Point Defects in Intermetallics using PAC

Washington State University, Pullman WA

Investigators

Abstract

0091681 Collins Point defects in intermetallic compounds are studied using perturbed angular correlation of gamma rays (PAC). Local environments of dilute indium probe atoms are resolved through measurement of characteristic quadrupole interactions. Two kinds of environment are studied: probe atoms as solutes on inequivalent lattice sites in a compound or mixture of phases, and probes bound in complexes with lattice vacancies. Research is in three areas that are natural extensions of recent work. Preferences of solute atoms for different lattice sites in compounds or in two-phase alloys are determined through measurement of fractions of probe atoms in the different sites. Sublattice sites are identified by comparing measured quadrupole interactions with those in a series of compounds having the same structure and with computations of quadrupole interactions. The goal is to develop a more systematic understanding of site preferences. The effect of applied pressure on the quadrupole interactions of probes bound in complexes with lattice vacancies are studied at room temperature using a diamond anvil cell. The expectation is that there will be large relaxations of atoms near vacancy complexes. Pressure dependencies will be correlated with temperature dependencies and with computations of the structures and electric-field gradients of the complexes. The goal is to understand how the shape and volume of vacancy clusters change under the influence of pressure and temperature. Four processes involving point defects will be studied: defect formation, binding of vacancies to probe atoms, conversion between different structural point defects in non-stoichiometric compounds, and vacancy migration. Activation volumes for the first three processes will be determined through measurement of site fractions of probe-vacancy complexes. The activation volume for vacancy migration will be determined from the pressure dependence of signal relaxation caused by stochastic jumping of vacancies near probe atoms. %%% These measurements will provide fundamental information about point defect processes. These will be the first high pressure PAC studies carried out using a diamond anvil cell and the first at high temperature. Results in this area should be useful to researchers studying increasingly complex systems using PAC and similar methods. ***