Structure analysis of amorphous Pd75Si25 alloy by electron diffraction and high-resolution electron microscopy

A structural investigation on sputter-deposited amorphous Pd75Si25 alloy thin films including the medium range order (MRO) structure has been made by means of (1) atomic pair distribution function (PDF) analysis by electron diffraction using an imaging plate (IP), (2) high-resolution electron microscopy (HREM) and (3) nano-probe electron diffraction. In the PDF profile a subpeak for the Pd-Si correlation at a distance of 0.24 nm was observed near the first main peak for the Pd-Pd correlation. MRO regions were clearly observed by HREM as lattice fringe regions with a size of 1 ~ 3 nm. Nano-probe diffraction patterns with various zone-axes beam incidences revealed that the MRO structure is hexagonal with the Pd2Si-type structure (lattice parameters: a = 0.715 and c = 0.312 nm, space-group: P2m). In the MRO, Si atoms are though to occupy their sites statistically to form the hexagonal structure with the Pd3Si composition. The nearest neighbor Pd-Si and Pd-Pd distances obtained from the nano-diffraction structure analysis coincided with those obtained from the PDF analysis. An examination was made on the local structure model of the present alloy.