Structure and superconductivity without apical oxygens in (Ca,Na${)}_2$${\mathrm{CuO}}_2$${\mathrm{Cl}}_2$

Recently superconductivity has been reported in (Ca,Na${)}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$ in which the apical anion position was proposed to be occupied by Cl [Hiroi et al., Nature 371, 139 (1994)]. In this paper we report a neutron powder diffraction investigation of nonsuperconducting ${\mathrm{Ca}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$ and superconducting (Ca,Na${)}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$ (${\mathit{T}}_{\mathit{c}}$=26 K). We confirm that in both compounds the apical anion position is fully occupied by Cl. Unlike the recently reported superconductor ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{F}}_{2+\mathrm{\delta }}$, in (Ca,Na${)}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$ we find little evidence for the presence of interstitial atoms. In our structural data the substitution of ${\mathrm{Na}}^{+}$ for ${\mathrm{Ca}}^{2+}$ is evidenced by a shortening of the Cu-O bond and lengthening of the Cu-Cl bond. The bond valence of the Cu atom increases, consistent with holes being created by the Na substitution.