Preparation of new superconductors by metal doping of two-dimensional layered materials using ethylenediamine

We have studied new superconductors prepared by metal doping of two-dimensional (2D) layered materials, FeSe and $\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$, using ethylenediamine (EDA). The superconducting transition temperatures (${T}_c\mathrm{s}$) of metal-doped FeSe and metal-doped $\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$, i.e., ${(\mathrm{EDA})}_y{M}_x\mathrm{FeSe}$ and ${(\mathrm{EDA})}_y{M}_x\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$ ($M$: Li, Na, and K), were 31--45 K and 19--25 K, respectively. The stoichiometry of each sample was clarified by energy dispersive x-ray (EDX) spectroscopy, and the x-ray powder diffraction pattern indicated a large expansion of lattice constant $c$, indicating the cointercalation of metal atoms and EDA. The pressure dependence of superconductivity in ${(\mathrm{EDA})}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$ has been investigated at a pressure of $0--0.8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, showing negative pressure dependence in the same manner as $(\mathrm{N}{\mathrm{H}}₃{)}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$. The ${T}_c\ensuremath{-}c$ phase diagrams of ${M}_x\mathrm{FeSe}$ and ${M}_x\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$ were drawn afresh from the ${T}_c$ and $c$ of ${(\mathrm{EDA})}_y{M}_x\mathrm{FeSe}$ and ${(\mathrm{EDA})}_y{M}_x\mathrm{FeS}{\mathrm{e}}_0.5\mathrm{T}{\mathrm{e}}_0.5$, showing that the ${T}_c$ increases with increasing $c$ but that extreme expansion of $c$ reverses the ${T}_c$ trend.