Surface Reconstruction Phase Transformations

Abstract

Clean single-crystal (metal) surfaces usually possess well-defined two-dimensional (2D) periodicities closely resembling the atomic ordering in the bulk. In some instances, however, the surfacelattice reconstructs into a phase with new symmetry. This can occur spontaneously with temperature, or it can be induced by small coverages of adsorbed atoms or molecules. The phase transformation may be reversible or irreversible. For example, Au (110) exhibits an irreversible reconstruction into a (2×1) superstructure once the clean surface has been prepared. This surface then undergoes a reversible order-disorder phase transition when the temperature is raised above a critical value, T_c = 700 [K]. Similarly, a carefully cleaned W (100) surface reconstructs into a (√2 × √2) lattice¹ which also disorders reversibly at T_c = 300 [K]. A small quantity of chemisorbed hydrogen switches the symmetry of the atomic displacements from domains with p2mg space group symmetry to domains with c2mm symmetry, while maintaining a (√2 × √2) translational periodicity [2]. Increasing the amount of hydrogen produces a commensurate to incommensurate phase transformation which is reversible with coverage [3]. Clean Mo(100) reconstructs into a surface lattice which is incommensurate with the bulk substrate structure over a wide temperature range below 300 [K] [4]. Si (111) forms a (2×1) reconstructed lattice which transforms upon heating irreversibly into a (7×7) superlattice [5]. Surprisingly, the (7×7) structure exhibits a reversible order-disorder phase transition.