Electronic and spin configurations of Co3+ and Ni3+ ions in oxides of K2NiF4 structure: A magnetic susceptibility study

In La4LiCoO8, Li+ and Co3+ ions are ordered in two dimensions and Co3+ ions undergo transitions from the low-spin to the intermediate as well as the high-spin states. Both Sr4TaCoO8 and Sr4NbCoO8 exhibit low to intermediate-spin state transitions of Co3+ ions. In the system LaSr1−xBaxNiO4, the eg electrons are essentially in extended states forming a σ∗ x 2−y 2 band. With increase in x, the band width decreases accompanying an increase in unit cell volume; high-spin Ni3+ ions are formed to a small extent with increasing x, but there is no spin-state transition. In LaSrAl1−xNixO4, at small x, there is a small proportion of high-spin Ni3+; when x ≈ 0.6, there is an abrupt decrease in the c / a ratio, signalling the formation of the σ∗ x 2−y 2 band. In LnSrNiO4, the c / a ratio decreases sharply between Ln = La and Nd; this is likely to be accompanied by a broadening of the σ∗ x 2−y 2 band.