Achieving coherent phase transition in palladium–hydrogen thin films

The thermodynamics of structural phase transformations in thin films depends on the mechanical stress that can be released by plastic deformation. For thin films below a critical film thickness, plastic deformation is energetically unfavourable: thus, the system stays coherent and stress remains. For PdHc films less than 22 nm thick, a new situation emerges: while the interfaces between matrix and hydride precipitates remain coherent throughout the complete phase transition, misfit dislocations form between the hydride phase and the substrate.

► Stress evolving during phase transformation changes thermodynamic properties of thin films. ► We investigate stress relaxation mechanisms inPdHc thin films during hydrogen loading. ► Below 22 nm film thickness misfit dislocations emerge between hydride phase and substrate. ► Below 22 nm film thickness misfit dislocation loop formation around precipitates is not possible. ► This yields a new state of partial coherency in thin films.