The structure and dynamics of magnesium surfaces

Author

Ismail

Date of Award

8-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

E. Ward Plummer

Committee Members

T. A. Callcott, C. H, Feigerle

Abstract

Low-energy electron diffraction (LEED) 1-V and momentum resolved inelasticelectron scattering have been utilized to study the structure and dynamics of the Mg(0001)and Mg(1010) surfaces. Magnesium was chosen since it is a free-electron-like metal,which is an ideal test for first principles calculations.No reconstruction was found on the Mg(0001) and Mg(1010) surfaces fromtemperatures 120 K to 400 K. For the Mg(0001) surface, the first interlayer spacing isexpanded by +1.96 % at T=130 K, +2.34 % at T=300 K, and +2.48 % at T=400 K. Thethermal expansion of this surface is almost the same as in the bulk Mg. The temperaturedependent measurements of the atomic structure of the Mg(1010) surface revealed that theinterlayer spacings d_i,i+j, exhibited a remarkable oscillatory thermal expansion. As the temperature increases, the short interplanar spacings d_1,2’ and d_3,4’ and d_5,6 contract, while the long interplanar spacings d_2,3’, d_4,5’ and d_6,7 expand. The surface thermal expansion for the first short interlayer spacing is -7 x α_B’, where ~ 25x10^-6 K^-1 (bulk thermal expansion);while the first long interlayer spacing d₂₃ exhibits a large thermal expansion +5 x α_B. The Second short interlayer spacing displays a negative thermal expansion of -10 x α_B, the second long interlayer d_{45 shows a positive thermal expansion of 2 x αB’, the third shortinterlayer dB’ shows a negative thermal expansion (-1 x αB ), and the third long interlayer spacing d67displays a positive thermal expansion (+3 x α). The net thermal expansion of the whole surface region, defined as the spacing between plane 1 and 7 is about the same as for the bulk. This is the first time such deep oscillatory interplanar thermal expansion has been observed at a surface. First-principles density-functional calculations indicate that theunusual oscillatory thermal relaxation is driven by the tendency of the short interlayer orbilayer to collapse into a single layer. For both surfaces, when the temperature dependent measurements are extrapolated to T=0 K, they are in excellent agreement with the T=0 K calculations.For Mg(0001), there are three layers involved in the surface dynamics; while forMg(1010), the surface dynamics involve up to six layers (deep layers). The number of players involved in the surface dynamics is strongly related to the interlayer spacing.Qualitatively, the bulk force constant model is able to reproduce the measured Rayleigh Wave dispersion along the zone boundary K-M for the Mg(0001) surface, which is similar to the case of the Al(111) surface, but contradicted to the case of the Be(0001)surface. In general, the surface lattice dynamics of Mg(0001) are similar to those ofBe(0001) and Al(111), which are consistent with their electronic properties. Nonetheless,the nature of the surface force constants for an open surface metal, i.e., the Mg(1010)surface, is significantly different than in the bulk.No enhanced surface anharmonicity at the Mg(0001) and Mg(1010) surfaces was observed. For the Mg(1010) surface, the enhanced surface anharmonicity could be in the sublayers, which is difficult to be detected experimentally. The vibrational modes localized in the sublayers could be important to explain the oscillatory thermal expansion at theMg(1010) surface.}