Finding New Crystalline Phases by Electronic Theory and Electron Diffraction

Suny At Stony Brook, Stony Brook NY

Investigators

Abstract

This project concerns the study of phases of condensed matter systems and their stability and metastability with respect to applied stresses. The goals of the present project are: (1) to identify metastable phases by calculation; and (2) to stabilize them at room temperature in the laboratory. The theoretical search for new phases has become possible within the last few years by modern electronic-structure computer programs that can calculate total energies for any configuration of atoms in the unit cell of a crystal. Hence the programs can search for configurations that minimize the energy. Moreover, the calculations are made from first principles, i.e., require no empirically determined quantities, and are equally reliable for all configurations. The metastable phases thus found will be stabilized by coherent (pseudomorphic) epitaxy on suitable substrates. Each phase will be made as an ultrathin film with nanoscale thicknesses of perhaps 10 to 20 atomic layers. Usually the film will be strained away from its equilibrium state, since the much thicker substrate strains the surface mesh of the film to the surface mesh of the substrate. If ten or more layers can be grown, inner layers are strained bulk layers independent of surface effects, and are examples of the new metastable bulk phase. The crystal structure of these bulk layers will then be determined by quantitative low-energy electron diffraction and compared with the theoretical predictions. Probably there are many more metastable than stable phases, since even for pure elements all minima in a many-dimensional space are being sought. This research will be conducted with graduate students who will be trained for future employement in an active area of materials science %%% many materials the atoms are arranged in regular, periodic arrays which form a crystal. Crystals occur in nature with a particular atomic structure which is stable with respect to impressed forces. A given material may have different atomic structures, which have limited stability and which may have very different properties; such structures are said to be metastable. The word ``metastable'' means that if we only disturb it gently (for example, slightly deform it) the structure persists, but if we apply large enough deformations then the structure collapses. Some materials are found in nature both in a stable form (e.g., graphite) and in a metastable form (e.g, diamond), but most materials exhibit naturally only the stable form. Their metastable structures are mostly unknown, and, if known, can be fabricated only by special processes which stabilize them. Stabilization can be achieved, for example, by forcing the material to grow in the form of an ultra-thin film on top of another, stable, crystal (called the substrate). Metastable structures are far more numerous than stable ones, and their properties can be potentially very useful. The goals of the present project are: (1) to find metastable forms of several metals and alloys by calculation; and (2) to try to make and stabilize them experimentally in the laboratory. The former goal is made possible by modern electronic-structure computer programs, which can test any possible structure of a given material; the latter goal will be reached by growing nanoscale films with thicknesses of 10 to 20 atomic layers on suitable substrates in specially evacuated chambers. The structure of the films will be determined by well-tested electron-diffraction techniques and compared to the calculated structures. This combined experimental-theoretical project involves graduate and undergraduate students. They will receive training in an advanced area of contemporary materials science and are thereby prepared to enter scientific/technological workforce of the next few decades. ***