Structure and rare-earth magnetism in (${\mathrm{Nd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$${)}_2$${\mathrm{CuO}}_4$

Mixed compounds (${\mathrm{Nd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Gd}}_{\mathit{x}}$ ${)}_2$ ${\mathrm{CuO}}_4$ with T’ structure have been studied by x-ray diffraction, by measurements of the specific heat and of the magnetic susceptibility, and by ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ Mössbauer spectroscopy. For Gd contents larger than a critical concentration ${\mathit{x}}_{\mathrm{cr}}^{\mathit{s}}$=0.625, marked by a discontinuity in the concentration dependence of the lattice parameters a and c, the sites of O(1) atoms in the ${\mathrm{CuO}}_2$ planes deviate from the symmetrical position in the T’ structure. The deviations correspond to rotations of the O(1) squares around the central Cu sites. These distortions lower the local symmetry at Gd sites, which is orthorhombic for x>${\mathit{x}}_{\mathrm{cr}}^{\mathit{s}}$. However, x-ray-diffraction patterns show the global symmetry to be tetragonal for all compounds. The distortions provide an explanation for the magnetic properties of Gd-rich compounds, in particular for their weak ferromagnetism. A second critical concentration, ${\mathit{x}}_{\mathrm{cr}}^{\mathit{m}}$∼0.4, appears in the concentration dependence of the ordering temperature ${\mathit{T}}_{\mathit{N}}^{\mathit{R}}$ of rare-earth moments. For x>${\mathit{x}}_{\mathrm{cr}}^{\mathit{m}}$, ${\mathit{T}}_{\mathit{N}}^{\mathit{R}}$ increases linearly with x, from ${\mathit{T}}_{\mathit{N}}^{\mathit{R}}$=1.95 K for x=0.46 to ${\mathit{T}}_{\mathit{N}}^{\mathit{R}}$=6.5 K for x=1. For all x<${\mathit{x}}_{\mathrm{cr}}^{\mathit{m}}$, a Schottky maximum of the specific heat near 1.6 K indicates induced ordering of the rare-earth moments due to interactions with the ordered Cu moments.