Tracking the Formation of High Entropy Solid Solutions and High Entropy Intermetallics by In Situ X-ray Diffraction and Spectroscopy

The recent focus on high entropy alloys in the field of electrocatalysis has led to a corresponding increase in the interest in nanosizing these materials. Nevertheless, the precise mechanism by which they are formed at low temperatures remains unclear. In this study, we investigate the formation and subsequent growth of PtFeCoNiPd particles following a facile and industrial-scalable incipient wetness impregnation approach using quasi-simultaneous in situ powder X-ray diffraction and X-ray absorption near-edge structure spectroscopy. The initial formation of crystallite domains with a size of 2-3 nm and an fcc structure, which are rich in Pt and Pd, was observed. A continuous incorporation of the nonprecious elements at elevated temperatures leads to crystallite domain growth while maintaining an fcc structure. Upon reaching a temperature of 330 °C, Bragg peaks corresponding to a face-centered tetragonal phase emerge, indicating a transition to an intermetallic species. The degree of ordering was found to be dependent on the atomic ratio of precious to nonprecious elements in the samples, with the synthesis temperature program also influencing this degree of ordering. This suggests the possibility of a synthetic control.