Structure and Entropy of Materials Studied by Nuclear Resonant Scattering

California Institute Of Technology, Pasadena CA

Investigators

Abstract

The objective of this research is the development of reliable and accurate techniques for the determination of structural and vibrational properties of complex materials. A major goal of the study is to investigate the disorder in metallic alloys with the unique chemical environment selectivity of Mossbauer diffractometry. The unique feature of the technique is its capability to select iron atoms in a particular chemical environment, and obtain a diffraction pattern from those iron atoms alone. With the new Mossbauer diffractometer, it is possible to measure the periodicity of an irregular chemical site of iron atoms with 3 aluminum first-nearest neighbors in ordered FeAl. Studies will also be made on alloys of FeRh and PdFe with emphasis on investigating the inelastic spectra to obtain the phonon partial density of states of iron atoms. Extreme high pressures using diamond anvil cells will be used to investigate the influence of pressure on interatomic force constants. In a different style of experiment, the Mossbauer diffractometer will be used to study the spatial scale of thermal relaxations in magnetic nanoparticles. A specific experiment on thermal relaxations on hematite nanoparticles will be developed over the course of the work. Such results show how the vibrational entropy of alloys depends on interatomic separations, and can elucidate the reasons for the P(V,T) equation of state of iron alloys. The broader impact of the proposed work lies not only in developing reliable techniques for the structural determination of complex materials but also in obtaining detailed information on interactions in solids at the atomic level. Thermodynamics and atom arrangements are at the core of materials science, and the proposed work will apply new experimental methods for studying them. The methods are specialized, but have unique capabilities for resolving a high level of detail about the structure and dynamics of materials. The structure of materials is the focus of the work on Mossbauer diffractometry. The entropy of materials is the focus of the work on inelastic nuclear resonant scattering. The basic issues in diffraction and scattering are also of broad academic interest in materials science.