High-Resolution Leed Study of Structural Disordering on Low-Index Nickel Surfaces.

High q Resolution Low Energy Electron Diffraction has been used to investigate the thermal disordering of low index Ni(110) and Ni(001) surfaces. For both surfaces, deviation from simple Debye-Waller theory of temperature dependence of diffraction intensity has been observed above 900K. It was found that the anharmonicity in the Ni-Ni potential was responsible for the anomalous decay of diffraction intensity between 900K to 1200K for both Ni(110) and Ni(001). The vibrations were found to be anisotropic between parallel and perpendicular to the surface plane in the (110) face and isotropic in the (001). Diffraction lineshape analysis shows that above 1200K, thermal activated steps form on the (110) surface and at about 1400K, the surface undergoes a well defined preroughening transition that can be described by a RSOS model. No step formation has been detected on the (001) surface up to 1400K. However, as a result of surface anharmonicity, the I-V profile indicates that an enhanced thermal expansion coefficient causes the top layer of the (001) surface to be lifted up from the substrate. Subsequently, at about 1300K, the (001) surface loses lateral order, which suggests the possibility of a surface melting.