Correlation effects in the high formal oxidation-state compound Sr2CoO4

Two recent reports confirm that the newly synthesized Sr2CoO4 (formal oxidation state Co4+) shows a high Curie temperature (similar to 250 K), but they report different moments of 1.8 mu(B) and 1 mu(B) per Co. Using both commonly used functionals in the correlated band approach (LDA+U) as well as the local spin density approximation (LSDA), the combined effects of correlation and hybridization with O 2p states are calculated and analyzed. Sr2CoO2 is already ferromagnetic within LSDA (M=1.95 mu(B)). Increasing U from zero, the two LDA+U schemes affect the moment oppositely out to a critical value U-c=2.5 eV, at which point they transform discontinuously from different states to the same large U state. Fixing U at U-c, fixed spin moment calculations show similar behavior out to a minimum at 1 mu(B) (a half metallic state), beyond which the fully localized-limit scheme jumps to a state with energy minimum very near 2 mu(B) (very close to the LSDA moment). Although the energy minima occur very near integer values of the moment/Co (1 mu(B),2 mu(B)), the strong 3d-2p mixing and resulting 3d orbital occupations seem to preclude any meaningful S=1/2 or S=1 assignment to the Co ion.