The electronic structure of K2NiF4 and K2CoF4: beyond LSDA

In the present work the electronic structures of the correlated perovskites K2NiF4 and K2CoF4 are investigated within the (spin-) density-functional t heory by using different approximations to the exchange-correlation potenti al. It is shown that by including the strong on-site Coulomb interaction ex plicitly in the Hamiltonian (LSDA + U method) an energy gap of the experime ntal size is obtained for both compounds. Moreover, the inclusion of the U increases magnetic moments in agreement with experiments. Calculated densit y of states are compared to spectroscopy results and quite a good agreement is found. It is shown that energy gap and spin-moment are only slightly de pendent on the radius of the transition-metal sphere and on the form of the LSDA + U-functional. The properties of the perovskites K2NiF4 and K2CoF4 s how equal trends as a function of U and both compounds are characterized as Mott-Hubbard insulators in which the on-site Coulomb interaction primarily determines the size of the energy gap. (C) 2001 Elsevier Science Ltd. All rights reserved.