Structure and superconductivity without apical oxygens in (Ca,Na)2CuO2Cl2

Recently superconductivity has been reported in (Ca,Na)2CuO2Cl2 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 Ca2CuO2Cl2 and superconducting (Ca,Na)2CuO2Cl2 (Tc=26 K). We confirm that in both compounds the apical anion position is fully occupied by Cl. Unlike the recently reported superconductor Sr2CuO2F2+δ, in (Ca,Na)2CuO2Cl2 we find little evidence for the presence of interstitial atoms. In our structural data the substitution of Na+ for Ca2+ 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