生物化工基础幻灯片

1,Pr. J. Morillo CNRS/CEMES and UPS,Centre dElaboration de Matériaux et dEtudes Structurales (France),Université Paul Sabatier Toulouse (France),Structure, chemical order and magnetism of binary 3d/4d nanoparticles : Experiment and theory,2,INTRODUCTION EXPERIMENTAL RESULTS MODELLING,3,Magnetic particles for recording applications,Example: CoxRh1-x nanoparticles,4,Cobalt : hcp (T750K) Rhodium : fcc,Alloy: hcp (x50,5%) Disordered solid solution,W. Koster, Z. Metallkunde, 43 (1952),Bulk crystals: Co, Rh and Co1-xRhx,5,(fcc),Close packed structures : face centered cubic (fcc) and hexagonal compact (hcp),6,(fcc),Close packed structures : face centered cubic (fcc) and hexagonal compact (hcp),Tetrahedron and octahedron are the building blocks of close-packed structures 1st and 2d neighbours distances are characteristic of close-packed structures,7,Close-packed nanoparticles,8,INTRODUCTION EXPERIMENTAL RESULTS MODELLING,9,PVP- Co1Rh1 mean f = 1.8 nm,10,WAXS measurements of radial distribution function,11,Specific non periodic Structures associated to Co and Co rich particles,12,First neighbour distance dmm as deduced from WAXS measurements of radial distribution function,Bulk fcc,CoRh,Ru concentration (at. %),CoRu,13,HREM image showing a four fold symetry in the Co nanoparticle,14,EXAFS : oxidation at cobalt edge, Rh environment for Rh EFTEM : on bigger particles (5- 6 nm) synthesized in HDA (hcp-fcc),15,CoRh,CoRu,Magnetic moment measurements,16,Which structure? Which chemical order? Can they explain the magnetic moment enhancement?,17,INTRODUCTION EXPERIMENTAL RESULTS MODELLING,18,Introduction,Levels of materials modelling,10,102,103,104,105,106,force-field,number of atoms,accuracy,tight-binding,ab initio (parameter-free),quantum chemistry Hartree-Fock, configuration interaction density functional theory,empirical potentials molecular dynamics Monte-Carlo,Compromise between accuracy and computational tractability,our study,19,small clusters,large clusters,surfaces,now,Introduction,semi-empirical methods,Model larger particles Study properties that are not easily calculated in ab initio (CPU time),20,ab-initio Density Functional Theory calculations,Use of VASP code DFT-GGA (PW91) calculations : spin only, colinear magnetism PAW method (Projector Augmented Wave) advantages : very good accuracy, espec. for magnetic systems reasonable computation time Calculation of ground state properties: cohesive energy Ecoh = E(system) - E(isolated atoms) (eV/at.) geometry local / average magnetic moments µ (µB/at.) Tests on bulks and small clusters of pure materials: OK Tests of different XC functionals,21, 4*3 parameters: , ij = CoCo, RhRh, CoRh rij0 : normalisation factors (first neighbours distances),N-body central force field model : second-moment approximation of the tight-binding model: d-electron band contribution (n-body) Born-Mayer short distance electron core-core repulsion (pair interaction),Semi-empirical calculations,22,Parameters fit: Co-Co and Rh-Rh direct interactions from F. Cleri, V. Rosato, (Phys. Rev. B, 48 (1993): Cohesive energy: Ec Elastic constants: Cij, Lattice parameter(s): a (c). Co-Rh mixed interactions: Cohesive energies Lattice parameters,hypothetical ordered alloys at the studied compositions,of,ab-initio,Semi-empirical calculations,23,Which structure? Which chemical order? Can they explain the magnetic moment enhancement?,24,Which structure for the pure Co particles?,Polytetrahedral 100 150 at. Apparent four fold symetry axis Reproducing the WAXS curves,25,Polytetrahedral geometrical models,26,Co particles (100N150),Selection of the lowest energy configurations,27,0,2,4,6,8,10,12,300 K,1200 K,a. u.,d (Å),1200 K,Co particle (105 at.),60-70 meV,Radial distribution function,28,RELATIVE STABILITIES OF THE DIFFERENT Co MODELS,50-60 meV,30 meV,Perfect polytetrahedral structures are unstable relative to the defective octahedral structures,Most probable: Metastable glass like high temperature disordered polyhedral structures,29,30,Which structure? Which chemical order? Can they explain the magnetic moment enhancement?,31,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,32,distance to centre (Å),300K MCM Relaxation and Exchange,Structure and chemical order,33,Co1Rh1,Co1Rh3, Co Rh,distance to centre (Å),Co3Rh1,1600K TO 300 K MCM Relaxation and Exchange Simulated Annealing,Structure and chemical order,34,Radial Distribution Functions,1600 K Simulated annealing,300K,Structure and chemical order,35,Intermetallic distance dmm(nm),Structure and chemical order,Bulk :,36,Conclu