Synthesis and characterization of La_2-yY_yCuO₄ with T'-structure

Superconductivity is a phenomenon when the resistivity becomes zero below the critical temperature (T _c). The appearance of superconductivity in undoped bulk samples of T'-La_1.8-xEu_0.2 M_x CuO₄ (M = Sr, Ca; x = 0.05, 0.10, 0.15) has altered the existing phase diagram in superconducting cuprates and raised big questions about the mechanism of superconductivity. Here, new materials of T'-La_2-yY _y CuO₄ with y = 0.05, 0.10, 0.15 have been synthesized by the low-temperature synthesis method in order to grow undoped superconductors with the single phase of the T'-structure. The powder x-ray diffraction analysis was performed to identify of the crystal structure, while magnetic susceptibility measurements were carried out using a SQUID magnetometer at low temperature down to 2 K. It has been found that the T'-structure is stabilized when it was annealed at 500 - 600°C. The c-axis length decreases with increasing Yttrium-concentration. For y = 0.10, the single phase of the T'-structure was obtained by the oxidation annealing at 300°C and the succeeding reduction annealing at 500°C for 24 hours. From magnetic susceptibility measurements, it has been found that no trace superconductivity is observed in all samples, which is probably due to the remaining excess oxygen suppressing the appearance of superconductivity in the sample.