Solubility Limit Of Ce Research Articles

Formation kinetics of polycrystalline Eu2−xCexCuO4−y obtained from a sol-gel precursor

Polycrystalline samples of Eu2−xCexCuO4−y (0.0≤x≤0.18) were prepared from a sol-gel precursor and sintered in air at different temperatures. From the x-ray diffraction results, the beginning of Eu2−xCexCuO4−y phase formation is observed by increasing the sintering temperature up to 500 °C, as indicated by the presence of very broad Bragg reflections belonging to the desired phase. At 700 °C, Eu2−xCexCuO4−y coexists with Eu2O3, CeO2, and CuO. Additional sintering at 950 °C in air for 20 h results in single phase materials. The effectiveness of Eu replacement by Ce in the Eu2CuO4 pristine phase has been confirmed by a decrease in the lattice parameter c with increasing x. Also, the solubility limit of Ce in these series was found to be higher than x=0.15. The results of thermal analysis reveal that the eutectic temperature Te≊1020 °C is Ce independent. On the other hand, the peritectic temperature Tp increases significantly with increasing Ce concentration. It is close to Tp≊1180 °C for Eu2CuO4 and Tp≊1195 °C for Eu1.82Ce0.18CuO4−y. Thermogravimetric analysis performed during the heating process in all samples studied revealed a weight loss of ≊1.5% at the peritectic temperature. This weight loss has been attributed to an oxygen removal which is partially recovered during the cooling process.

Role of bond-length mismatch in L2-xCexCuO4 (L=lanthanide).

The electron-doped ${\mathit{L}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (L=lanthanide) superconductors have intergrowth structures in which ${\mathrm{CuO}}_{2}$ sheets alternate with (L,Ce${)}_{2}$${\mathrm{O}}_{2}$ fluorite layers along the c axis. Stabilization of such intergrowth structures requires bond-length matching between Cu-O and (L,Ce)-O bonds. Any bond-length mismatch will result in the buildup of compressive or tensile stresses in the Cu-O and (L,Ce)-O bonds. The consequences of such internal stresses in ${\mathit{L}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ are investigated by a systematic variation through ${\mathit{L}}^{3+}$ size of the lattice parameter a. A decrease in the degree of bond-length mismatch or internal stresses with decreasing ${\mathit{L}}^{3+}$ size causes a systematic decrease in the Ce solubility limit and in the ease with which oxygen vacancies can be created. The concentration of oxygen vacancies decreases---or the oxygen content increases---with decreasing ${\mathit{L}}^{3+}$ size for a given ${\mathrm{N}}_{2}$-annealing temperature and Ce content; it also decreases with increasing Ce content for a given ${\mathit{L}}^{3+}$ ion. The decreasing oxygen-vacancy concentration with decreasing size of ${\mathit{L}}^{3+}$ causes an apparent increase in the critical Ce concentration ${\mathit{x}}_{\mathit{c}}$ required to induce the antiferromagnetic semiconductor to superconductor transition as the size of ${\mathit{L}}^{3+}$ decreases, although the transition seems to occur at a fixed critical electron concentration ${\mathit{n}}_{\mathit{c}}$=0.175\ifmmode\pm\else\textpm\fi{}0.005 irrespective of the ${\mathit{L}}^{3+}$ size.

Double resistive superconducting transition in Sm2-xCexCuO4-y.

We report a systematic study of the double resistive superconducting transition in carefully prepared polycrystalline samples of the electron-doped superconducting series ${\mathrm{Sm}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ for 0.12\ensuremath{\le}x\ensuremath{\le}0.20. From high-resolution x-ray-diffraction measurements, the solubility limit of Ce in this series is slightly greater than 0.16. An intrinsic double resistive transition present in all superconducting compounds is attributed to the granularity of these polycrystalline samples. There is a partial transition when the grains become superconducting at a temperature ${\mathit{T}}_{\mathit{c}1}$, and coupling between grains at a lower temperature ${\mathit{T}}_{\mathit{c}2}$ apparently completes the transition. Only the transition associated with coupling between grains is observed in magnetic-susceptibility measurements due to inhomogeneous grains and large penetration depths. From compositional dependences of resistive and magnetic measurements, the superconducting volume fraction increases linearly with increasing dopant concentration up to the solubility limit, while the best coupling between grains is at a dopant concentration of 0.15. Thus, there is a subtle relationship between doping and coupling in this series. Also, arguments for a true thermodynamic phase transition at ${\mathit{T}}_{\mathit{c}2}$ are given.

Depression of the weak-ferromagnetism of CuO2 planes in Gd2CuO4 through Ce and Th doping

Abstract Gd 2−x Ce x CuO oxides belong to a family of recently discovered electron doped cuprate superconductors. The undoped material Gd 2 CuO 4 shows antiferromagnetic ordering of the Cu moments about room temperature. We report here ESR and static magnetization measurements made in ceramic samples which indicate a depression of the ordering temperature at a rate of 18 K/at%Ce, down to the Ce solubility limit of x∽0.16, where the material orders below 180 K. A similar rate of depression has been observed for a sample with Th substitution for Gd ions.

Synthesis and x-ray characterization of superconducting and related Ln 2− xCe xCuO 4− y (Ln=Nd and Gd) compounds

In view of the recent attention focused on electron superconductivity, we have prepared and analyzed the crystalline structures of Ce substituted Ln 2− x Ce x CuO 4− y (Ln=Nd and Gd) compounds. X-ray powder diffraction analysis shows that the Ce ions have been succesfully doped into the Nd sites of the tetragonal Nd 2CuO 4 lattice and the maximum Ce solubility limit is x c = 0.20. Linear decreases in both the c lattice dimension and the unit cell volume, V, with increasing x up to the solubility limit indicate the smaller Ce 4+ ions, not the larger Ce 3+ ions are present. The tetragonal unit cell dimensions of the 21 K superconducting Nd 1.85Ce 0.15CuO 4− y compound refined from a high quality powder diffraction pattern with a figure of merit F 30=267.3 (0.003, 35) are: a = 3.9463 (1) A ̊ , c = 12.0809 (2) A ̊ and V = 188.14 (1) A ̊ . X-ray results also show that the maximum Ce solubility limit in Gd 2CuO 4 is significantly lower, with x c=0.15. Linear decreases in c and V with increasing x in Gd 2- x Ce x CuO 4− y have also been obtained indicating the presence of Ce 4+ ions. The unit cell dimensions of the semiconducting Gd 1.85Ce 0.15CuO 4− y compound are a = 3.9023 (1) A ̊ , c = 11.8310 (2) A ̊ and V = 180.16 (1) A ̊ 3 .