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 theory by using different approximations to the exchange-correlation potential. It is shown that by including the strong on-site Coulomb interaction explicitly in the Hamiltonian (LSDA+U method) an energy gap of the experimental size is obtained for both compounds. Moreover, the inclusion of the U increases magnetic moments in agreement with experiments. Calculated density 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 dependent 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 show 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.