A microscopic model is presented for the structure and superconductivity of $A_x{T}_{1+y}{C}_2$ compounds, where $A$ is a monovalent atom or radical, $T$ is an even-valence transition metal, $C$ is a chalcogenide, and the parameters $x$ and $y$ lie in the ranges $0.1<x\lesssim 0.3$, $0<~y\lesssim 0.5$. The model explains why the maximum superconducting transition temperature ${T_S}^{max}$ is higher than 12°K in bridge compounds with $y=0.1\mathrm{}$, although the maximum value of $T_S$ observed in related layer compounds with $y=0.0\mathrm{}$ is about 6°K.

• Received 15 March 1972

DOI:https://doi.org/10.1103/PhysRevLett.28.1196