Systematic study of (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Gd</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>1.85</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mn>0.15</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>(<i>0≤x≤1</i>): Structure, superconductivity, resistivity, and magnetic properties

A room-temperature structural phase diagram has been determined in (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$${)}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ system (0\ensuremath{\le}x\ensuremath{\le}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}$${\mathrm{CuO}}_{4}$, the initial susceptibility indicates a N\'eel state in the Cu-${\mathrm{O}}_{2}$ plane at ${T}_{N}$=285 K and another magnetic transition at low temperature. ${T}_{N}$ is not sensitive to the Sr doping at all, indicating that extra holes cannot be doped onto the Cu-${\mathrm{O}}_{2}$ plane. While the ${T}^{\mathrm{*}}$ and T' phases are insulating, exhibiting a variable-range hopping behavior, the Gd-doped (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$${)}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ (x\ensuremath{\le}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.

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