ANOMALOUS RELAXATIONS OF (0001) AND (1010) SURFACES IN HCP METALS

We have used local-volume potentials (LVP) to study the (0001) and (1010BAR) surfaces of 13 hcp metals. The calculated structures and energies of (0001) and (1010BAR) surfaces show some agreement with experiment, particularly as to the sign of the relaxation. All the metals show deep changes in interlayer spacings. On (0001) surfaces Co, Gd, Hf, Mg, Re, Ru, Ti and Tl show first layer-spacing contraction while Be, Dy, Er, Sc and Zr show anomalous first layer-spacing expansion. Similarly, on (1010BAR) surfaces Co, Dy, Gd, Mg, Re, Ru, Ti and Tl show first layer-spacing contraction while Be, Er, Hf, Sc and Zr show anomalous first layer-spacing expansions. The prediction of first layer-spacing expansion of the Be(0001) surface agrees with a recent low-energy electron diffraction (LEED) experiment. The results on hcp (0001) surfaces contrast with relaxation on fcc (111) surfaces, also reported here, where the model always give contractions.