Lattice constant, bond-orientational order, and solid solubility of PdPt bimetallic nanoparticles

Crystal structure deviations of bimetallic Pd-Pt nanoparticles (NPs) were investigated using synchrotron high-energy X-ray diffraction. The samples comprised Pd NPs and bimetallic Pd-Pt NPs featuring those with a Pd-core/Pt-shell structure as well as PdPt solid-solution NPs with varying diameters of 6.1, 6.7, 8.1, and 11.2 nm. The Rietveld method was used to find the lattice constant of the Pd NPs, which was larger than the bulk value, while reverse Monte Carlo modeling revealed that they possessed a highly disordered structure. The lattice constants of the Pd-Pt bimetallic NPs were found to vary with the Pt content. For Pd-core/Pt-shell structure NPs, the lattice constants decreased with the Pt content owing to lattice relaxation at the core/shell interface. After a reaction process of hydrogen absorption/desorption, the lattice constant of the resulting PdPt solid-solution NPs increased with the NP Pt content, which followed Vegard's law. The degree of disorder of the solid-solution NPs was evaluated by the P_BOO values (i.e., structural parameters describing the deviation of the local structure from an ideal crystal structure) and was found to decrease with the Pt content. A short-range-order parameter was introduced using a three-dimensional reverse Monte Carlo configuration structure to evaluate the atomic-scale solid solubility of the Pd_0.92Pt_0.08 and Pd_0.79Pt_0.21 NPs. The parameter values obtained were close to 0, indicating that the Pd and Pt atoms were essentially randomly dispersed in the solid-solution NPs.