Electronic and Spin Configurations of $Co^{3+}$ and $Ni^{3+}$ Ions in Oxides of $K_2NiF_4$ Structure: A Magnetic Susceptibility Study

In $La_4LiCoO_8$, $Li^+$ and $Co^{3+}$ ions are ordered in two dimensions and $Co^{3+}$ ions undergo transitions from the low-spin to the intermediate as well as the high-spin states. Both $Sr_4TaCoO_8$ and $Sr_4NbCoO_8$ exhibit low to intermediate-spin state transitions of $Co^{3+}$ ions. In the system $LaSr_{1-x}Ba_xNiO_4$, the $e_g$ electrons are essentially in extended states forming a $\sigma\asterik x^2-y^2$ band. With increase in x, the band width decreases accompanying an increase in unit cell volume; high-spin $Ni^{3+}$ ions are formed to a small extent with increasing x, but there is no spin-state transition. In $LaSrAl_{1-x}Ni_xO_4$, at small x, there is a small proportion of high-spin $Ni^{3+}$; when $x \approx 0.6$, there is an abrupt decrease in the c/a ratio, signalling the formation of the $\sigma^{\ast} x^2-y^2$ band. In $LnSrNiO_4$, the c/a ratio decreases sharply between Ln = La and Nd; this is likely to be accompanied by a broadening of the $\sigma^{\ast} x^2-y^2$ band