Solving the “Coloring Problem” in InPd3–xAgx (x = 0–0.7) by Phase Diagrams Modeling and Diffraction Experiments

Abstract

A series of InPd3–xAgx (x = 0–1) compositions were synthesized by conventional high-temperature synthesis, and as-synthesized samples were characterized by powder X-ray diffraction experiments. Up to x = 0.7, InPd3–xAgx adopts the ternary substitutional variant of the InPd3 structure (TiAl3-type), when x > 0.7, elemental Ag starts to segregate along with the main phase. Accurate structural characterization in InPd3–xAgx faces a critical challenge due to the narrow X-ray scattering contrast among constituents In, Pd, and Ag and nearly identical neutron scattering lengths of Pd and Ag. To overcome this “coloring problem”, a combination of calculation of phase diagrams modeling (CALPHAD) and diffraction techniques (X-ray and neutron) was employed. In the compositional range 0 ≤ x ≤ 0.7, InPd3–xAgx presents a ternary variant of the TiAl3-type structure, where Ag atoms selectively substitute one (the 2b Wyckoff site) of the two Pd sites in InPd3. Notably, in contrast to the isologous InPd3–xCux (x = 0–1) system, Ag substitution does not form an ordered VRh2Sn-type structure at the limiting composition. The distinct site preference in InPd3–xAgx is elucidated by charge population analysis, electronic structure calculations, and orbital-resolved chemical bonding investigations, and the extent of substitution is supported by formation free energy calculations.