Element-specific quantitative determination of the local atomic order in CoPt alloy nanoparticles: Experiment and theory

An annealing-driven transition from a chemically disordered $A1$-like structure to a chemically ordered $L{1}_0$-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios $c/a$ are inferred. This ratio is found to be remarkably different depending on the approach used ($c/a>1$ from x-ray absorption at the Co $K$ edge and $c/a\simeq 0.94$ from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys.

Figure 1

XANES spectra for the S${}_2$, S${}_3$, and S${}_4$ samples before annealing (red dashed lines) and after annealing (solid lines). The XANES spectrum of a CoPt film is shown for the sake of comparison. The vertical dashed lines indicate the position of the $B1$ and $B2$ structures. The inset shows an enlargement of the $A$ shoulder for the 4 nm sample, before and after annealing.

Figure 2

Modulus and real part of the Fourier transform of the EXAFS signal in the various samples: (a) S${}_2$, (b) S${}_3$, and (c) S${}_4$. Dashed (solid) curves refers to measurements before (after) annealing. The insets show the corresponding $k^2$-weighted EXAFS signals.

Figure 3

Comparison between the experimental EXAFS signal (dots, contribution of the NN peak only) and theoretical fits (solid lines) with $c/a\left(\mathrm{XAS}\right)=1.03$ (a) and $c/a\left(\mathrm{XAS}\right)=0.94$ (b) for the annealed S${}_3$ sample.

Figure 4

Illustration of the relaxed $L{1}_0$-like Co${}_{300}$Pt${}_{286}$ cluster (a), where dark and light spheres represent Co and Pt atoms, respectively. (b) Top view along the (001) direction of the uppermost Co facet [highlighted in (a)]. The interatomic distances are given in Å. (c) Side view of the three uppermost Co and Pt planes showing the important Pt-layer corrugation. (d) Top view of the three uppermost Co and Pt layers along the (001) direction. Notice the tendency of the Co atoms to form short-bond-length squares (tetramerization).