
• Previous studies have explored palladium-hydrogen interactions, but detailed kinetic parameters for diffusion are less understood.

Purpose of the Study:

• To investigate the self-diffusion coefficients of palladium in palladium hydride (PdHx) across various hydrogen concentrations.
• To determine the thermodynamic and kinetic parameters governing palladium diffusion in PdHx using first-principles calculations.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model PdHx systems.
• Thermodynamic parameters (enthalpy of formation and migration) and kinetic parameters (Arrhenius pre-exponential factor) were computed.

Main Results:

• Enthalpy of migration decreased from 0.95 eV in Pd to 0.78 eV in PdH.
• Enthalpy of vacancy formation significantly dropped from ~1.1 eV in Pd to 0.4 eV in PdH due to filling of antibonding states.
• Arrhenius pre-exponential factor reduced substantially from 4.75 × 10⁻³ cm² s⁻¹ in Pd to 5.67 × 10⁻⁹ cm² s⁻¹ in PdH.
• Observed enthalpy-entropy compensation, consistent with the Meyer-Neldel rule.

Conclusions:

• Hydrogen significantly alters palladium self-diffusion by weakening Pd-Pd bonds and softening vibrational modes.
• Pd self-diffusion coefficients in PdHx remain comparable to pure palladium above 200 °C under relevant hydrogen pressures.
• The findings provide fundamental insights into diffusion mechanisms in metal hydrides.