Change from a bulk discontinuous phase transition in V₂H to a continuous transition in a defective near-surface skin layer

Using high- and low-energy x-ray scattering techniques, we have studied the effect of defects on the β₁-β₂ phase transition in V₂H. Since we earlier observed two length scales in this crystal, we focus here exclusively on either the pure bulk or the defective skin layer, which is several micrometres thick. While we found a strong and narrow discontinuous transition in the bulk, there is a continuous transition in this defective skin layer in which the mosaic spread decreases with depth. We suggest that the strain field associated with the change of the mosaic spread most likely prompts the conversion of the order of the phase transition.