Nanometre fcc clusters versus bulk bcc alloy: the structure of Cu-Pd catalysts

We present a simple method to improve the accuracy of the calculated heat o f mixing for the Cu-Pd alloy within the formalism of the molecular dynamics /Monte Carlo-corrected effective medium (MD/MC-CEM) theory by adding a fitt ed Morse potential to the pair interaction between Cu and Pd atoms. This le ads to a much better agreement between the theoretical and experimental Val ues of heats of mixing for five different compositions of the Cu-Pd alloy i n the bulk phases. Using this newly fitted model, we have performed simulat ions on CuPd clusters consisting of 50-10 000 atoms with fee and bce struct ures. Our calculations show that in the range of cluster sizes of several t housand atoms, the fee structure is energetically favoured over the bcc str ucture. We estimate an approximate size for the fcc to bcc (CsCl, known bul k structure for CuPd) transition in these clusters to be around 10 000 atom s. Additionally, we have also performed calculations of the X-ray diffracti on patterns of a variety of cluster geometries and sizes. The calculated X- ray diffraction pattern of a slightly distorted fee cluster exhibits the ma in features observed in the available experimental diffraction patterns of colloidal bimetallic catalysts of CuPd. The calculated diffraction patterns of bcc clusters are quite different from the experimental data.