Semi-empirical and ab-initio calculations of the crystal field interactionin rare earth cuprates

Semi-empirical and ab-initio methods useful for theoretical investigations of the crystal field (CF) interaction in rare earth (RE) cuprates are descr ibed. Concrete calculations are performed for Sm1+xBa2-xCu3O6+y a system pr oviding detailed data on intermultiplet infrared-active CF transitions for Sm3+ ions on the regular D-4h symmetry sites as well as on the substituted C-4v. symmetry Ba sites. Within this study, the main attention is paid to t he second order CF parameters calculated using a parameter free first princ iples method based on the density functional theory (DFT). A general potent ial linearized augmented plane wave (LAPW) computational method is used to obtain the ground state charge density. The calculated value of B-20 = 320 cm(-1) in D-4h symmetry sites of Sm is in a good agreement with the phenome nological value of 282 cm(-1) obtained from a fit to infrared transmission spectroscopy data. For Sm in the C-4v symmetry sites, the DFT value of B-20 = -227 cm(-1) together with the standard superposition model values, B-40 =24, B-44 = -331, B-60 = - 427 and B-64 =624 cm(-1), allow us to interpret the main features of the available infrared absorption data. It is shown th at the sign of the B-20 in SmBa2Cu3O6+y, positive in the regular sites and negative in the Ba sites, is governed by the shape of the crystal potential within the LAPW atomic sphere of Sm atom. (C) 2001 Elsevier Science B.V. A ll rights reserved.