生物化工基础幻灯片.ppt

* Which structure? Which chemical order? Can they explain the magnetic moment enhancement? * Which chemical order for the CoxRh1-x particles ? Co rich: Polytetrahedral Rh rich: with octahedral sites 100 – 500 at. Strong non linear dmm dilation with x Reproducing the WAXS curves Monte Carlo Metropolis study: Simulated Annealing with Atomic Relaxation and Exchanges between Co and Rh atoms. A large variety of initial states: Disordered, Ordered, Core/Shell fcc, hcp, Cuboctahedral, … Different initial temperatures * Co3Rh1(hcp) ? Co ? Rh distance to centre (?) Co1Rh1(fcc) Co1Rh3(fcc) 300K MCM Relaxation and Exchange Structure and chemical order * Co1Rh1 Co1Rh3 ? Co ? Rh distance to centre (?) Co3Rh1 1600K TO 300 K MCM Relaxation and Exchange Simulated Annealing Structure and chemical order * Radial Distribution Functions Experimental Model 1600 K Simulated annealing 300K Structure and chemical order * Intermetallic distance dmm(nm) Structure and chemical order Bulk : fcc model fcc exp. hcp exp. * Conclusion on structure and chemical order Co rich particles : Co, Co3Rh1, CO1Rh1 Structure Metastable Disordered glass like polytetrahedral structure Chemical order Equilibrium distribution Rh core – Co surface segregation structure Small Co core Rh rich particles : Rh, Co1Rh3 Structure Stable fcc ordered Chemical order Rh core – Co surface segregation structure Possible very small Co core * Which structure? Which chemical order? Can they explain the magnetic moment enhancement? * magnetism magnetism enhanced by geometrical dilation interatomic distance magnetism enhanced by coordination reduction coordination Geometrical contraction at reduced coordination z = 2 z = 6 z = 12 Magnetism: from the single atom to the infinite cristal Ecoh Magnetic moment (μB) Monolayer (111) Linear chain bulk cfc Isolated atom z = 12 z = 2 z = 6 Cobalt example * 3 1.57 1.93 μCo Co From the atoms up to the bulks 3 1.82 μRh Rh 0.00 Magnetic moment per atom (μB/atom) CoRh