Strain is known to enhance the activity of the oxygen reduction reaction in catalytic platinum alloy nanoparticles, whose inactivity is the primary impediment to efficient fuel cells and metal−air batteries. Bragg coherent diffraction imaging (BCDI) was employed to reveal the strain evolution during the voltammetric cycling in Pt−Ni alloy nanoparticles composed of

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Bragg coherent diffraction imaging (BCDI), the well-established technique for imaging internal strain of nanoparticles, was used to image the internal compositional distribution of binary alloys in thermal equilibrium. The images experimentally obtained for Pd-Rh alloy nanoparticles are presented and discussed. The direct correspondence between the lattice strain and the compositional deviation is discussed with the derivation of the BCDI displacement field aided by illustrations. The correspondence suggests that the longitudinal derivative of the displacement field, the strain induced by compositional heterogeneity, can be quantitatively converted to the 3D images of compositional deviation from the particle average using Vegard's law. It also suggests that the transverse derivative can be qualitatively associated with the disorder of Bragg planes. The studied Pd-Rh alloy nanoparticle exhibited internal composition heterogeneity; Rh composition tends to be high at edges and corners between facets and gradually decreased from the surface to core of the particle.

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