Electronic structure of the mixed valence system(YM)2BaNiO5 (M=Ca,Sr)

Electronic structure of the system ${\mathrm{Y}}_{2-x}{M}_x{\mathrm{BaNiO}}_5\hspace{0.5em}(M=\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r})$ was calculated for $x=0,$ 0.2, and 0.5 using the full-potential linearized augmented-plane-wave method. To describe the exchange and correlation the local spin-density approximation (LSDA), generalized-gradient approximation (GGA), and two versions of the LSDA+U method were employed. Independently of the method used, the ground state of the parent compound corresponds to an antiferromagnetic insulator. The gap as obtained by LSDA and GGA is smaller than the experimental gap, while LSDA+U methods yield the correct value. To calculate the electronic structure in the mixed-valence region $\left(x\ne 0\right)$ the virtual-crystal approach was used. The ground state of the system is still antiferromagnetic, but a finite density of states appears at the Fermi level. The oxygen K-edge x-ray absorption spectra calculated using the LSDA+U methods agree well with the experiment, in particular, the appearance of an additional peak when ${\mathrm{Y}}^{3+}$ is substituted by the $M^{2+}$ cation is correctly described.