X-ray technique for determining chemical disorders in complex alloys

Abstract

Complex alloys and compounds possess an impressive array of properties and functionalities. These features often emerge from the particular ordering of constituent atoms within the crystal lattice (i.e., the chemical ordering). One key challenge for studying these materials is the ability to characterize chemical disorders that can alter, suppress, or enhance such unique functionalities. An ability to determine the distribution of the constituent atoms in complex alloys is therefore of critical importance for the materials community. Such measurements have been nearly impossible to perform in alloys that contain atoms with comparable sizes (‘similar’ atoms, i.e., in terms of atomic number and bond length). This difficulty arises primarily because conventional charge-scattering techniques (e.g., x-ray and electron) lack the sensitivity required to differentiate between similar atoms. For this reason, there is confusion in the literature regarding various ‘related’ or ‘indistinguishable’ structures (i.e., where similar atoms that occupy different lattice sites correspond to different crystalline symmetries but the structural differences may or may not be detectable experimentally). The problem is further amplified because there are a large number of alloys with two or more constituent elements that belong to the same row in the periodic table. To overcome these issues, we have recently developed an x-ray diffraction (XRD) technique called multiple-edge anomalous diffraction (MEAD).1 Our approach is based on tracking the diffraction intensity versus the x-ray energy through multiple absorption edges of the constituent elements. At energies near the absorption edge, anomalous dispersion and absorption cause variations in the atomic form factor, effectively causing the Figure 1. The Heusler compounds and related lattice structures. Four interpenetrating face-centered cubic (FCC) sub-lattices are each occupied by a specific element positioned at [000], [ 12 00], [ 1 4 1 4 1 4 ], and [ 14 1 4 3 4 ]. In the full Heusler L21 structure (Cu2MnAl-type, i.e., two parts copper, one part manganese, and one part aluminum), copper atoms occupy the Aand C-sites, and manganese and aluminum atoms occupy the Band D-sites, respectively. In the ‘inverse’ Heusler X structure (CuHg2Ti-type, i.e., one part copper, two parts mercury, and one part titanium), mercury atoms occupy the Aand B-sites, and copper and titanium atoms occupy the Cand D-sites, respectively. In the quaternary Y structure (LiMgPdSn-type, i.e., one part lithium, one part manganese, one part palladium, and one part tin), each element occupies a specific FCC sub-lattice.