Electronic properties of ${\mathrm{Ca}}_2$${\mathrm{CuO}}_2$${\mathrm{Cl}}_2$ and ${\mathrm{Ca}}_2$${\mathrm{CuO}}_2$${\mathrm{Br}}_2$

The linear augmented-plane-wave method has been applied to calculate the electronic band properties of the halo-oxocuprates ${\mathrm{Ca}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$ and ${\mathrm{Ca}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Br}}_2$, a class of quaternaries that form with the same ${\mathrm{K}}_2$ ${\mathrm{NiF}}_4$-type structure as ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. The results exhibit the same generic features that characterize previous cuprate superconductor parent compounds such as ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$, including a single half-filled Cu d(${\mathit{x}}^2$-${\mathit{y}}^2$)–O p(x,y) σ-antibonding subband at ${\mathit{E}}_{\mathit{F}}$. A tight-binding analysis shows that the qualitative differences between the valence-band results at lower energies for the halo-oxocuprates and ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ are due to the increased binding energies [relative to Cu(3d)] of the Br(4p) (∼0.5 eV) and Cl(3p) (∼1.3 eV) states as compared with the apical O(2p) (∼0.1 eV) states.