Structural study of the submonolayer phases of potassium overlayers on Ni(100)

Low-energy electron-diffraction measurements from potassium overlayers on Ni(100) indicate that there are several submonolayer phases for K/Ni(100) below room temperature. Two commensurate solid phases are formed: a c(4×2) at coverage =0.25 and a ${(}_1^2$ ${}_{5/3}{}^0{}_{}{}^{}$) phase at a coverage of 0.30. In the coverage range 0.25–0.32, the overlayer undergoes a uniaxial compression, with the potassium adatoms remaining commensurate in the Ni [110] direction and the contraction taking place in the [11¯0] direction. This produces a uniaxially incommensurate phase except at coverages of 0.25 and 0.30. Comparison of diffracted intensities from this phase and the results of a ground-state calculation indicates that the lateral energy variation due to the substrate in this phase is about 0.10 eV. At higher coverages, the overlayer becomes hexagonal and rotates away from alignment with the Ni [110] direction as the overlayer lattice parameter decreases. The results are compared with the predictions of various models of rotational epitaxy and they are found to be most consistent with the Novaco-McTague model. The disordering transitions of the uniaxial and rotated phases are studied and a submonolayer phase diagram is presented for this system.