Systematic study of (La1−xGdx)1.85Sr0.15CuO4(0≤x≤1): Structure, superconductivity, resistivity, and magnetic properties

A room-temperature structural phase diagram has been determined in (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Gd}}_{\mathit{x}}$ ${)}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ system (0≤x≤1). There exist three stable phases (T, $T^{\mathrm{*}}$ and T’), in which the local Cu-O unit is an octahedron, a pyramid, and a square, respectively. The Jahn-Teller distortion is reduced in the order of T, $T^{\mathrm{*}}$, and T’. For each phase, there is a solubility region. No magnetic ordering is found in the T and $T^{\mathrm{*}}$ phase, both of which exhibit paramagnetism with a constant Gd magnetic moment consistent with that of ${\mathrm{Gd}}^{3+}$. In ${\mathrm{Gd}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Gd}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$, the initial susceptibility indicates a Néel state in the Cu-${\mathrm{O}}_2$ plane at $T_N$=285 K and another magnetic transition at low temperature. $T_N$ is not sensitive to the Sr doping at all, indicating that extra holes cannot be doped onto the Cu-${\mathrm{O}}_2$ plane. While the $T^{\mathrm{*}}$ and T’ phases are insulating, exhibiting a variable-range hopping behavior, the Gd-doped (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Gd}}_{\mathit{x}}$ ${)}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ (x≤0.1) is superconducting with $T_c$ reducing with increasing Gd concentration. The suppression of $T_c$ is not due to a variation of the electron-boson coupling strength which remains unchanged in the system, but correlates closely with the low-temperature resistivity anomaly. Such an anomaly can be best described by a logarithmic temperature dependence.