Phase transformation reconstructive

The tungsten (110) surface is one of the best studied of all surfaces, especially in field emission and field ion microscopy for many reasons. It is a very stable surface without surface reconstruction or phase transformation. It is also inert to contaminations. For the study of adatom-adatom interactions, it is a very smooth plane with the largest density of adsorption sites available of any W surface. Lesser restrictions are imposed on the adatom-adatom separation. As the surface is structurally very smooth, wave mechanical interference effects are least affected by the surface atomic structure. 

We then have a list of indices b, k, /, the associated intensities, and their standard deviations. All we now need are the phases to reconstruct the electron density by a Fourier transformation. 

As single-phase substances are heated or cooled, they can undergo a number of polymorphic transformations. Polymorphs are different crystalline modifications of the same chemical substance. These transformations are quite common and include crystallization of glasses, melting, and many solid-solid phase transformations, some of which are described below. In general, there are two types of polymorphic transformations, displacive and reconstructive. 

The phase transformation of SmCoC2 from the high-temperature orthorhombic form to the low-temperature monoclinic form can be better regarded as a reconstructive transformation as opposed to a displacive one because this orthorhombic modification can be obtained by quenching to room temperature, however, the mechanism of this phase transformation remains to be studied in detail. 

The classic example of a reconstructive phase transformation in ceramics is the transformation between the low and high forms of SiOi the distorted form of quartz structure is stable at the lower temperature. Twins are again often formed during reconstructive phase transformations when these lead to a decrease in symmetry since the change can often occur in symmetry-related ways the twins are then related by the lost symmetry element. 

The nature and cause of these surface phase transformations are not well established at present. The case of structural change from metastable to stable on adsorption or removal of adsorbates indicates the likelihood of electronic transitions that accompany reconstruction. At the surface there are fewer nearest neighbors as compared to atoms in the bulk. The electronic structure that is the most stable in this reduced-symmetry environment may be substantially different from that of the bulk metal. Since the surface atoms are surrounded by atoms only on one side and there is vacuum on the other, they may change their coordination number by slight relocation with simultaneous changes in the electronic structure. It is indeed surprising that more surfaces do not show reconstruction. 

In the past several years, we have already succeeded in demonstrating that the technique can be used to monitor molecular adsorption and desorption at a variety of interfaces, to probe the spectrum of submonolayers of molecules adsorbed on surfaces,3 to measure the orientation and distribution of adsorbed molecules,to study surface reconstruction and phase transformation of semiconductors,5 and so on. In this paper, we describe a few additional experiments that we have recently carried out in our laboratory to further explore the applicability of surface SHG. 

Figure 3.19 Summary of reconstructive and displacive phase transformations of silica at ambient pressure.

---