ELECTROSTATIC LATTICE COEFFICIENTS AND BINDING-ENERGY OF ORTHORHOMBICLA2-XSRXCUO4

Three valency models for orthorhombic La2-xSrxCuO4 were investigated f or increasing Sr concentrations x (0 less than or equal to x less than or equal to 0.21): 1. Cu2+ --> Cu3+, 2. apex O2- --> O- and 3. in-pla ne O2- --> O-. All calculations were done by using structural paramete rs valid for the temperature range from 10 to 22 K. We thereby calcula ted the electrostatic interaction energy which, next to ionization pot entials and electron affinities, comprises a major of the binding ener gy E(B) of crystals. Second-order effects were accounted for by calcul ating the strength of ionic dipole moments induced by crystal electric fields at relevant lattice sites. Their largest strengths are compara ble to the dipole moment of the water molecule. Three out of five dipo les in La2-xSrxCuO4 vanish during the transition from the orthorhombic to the tetragonal phase. The binding energy differences between the d ifferent models suggest that the system is in a state of model 1. Howe ver, the differences are very small, being in the order of 0.3 to 0.76 eV at x = 0.13.