Kinetically Limited Phase Formation of Pt-Ir Based Compositionally Complex Thin Films

Area of Science:

• Materials Science
• Solid State Chemistry
• Thin Film Technology

Background:

• High entropy alloys (HEAs) are typically designed using criteria like configurational entropy, enthalpy of mixing, and atomic size difference.
• Predicting single-phase HEA formation is crucial for developing advanced materials with tailored properties.

Purpose of the Study:

• To investigate the phase formation of PtIrCuAuX (X = Ag, Pd) compositionally complex thin films.
• To critically appraise the established criteria for predicting HEA formation.
• To explore the influence of kinetics on phase formation in these systems.

Main Methods:

• Synthesis of equiatomic PtIrCuAuX (X = Ag, Pd) thin films.
• X-ray diffraction (XRD) for phase analysis.
• Energy-dispersive X-ray spectroscopy (EDX) for elemental composition.
• Calculation of bond strengths and surface diffusion activation energy barriers.

Main Results:

• Equiatomic PtIrCuAuPd films formed a single-phase solid solution, consistent with HEA design rules.
• Equiatomic PtIrCuAuAg films exhibited two-phase formation, contradicting standard HEA predictions.
• Substitution of Pd with Ag decreased bond strength and lowered surface diffusion activation energy, promoting multi-phase formation.

Conclusions:

• Established HEA design rules based on thermodynamics alone are insufficient for predicting phase formation.
• Kinetic factors, specifically bond strengths and surface diffusion barriers, play a critical role in phase selection.
• Phase formation in PtIrCuAuX systems is governed by an interplay between energetics and kinetics, not solely configurational entropy.