We have used electron microscopy, scanning electron diffraction, and structure modeling techniques to investigate crystallization behavior and structure of ion-mixed and codeposited films of Pt-Si of various compositions deposited on Si and on NaCl. We have determined that the amorphous alloys of ion-mixed Pt2Si3 have lateral compositional fluctuations on the atomic scale of 80-160 Å in diameter. We show that the amorphous phase of Pt2Si3 is a new phase which cannot be fully described by the simple binary dense random packing of hard spheres model (DRPHS). We propose, but do not prove unequivocably, that the amorphous structure can be described by a DRPHS model with lateral compositional fluctuations on the atomic scale of 80-160 Å, which contains very small nuclei on the order of 5-7 Å in size. We further propose that these nuclei have a composition exactly equal to the metastable crystalline Pt 2Si3 phase, and that they cannot grow without a high temperature annealing because of the surrrounding amorphous material of slightly different composition. We find a long range diffusion process for amorphous Pt2Si3 at temperatures below crystallization. The activation energy for the process is 1.56±0.2 eV, with an average 〈D〉=2.0×10-15 cm 2/sec at 385 °C. The process can relax and homogenize the amorphous structure by atomic displacements over a distance of 80-160 Å, and is probably possible because of micropores in the film. The structures of Pt62Si38 and Pt26Si74 on NaCl were amorphous, while that of Pt80Si20 was microcrystalline.