Improved conductivity observed in doped layered oxysulfides [Cu$_{2}$S$_{2}$][Sr$_{n+1}M_{n}$O$_{3n-1}$]

[Cu$_{2}$S$_{2}$][Sr$_{n+1}M_{n}$O$_{3n-1}$] is a group of semiconducting oxyslfides which are composed of alternate stackings of Cu$_{2}$S$_{2}$ antifluorite layers and perovskite-based $M$O$_{2}$ planes$^{[1]}$. Perovskite structures are expected to show interesting properties such as high-temperature superconductivity and magnetoresistance. However they have attracted less attention because they have relatively high resistivity and there are only a few reports of career doping$^{[2]}$. In this study, we found Na substitution was especially effective on [Cu$_{2}$S$_{2}$][Sr$_{2}$CoO$_{2}$]. Systematic decreases in lattice parameter with increasing doping levels indicated that Na was successfully substituted at the Sr site. Remarkable decrease in resistivity was observed especially at low temperatures, from 10$^{3} \quad \Omega $cm (undoped) to 0.15 $\Omega $cm at 50 K. This value is lowest ever reported in the [Cu$_{2}$S$_{2}$][Sr$_{n+1}M_{n}$O$_{3n-1}$] oxysulfides. [1] K. Ueda\textit{ et al.}, \textit{Chem. Mater.} \textbf{13} (2001) 1880 [2] K. Ito \textit{et al.}, \textit{J. Appl. Phys.} \textbf{99} (2006) 08F705