MONTE-CARLO AND HYBRID MONTE-CARLO MOLECULAR-DYNAMICS APPROACHES TO ORDER-DISORDER IN ALLOYS, OXIDES, AND SILICATES

Order-disorder in alloys, oxides, and silicates is studied by means of Monte Carlo methods. The key objective is to take explicit account of atom relaxation and cluster formation without resorting to the use of a parametrized Hamiltonian. We use a variety of Monte Carlo technique s, ranging from an Ising model to a hybrid Monte Carlo/molecular dynam ics scheme. It is crucial to lake account of relaxation of the local a tomic environment and vibrational effects. We have investigated both c onvergent and nonconvergent ordering. Applications include (i) Cu/Au o rdering in Cu-Au alloys, (ii) Mg/Mn ordering in olivine MgMnSiO4, and (iii) La3+/M2+ order-disorder in the quaternary cuprates La2MCu2O6 (M = Ca, Sr). These examples include the interchange of atoms in a system where only short-range forces need to be considered, the Interchange of isovalent ions, and the interchange of heterovalent ions, respectiv ely. The importance of relaxation effects increases in the order i-iii , i.e., as the size/charge mismatch between the interchanged atoms or ions increases.