Structural Phase Transitions on W and Mo Surfaces

Abstract

In chemisorption systems, a rich variety of surface structures has been found as a function of temperature and adsorbate coverage. The detailed investigation of these surfaces phases and the transitions between them has been the subject of many recent studies both experimentally and theoretically. In most chemisorption systems,the occurrence of different phases can be understood, at least qualitatively, with a lattice gas model, in which the adsorption is site specific and the ordered phases are due to the lateral adsorbate-adsorbate interaction [1]. However, recent LEED studies [2] first indicated that in the case of W(001) and Mo(001) even the clean surfaces undergo a reversible structural phase transition as the temperature is lowered. The transition is from the (1×1) structure to c(2×2) structure on W(001) and an incommensurate structure in the case of Mo(001). The role of the adsorbate at low coverage is to enhance or suppress the inherent instability of the surface towards reconstruction as well as to alter the relative stability of different structures which are close in energy [3]. These LEED findings were corroborated subsequently by many other types of experiment, such as high energy ion scattering [4], electron energy-loss spectroscopy [5] and electron stimulated desorption [6]. By now, the existence of structural phase transitions for clean W (001) and Mo(OOl) surfaces is clearly established. These phase transitions are of a different nature from those driven by adsorbate-adsorbate interactions. In the subsequent sections we shall discuss the nature of these new phases and current theoretical models under study for understanding these surface structural phase transitions.