Influences of Lattice Sinks and Defect Interactions on Solutes in Compounds
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Influences of Lattice Sinks and Defect Interactions on Solutes in Compounds Gary S. Collins∗ and Matthew O. Zacate Department of Physics, Washington State University Pullman, WA 99164, U.S.A. ABSTRACT A thermodynamic model for the site preference of dilute solutes in ordered compounds was developed recently. The model is extended here in two ways to help interpret experimental results from this laboratory. (1) A preference for solutes to occupy sites in 'sinks' such as grain boundaries rather than regular lattice sites is included using a simple model. A wide range of site preference behavior is found; for example, a solute may switch between substitutional and sink sites with changing composition. (2) The effect of an attractive interaction between solute and defect on the site preference is examined. Conditions are established under which a solute can be stabilized by association with a defect on a site where the solute would otherwise not be found. INTRODUCTION Preferences of solutes among inequivalent lattice sites in a compound affect a variety of physical and mechanical properties. One needs to understand the underlying thermodynamics in order to interpret site preference observations and make predictions. Recently, we developed a thermodynamic site-preference model applicable for compounds of any structure and encompassing both substitutional and interstitial sites [1]. The model has been used to help explain some aspects of experimental observations of the site preference of dilute indium solutes in five aluminides and galliumides having the Ni2Al3 crystal structure [2]. This crystal structure has one Ni-site and two inequivalent Al-sites. Observations were made using the method of perturbed angular correlation of gamma rays (PAC), in which lattice locations of indium solutes were determined through measurements of nuclear quadrupole interactions. Solutes in each of the five phases were found to switch sites as the composition changed from one side of the stoichiometric composition to the other. Indium solutes in transition-metal (TM) rich phases were found to occupy one of the two inequivalent sites of the trivalent element (Wyckoff notation 2d). Solutes in TM-poor galliumides were observed instead on the TM-sublattice [2], whereas solutes in TM-poor aluminides exhibited a strongly inhomogeneous signal indicating that they had been expelled from normal lattice sites to 'lattice sinks' such as grain boundaries [2]. The thermodynamic model [1] was developed for systems in which interactions among solutes and intrinsic defects (vacancies, antisite atoms and host interstitials) can be ignored. Such a model is applicable as a first approximation for intermetallic compounds in which coulomb interactions are screened by conduction electrons. The model may also apply for ionic and semiconducting compounds in which defects and solutes have the same charges as atoms they replace. ∗
Corresponding author, [email protected]. F8.19.1
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