  Pd-Zn intermetallic catalysts show encouraging combinations of activity and
selectivity on well-defined active site ensembles. Thermodynamic description of
the Pd-Zn system, delineating phase boundaries, and enumerating site
occupancies within intermediate alloy phases, are essential to determining the
ensembles of Pd-Zn atoms as a function of composition and temperature.
Combining the present extensive first-principles calculations based on density
functional theory (DFT) and available experimental data, the Pd-Zn system was
remodeled using the CALculation of PHAse Diagrams (CALPHAD) approach. High
throughput modeling tools with uncertainty quantification, i.e., ESPEI and
PyCalphad, were incorporated in the phase analysis. The site occupancies across
the {\gamma}-phase composition region were given special attention. A
four-sublattice model was used for the {\gamma}-phase owing to its four Wyckoff
positions, i.e., the outer tetrahedral (OT) site 8c, the inner tetrahedral (IT)
site 8c, the octahedral (OH) 12e, and the cuboctahedral (CO) site 24g. The site
fractions of Pd and Zn calculated from the present thermodynamic model show the
occupancy preference of Pd in the OT and OH sublattices in agreement with
experimental observations. The force constants obtained from DFT-based phonon
calculations further supports the tendency of Pd occupying the OH sublattice
compared with the IT and CO sublattice. The catalytic assembles changing from
Pd monomers (Pd1) to trimers (Pd3) on the surface of the {\gamma}-phase is
attributed to the increase of Pd occupancy in the OH sublattice.
