| Popis: |
Electron-diffraction patterns afforded by thin films of face-centred cubic metals prepared by evaporation, electrodeposition, chemical deposition and chemical displacement often show extra rings and bands which may be regarded as typical of the face-centred cubic structure rather than of the metal itself. It is shown that the extra rings are due to the fact that the first deposited layers crystallize in close-packed hexagonal crystals having an axial ratio c/a equal to 1.63, the effective atomic radius being the same as in the normal face-centred cubic form. Further, a gradual transition from close-packed hexagonal to face-centred cubic is postulated, and the prominent band is attributed to the progressive contraction of the (100) hexagonal spacing to the (200) cubic spacing during this transition. Certain very thin evaporated films yield structures intermediate between the hexagonal and cubic modifications, and from this it is inferred that the transition occurs gradually as deposition proceeds, and is not due to an atomic rearrangement occuring when a certain critical film-thickness is reached. These views are supported by the discovery of (i) an electrodeposited silver film consisting solely of close-packed hexagonal crystals, (ii) a similar gold specimen yielding a pattern in which the hexagonal and cubic diffractions are equally pronounced, and (iii) evaporated cobalt films consisting mainly of the hexagonal modification, together with a certain amount of the face-centred cubic form. |
