In-situ STM and XRD studies on Nb–H films: Coherent and incoherent phase transitions

• Hydride formation found in 25 nm Nb–H films.
• Critical film thickness for coherent-to-incoherent phase transition confirmed.
• Size and spatial distribution of hydrides controlled by the coherency state.
• Invisibility of small coherent hydrides in XRD pattern.

Hydride precipitation in 25 nm and 40 nm epitaxial Nb-films was studied by Scanning Tunnelling Microscopy (STM) supported by X-ray diffraction (XRD) measurements. In combination, these methods yield information about the phase transition, the coherency state, the hydride precipitates’ density and size as well as their lateral distribution, at 293 K. For both film thicknesses, hydride formation was detected with STM; it can be easily missed by XRD. While the 25 nm film showed a coherent phase transition, the phase transition was incoherent for the 40 nm film. This is in good accordance with theory. The phase transition features are found to strongly depend on the coherency state: a large number of small hydrides appear in the coherent regime while a small number of large hydrides evolve in the incoherent regime.