First-principle investigations of K₂NiF₄-type double perovskite oxides La₄B′B″O₈ (B′B″ = Fe, Co, Ni)

The K₂NiF₄-type structure La₄CoNiO ₈ (LCNO), La₄FeCoO₈ (LFCO), and La ₄FeNiO₈ (LFNO) are studied by using the first-principle electronic structure calculations. Our results indicate that the ground state of LCNO is a ferrimagnetism (FiM) with a large energy gap about 1.9 eV, LFCO and LFNO are antiferromagnetism with energy gaps about 1.3 and 1.4 eV, respectively. Their orthorhombic distortions, out-of-plane elongation, and tilting of octahedron are discussed. It is indicated that LFCO and LFNO have stronger crystal distortion than LCNO. Our calculations indicate that the in-plane magnetic exchange interaction of LCNO is much stronger than LFCO and LFNO, thus LCNO should have much higher magnetic ordering temperature than LFCO and LFNO.