Band analysis of the high-Tcprospects for CaNiN and related perovskite-type nitrides

Half-filled ${\mathrm{\sigma }}^{\mathrm{*}}$ bands, a distinctive characteristic of parent compounds for cuprate and bismuthate high-${\mathit{T}}_{\mathit{c}}$ superconductors, are utilized as an electronic ‘‘fingerprint’’ to identify possible high-${\mathit{T}}_{\mathit{c}}$ candidates among perovskite-type nitrides such as ${\mathrm{Ca}}_3$BiN and CaNiN. Band calculations for CaNiN confirm earlier results where the ${\mathrm{\sigma }}^{\mathrm{*}}$ bands lie below ${\mathit{E}}_{\mathit{F}}$, leaving conduction electrons with ${\mathrm{\pi }}^{\mathrm{*}}$ rather than ${\mathrm{\sigma }}^{\mathrm{*}}$ character. Model tight-binding calculations show that this ${\mathrm{\sigma }}^{\mathrm{*}}$-${\mathrm{\pi }}^{\mathrm{*}}$ reversal is due to the linear-chain feature of the CaNiN structure rather than the proximity of Ca(3d) bands. The ${\mathrm{\sigma }}^{\mathrm{*}}$ band survives intact in a hypothetical ${\mathrm{CaNiN}}_2$ phase that contains ${\mathrm{NiN}}_2$ planes. However, this band is empty rather than half filled, thus providing an opportunity for some creative materials synthesis. Because of this generally unfavorable combination of structural, electronic, and compositional deficiencies, it is concluded that nitrides are less-than-ideal candidates for high-${\mathit{T}}_{\mathit{c}}$ superconductivity.