A mechanism of inhibition of phase transitions in nano-grained close-packed Pd thin films

Analyzing experimental data and calculating corresponding energy barriers from first principles, we elucidate the feasibility of the experimentally observed phase transformation between the hcp and double hcp (dhcp) structures in the ( 11 2 ̄ 0 ) oriented hcp Pd thin films grown on W(001) and Nb(001) substrates and absence of the hcp–fcc transformation in those films. The hcp–dhcp transformation can be modeled by a transformation path which preserves the existing domain topology of the films and exhibits a sufficiently low energy barrier. On the other hand, this orthogonal pattern of rectangular domains induced by the four-fold symmetry of the substrate surface hinders the hcp Pd phase to convert back to the ground-state fcc phase, although there exists a transformation path exhibiting a very low energy barrier between the hcp and fcc structures. This path, however, would break the domain arrangement and, therefore, it cannot be accomplished. In this way, the hcp crystalline phase is locked inside of nanograins. The present case study constitutes an example how a higher-energy configuration can be stabilized by the topology of defects (here grain architecture/grain boundaries) and suggests a route for technological applications to prevent the destabilization of certain desirable properties (e.g. possible ferromagnetism) induced in the higher-energy (hcp Pd) phase.