Comment on "Superconductivity in electron-doped layered TiNCl with variable interlayer coupling"

In their article, Zhang et al. [Phys. Rev. B 86, 024516 (2012)] present a remarkable result for A(x)(S)(y)TiNCl compounds (alpha-phase TiNCl partially intercalated with alkali A and optionally co-intercalated molecular species S), finding the superconducting transition temperature T-c scales with d(-1), where the spacing d between TiNCl-layered structures depends on intercalant thickness. Recognizing that this behavior indicates interlayer coupling, Zhang et al. cite, among other papers, the interlayer Coulombic pairing mechanism picture [Harshman et al., J.Phys.: Condens. Matter 23, 295701 (2011)]. This Comment shows that superconductivity occurs by interactions between the chlorine layers of the TiNCl structure and the layers containing A(x), wherein the transverse A(x)-Cl separation distance zeta is smaller than d. In the absence of pair-breaking interactions, the optimal transition temperature is modeled by T-c0 proportional to (sigma/A)(1/2)zeta(-1), where sigma/A is the fractional charge per area per formula unit. Particularly noteworthy are the rather marginally metallic trends in resistivities of A(x)(S)(y)TiNCl, indicating high scattering rates, which are expected to partially originate from remote Coulomb scattering (RCS) from the Ax ions. By modeling a small fraction of the RCS as inducing pair breaking, taken to cut off exponentially with zeta, observations of T-c < T-c0 are quantitatively described for compounds with zeta < 4 angstrom and T-c approximate to T-c0 for Na-0.16(S)(y) TiNCl with propylene carbonate and butylene carbonate co-intercalants for which zeta > 7 angstrom. Since a spatially separated alkali-ion layer is not formed in Li0.13TiNCl, the observed T-c of 5.9 K is attributed to an intergrowth phase related to TiN (T-c = 5.6K).