METAL-COMPLEXES IN INORGANIC MATRICES .11. COMPOSITION OF HIGHLY DISPERSED BIMETALLIC NI, PD ALLOY PARTICLES PREPARED BY SOL-GEL PROCESSING- ELECTRON-MICROSCOPY AND FERROMAGNETIC-RESONANCE STUDY

The composites PdxNi1-x.15SiO(2) (x = 0.1, 0.3, 0.5, 0.7, or 0.9) with highly dispersed Ni/Pd alloy particles in a silica matrix were prepar ed by sol-gel processing, starting from Pd-(acac)(2), Ni(OAc)(2), Si(O Et)(4), and H2NCH2CH2NH(CH2)(3) Si(OEt)(3). The gels were calcined at 823 K in air and finally reduced with H-2 at 773 K. Transmission elect ron micrographs of PdxNi1-x.15SiO(2) revealed a bimodal particle size distribution, with 4-6 nm diameters for the smaller and 14-20 nm for t he larger particles. The fee lattice constants, determined by electron diffraction, showed the smaller particles to have about the nominal c omposition while the larger particles are richer in Pd. The particle c ompositions were independently determined from the Curie temperatures, obtained by thermomagnetic measurements, and the particle diameters f rom the I-rel(exp) values. By simulation of the FMR (ferromagnetic res onance) powder spectra, a third, Pd-rich, composition was identified ( for PdxNi1-x with x greater than or equal to 0.3). Since the Ni-rich p articles have approximately the nominal composition and the other part icles are richer in Pd, there has to be additional, probably unreduced nickel in the bulk of the material. To reach a good fit between exper imental and calculated spectra, one has to calculate the spectra with a B value smaller than B-0. This is interpreted by tension on the meta l particles and may be caused by the presence of unreduced Ni2+, which probably anchors the alloy particles to the matrix. This conclusion i s supported by the magnitude of the anisotropy field in the Ni-rich cr ystallites.