Structure and Electronic Anomalies of Amorphous Chalcogenides

University Of Houston, Houston TX

Investigators

Abstract

Vassiliy Lubchenko of the University of Houston is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry division to develop a theoretical and computational approach to studying an important class of inorganic solids, called the "chalcogenides," that contain elements from groups XVI (sulfur, selenium, tellurium), XV (phosphorus, arsenic, antimony, bismuth), and XIV (germanium). The ability of these solids to switch readily between ordered crystalline and amorphous ("glassy") forms allows one to tune the optical and electronic properties of these materials in an efficient way. This property makes the chalcogenides prime candidates for applications in optical drives, next generation computer memory and displays, smart optics, and novel computer architectures Amorphous chalcogenides are a special type of disordered solids called "glasses," which are ordinarily made by rapidly cooling a liquid, usually below its melting point One can convert a chalcogenide alloy between the glass and ordered crystalline form by first melting the material and, then, cooling the melt at a specified rate. Slow cooling yields the crystal, fast cooling yields the glass. The present study is establishing detailed connections between the molecular motions underlying the glass transition and quantum-chemical interactions in the chalcogenide alloys. Direct molecular modeling of the glass transition is difficult and is part of the greater challenge of predicting the structure of complex inorganic solids and designing materials with tailored properties. The study is exploiting the similarity of the interplay between distinct chemical interactions in the chalcogenides and the behavior of a well-defined model of mathematical physics to make the problem tractable. At the center of the broader impact activities is training of graduate, undergraduate, and, in particular, high school students. A strategy is in place to ensure the participating high school and undergraduate students complete meaningful subprojects that will contribute to peer-reviewed publications. The microscopic hypothesis of the proposed work is that the glass-forming ability of the chalcogenide alloys stems from a competition between distinct types of chemical bonding: covalent, multicenter, and secondary. The resulting complex interplay of interactions is captured by a classic model of statistical physics, viz., the 64-vertex model, which is being implemented. The most important outputs of the calculation are the structure and the configurational entropy of amorphous chalcogenides. The configurational entropy determines the complexity of the free energy landscape of the alloys and their glass-forming ability. To back up these calculations, a novel procedure is being implemented for generating dense glassy structures for particles with distorted-octahedral bonding typical of the chalcogenides. An additional coarse-graining description is being implemented with the help of a recently developed 6-component spin model based on the theory of elasticity of structurally-degenerate solids.