Small step graphs of cell data versus composition for ccp solid-solution binary alloys: Application to the (Pt,Ir), (Pt,Re) and (Pt,Ru) systems

It is shown here that crystallographic descriptions of hypothetical AB63, AB107, AB127, AB215 and especially AB255 stoichiometric compounds with cell edge, respectively 4, 3, 4, 6 and four times the (A,B) fcc subcell edge stick out as convenient models for ab initio studies of cell data versus composition for ccp solid-solution alloys. Their Wyckoff positions can be combined to generate most binary alloy compositions from 0% to 100% in multiples of 1/64, 1/108, 1/128, 1/216 and 1/256 while retaining the same periodicity and, respectively the same Fm  3¯m, Pm  3¯m, Im  3¯m, Fm  3¯m and Pm  3¯m space group symmetry. As an application, we model cell data for three cubic solid-solution alloys of Pt. (Pt,Ir) and (Pt,Ru) remain close to Vegard's law predictions with a slight convexity or concavity in the plot. That curvature is explainable by the magnitude and sign of the alloying energy. Modeling of (Pt,Re) between 0% and 45% Re in 50 steps of RenPt108−n stoichiometric compounds follows approximately non-Vegard experimental data but with large, unexplained spread. The method has been automated in Materials Toolkit (http://www.tothcanada.com/toolkit).