Mg/Ti multilayers: structural, optical and hydrogen absorption properties

TL;DR

This study investigates the structural, optical, and hydrogen absorption properties of engineered Mg/Ti multilayers with Pd capping, revealing a two-step hydrogenation process, significant layer expansion, and hydrogen trapping in Ti layers, advancing understanding of their microstructural behavior.

Contribution

It introduces a method to engineer short-range order in Mg/Ti multilayers and characterizes their hydrogenation behavior, providing new insights into their structural and optical properties.

Findings

Good structural coherence despite lattice mismatch

Two-step hydrogenation process with Ti hydride formation first

Large out-of-plane expansion indicating plastic deformation

Abstract

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a "spinodal-like" microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming the hydride before Mg. From in-situ measurements of the bilayer thickness L at different hydrogen pressures, we observe large out-of-plane expansions of the Mg and Ti layers upon hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. Upon unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces.