Superconductivity in electron-doped layered TiNCl with variable interlayer coupling

Titanium nitride chloride (TiNCl), a band semiconductor with the $\ensuremath{\alpha}$-form layered structure, becomes a superconductor with a transition temperature ${T}_{c}\ensuremath{\approx}$ 18.0 K by electron doping via alkali-metal intercalation. Upon cointercalation of various kinds of organic solvent molecules with alkali atoms, the superconducting layered crystals are swelled to different extents adjusting to the size of the molecules, and the ${T}_{c}$ decreases linearly down to 6.5 K as a function of 1/$d$, where $d$ is the interlayer separation (basal spacing) of the expanded nitride layers, implying the importance of the Coulomb interlayer coupling for superconductivity. This is in strong contrast to a previous finding that the ${T}_{c}$ of the electron-doped ZrNCl and HfNCl with the $\ensuremath{\beta}$-form layered structure rather increases with the increase of $d$ upon a similar cointercalation of solvent molecules.