Microphase separation in amorphous Pd-Si alloys studied by electron diffraction and HREM

Structures of amorphous Pd82Si18 and Pd75Si25 alloys have been investigated from a viewpoint of 'microphase separation'. In these alloys a local atomi c ordering called medium range order (MRO) with sizes of 1-2 nm was observe d by high-resolution electron microscopy (HREM). From nanodiffraction studi es, atomic structures of the MRO regions in the amorphous Pd82Si18 and Pd75 Si25 structures were determined as those of fcc-Pd-type and hexagonal-Pd2Si -type, respectively. Atomic pair distribution function (PDF) analyses for t hese alloys were performed by precise measurements of halo-electron diffrac tion intensities. In order to explain both the results of HREM and PDF stud ies,'microphase separation' structure models have been constructed for thes e alloys with the help of reverse Monte-Carlo calculations. Structure model s with MRO regions embedded in a dense-random-packing structure of Pd and S i were finally obtained, which could explain the experimental PDFs satisfac torily.