First-principles modelling of magnesium titanium hydrides.

Area of Science:

• Materials Science
• Solid State Chemistry
• Computational Materials Science

Background:

• Magnesium hydride (MgH2) exhibits poor hydrogenation kinetics.
• Alloying Mg with Ti to form Mg(x)Ti((1-x))H(2) improves (de)hydrogenation properties.
• Mg(x)Ti((1-x))H(2) films display an unusual black appearance, unlike typical hydrides.

Purpose of the Study:

• To investigate the structure and stability of Mg(x)Ti((1-x))H(2) across the full composition range (x=0-1).
• To understand the correlation between phase transitions and the observed changes in (de)hydrogenation properties.
• To explain the origin of the black optical appearance of these hydride films.

Main Methods:

• First-principles calculations based on density functional theory (DFT).
• Analysis of structural phase transitions and their dependence on composition.
• Calculation of electronic density of states and optical absorption spectra.

Main Results:

• Evidence for a fluorite to rutile phase transition at a critical composition x(c) = 0.8-0.9.
• This phase transition correlates with a sharp decrease in (de)hydrogenation rates.
• The black appearance is attributed to interband transitions causing broad optical absorption, with Ti d-states contributing to a Fermi level peak, which is sensitive to atomic disorder.

Conclusions:

• The Mg-Ti-H system exhibits a composition-dependent phase transition impacting hydrogenation performance.
• The unique optical properties and black color arise from electronic structure modifications and disorder effects.
• DFT calculations provide crucial insights into the structure-property relationships in Mg(x)Ti((1-x))H(2).