Exchange coupling in CaMnO3_3 and LaMnO3_3: configuration interaction and the coupling mechanism

The equilibrium structure and exchange constants of CaMnO$_3$ and LaMnO$_3$ have been investigated using total energy unrestricted Hartree-Fock (UHF) and localised orbital configuration interaction (CI) calculations on the bulk compounds and Mn$_2$O$_{11}^{14-}$ and Mn$_2$O$_{11}^{16-}$ clusters. The predicted structure and exchange constants for CaMnO$_3$ are in reasonable agreement with estimates based on its N\'eel temperature. A series of calculations on LaMnO$_3$ in the cubic perovskite structure shows that a Hamiltonian with independent orbital ordering and exchange terms accounts for the total energies of cubic LaMnO$_3$ with various spin and orbital orderings. Computed exchange constants depend on orbital ordering. UHF calculations tend to underestimate exchange constants in LaMnO$_3$, but have the correct sign when compared with values obtained by neutron scattering; exchange constants obtained from CI calculations are in good agreement with neutron scattering data provided the Madelung potential of the cluster is appropriate. Cluster CI calculations reveal a strong dependence of exchange constants on Mn d e$_g$ orbital populations in both compounds. CI wave functions are analysed in order to determine which exchange processes are important in exchange coupling in CaMnO$_3$ and LaMnO$_3$.Comment: 25 pages and 9 postscript figure

Structural, Electronic, and Magnetic Properties of MnO

We calculate the structural, electronic, and magnetic properties of MnO from first principles, using the full-potential linearized augmented planewave method, with both local-density and generalized-gradient approximations to exchange and correlation. We find the ground state to be of rhombohedrally distorted B1 structure with compression along the [111] direction, antiferromagnetic with type-II ordering, and insulating, consistent with experiment. We show that the distortion can be understood in terms of a Heisenberg model with distance dependent nearest-neighbor and next-nearest-neighbor couplings determined from first principles. Finally, we show that magnetic ordering can induce significant charge anisotropy, and give predictions for electric field gradients in the ground-state rhombohedrally distorted structure.Comment: Submitted to Physical Review B. Replaced: regenerated figures to resolve font problems in automatically generated pd

POINT DEFECTS, GENERAL.THE CALCULATION OF LATTICE DEFECTS AT SURFACES FOR CUBIC IONIC CRYSTALS

On présente un calcul des propriétés des défauts de surface pour un certain nombre d'oxydes cubiques, dans le cadre de la théorie de Mott-Littleton. Pour MgO et MnO les énergies des défauts ponctuels dans le bulk et sur les faces (001) et (011) sont données. On examine également l'effet du dopage en impuretés cationiques pour tous les substrats considérés dans ce travail. On étudie en particulier l'effet du remplacement de l'ion Mn2+ par des ions à charge plus élevée tels que U4+ dans MnO, aussi bien dans le bulk qu'en surface. L'effet du remplacement en surface de Mg2+ par CO4+ dans MgO est également examiné à la lumière de résultats expérimentaux récents.Calculations carried out within a Mott-Littleton framework are presented for the surface defect properties of a number of cubic oxides. For MgO and MnO the energies of the point defects in the bulk and at the (001) and (011) faces are given, while for the full range of substrates considered here, the doping by foreign cations is examined. Particular attention is paid to the bulk and surface substitution in MnO by a highly-charged species such as U4+, and to the surface substitution of Mg2+ by Co2+ in MgO in relation to recent experimental work

The calculated defect structure of ZnO

Les énergies des défauts fondamentaux dans ZnO sont calculées et comparées avec les résultats expérimentaux obtenus par Kroger. L'énergie correspondante à la bande de conduction est calculée en considérant des défauts neutres ou chargés. Les effets de dopage avec des impuretés cationiques : Li+, Na+, Al3+, Ga3+, In3+ et H+ sont analysés. Enfin les énergies associées aux centres F+ et F sont calculées théoriquement.We report the energies of the fundamental defects in ZnO and compare these with values that can be deduced from Kroger's analysis of the experimental data. We consider neutral, singly-charged and doubly-charged defects and from our analysis estimate the edge of the conduction band. We also consider the effect of dopants such as Li+, Na+, Al3+, Ga3+, In3+ and H+. Finally we consider the energetics involved in the formation of the F+ and F centres