Structural changes to epitaxial (0001) holmium layers during hydrogen loading

HoHy exhibits dramatic changes in both structural and optical properties as y is varied from zero to three by hydrogen loading. This work reports on t he effect of such loading upon epitaxial single-crystal Ho films grown by m olecular beam epitaxy on (110) Nb parallel to(11(2) over bar 0) Al2O3 subst rates. Upon loading, the film undergoes a structural transition from hcp me tal (alpha-phase, H in solid solution) to the fee dihydride (beta-phase). T here is a further transformation between the dihydride phase and the hexago nal trihydride (gamma-phase). Three films of HoHy, where, nominally, y = 0, 2, 3, were studied by XRD, AFM, UFM, SEM and TEM. Triangular networks of f eatures of width similar to 300 nm and height similar to 20-30 nm that alig n with the [0(1) over bar 1], [(1) over bar 01] and [1(1) over bar 0] direc tions of the dihydride sample are seen on the surface of both the dihydride and the trihydride samples, but have a much greater density on the surface of the latter Such features are not present on the as-grown metal layer. I n situ controlled-environment transmission electron microscopy (CETEM) stud ied the effects of hydrogen loading within the dihydride phase, in cross se ctional geometry. During loading, slip within the dihydride phase was obser ved. The hypothesis that the triangular features are due to the precipitati on of the di- and trihydride compounds, with expanded lattices, on the slip planes within the alpha- or beta-phase host crystal is discussed.