Study of the Pt-rich nanostructured FePt and CoPt alloys: oddities of phase composition

Nanoparticles of the FePt and CoPt alloys are studied. Detailed structural-phase data on the platinum-rich nanosystems has been obtained using a combination of X-ray diffraction (XRD), selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HR-TEM) in a scanning transmission electron microscope, in addition with derivatography in coupled with the simultaneous mass spectrometry, and elemental analysis of the samples. The upper limits of Fe and Co solubility in Pt and their temperature dependencies have been defined. It is shown that, if Fe or Co contents are above the solubility limits, there is not only the diffraction-detectable phases of FCC solid solutions but also undetectable phases by XRD-method, probably, it is the intermetallic compound of L12 structure (Fe1−xPt3+x or Co1−xPt3+x) or solid solution containing more amount of Fe and Co. A model of low-temperature transformation [A1‑type (FCC FeaPt1−a) → A1 (FCC Fea−xPt1−a+x) + A1undetectable phase (Fea+xPt1−a−x)] when samples are heated (100 – 500°C) is proposed. The model is based on that the crystals are nano-sized and the diffusion of Fe and Co in comparison with Pt is predominant. Therefore, diffraction-undetectable phases can be existed. According to the results of derivatography and mass spectrometry methods, there is no partial oxidation of Fe and Co in the FCC phase. Therefore, oxidation is not the cause of the observed effects.