| Comprehensive Modeling of Cu-Ni Nanoparticle Phase-Diagrams Based on Modified Cohesion and Coordination-Dependent Bond Energies Micha Polak1; Leonid Rubinovich1; 1BEN-GURION UNIVERSITY OF THE NEGEV, Beer-Sheva, Israel; PAPER: 104/SISAM/Regular (Oral) SCHEDULED: 11:20/Sat. 26 Oct. 2019/Dr. Christian Bernard ABSTRACT: Cu-Ni alloys have gained interest as bulk and nanoparticles (NPs) primarily due to their catalytic and magnetic properties. Different modeling methods employed before for computations of chemical-order were limited to Cu-Ni NPs consisting of few hundred atoms. The present study uses the highly efficient statistical-mechanical free-energy concentration expansion method (FCEM [1]), combined with coordination-dependent bond-energy variations (CBEV [2]), and the coarse-grained layer (CGLM [3]) models for the case of CuNi truncated-octahedrons (TO). This quite efficient semi-analytical methodology enables the exploration of chemical-order configurations and transitions between them in much larger particle sizes and over broad ranges of composition and temperature. Furthermore, in spite of free-atom electronic-relaxation contributions to transition-metal cohesive-energies, numerous studies have misused the latter instead of using genuine bond-energies in modeling NP properties [4].Using the corresponding modified cohesive-energies, and depending strongly on size and composition, the following findings regarding chemical-order configurations are obtained: due to the CBEV, asymmetric Janus-like configuration (JA) is expected to be the most stable for all compositions only for the 201, 586 and 1029-atom TO sizes. At elevated temperatures, they transform into quasi-mixed configurations (QM). For larger TOs, core-shell (CS) configurations start to stabilize in narrow ranges of elevated temperatures and intermediate compositions, and become progressively stable at increasingly wider ranges. Three types of transitions are revealed: JA-CS, CS-QM, and JA-QM, yielding the first comprehensive Cu-Ni nanophase-separation diagrams. The use of unmodified cohesive energies leads to significantly altered transition temperatures, demonstrating the importance of the commonly ignored modification. Preliminary results for Cu-Ni-Pd TOs reveal a considerable impact of Pd alloying on the chemical-order diagrams, particularly the suppression of JA in favor of CS configurations. References: [1] M. Polak and L. Rubinovich, Surf. Sci. Reports 38 (2000) 127-194. [2] L. Rubinovich and M. Polak, Phys. Rev. B 80 (2009) 045404. [3] M. Polak and L. Rubinovich, Phys. Chem. Chem. Phys., 17 (2015) 28211-28218. [4] M. Polak, L. Rubinovich, J. Phys.: Condens. Matter, 31 (2019) 215402. |
