Growth, structural, and physical properties of superconducting Nd2−xCexCuO4 crystals

Abstract

Single crystals of the n-type Nd-Ce-Cu-O superconducting materials have been grown via a flux technique, and their structural and physical properties characterized. Optimum crystal growth conditions were arrived at from a survey of various compositions and temperatures. We determine that the charge composition (50 mol % ${\mathrm{Nd}}_2$ ${\mathrm{O}}_3$–${\mathrm{CeO}}_2$, 50 mol % CuO) held at a temperature of 1300 °C and followed by a slow cooling to 1000 °C produces plateletlike crystals of the n-type phase ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$, which, after being annealed at 900 °C in nitrogen, become superconducting. The limit of the Ce solubility is x=0.18, and only crystals having a Ce content between 0.14 and 0.17 were found to superconductors, with the maximum $T_c$ occurring at x=0.14. The oxygen uptake and loss in the Nd material is similar to that measured on ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ but occurs at higher temperatures (750 °C instead of 500 °C). The large Meissner fraction indicates bulk superconductivity. However, due to a complex microstructure or compositional (e.g., oxygen) inhomogeneities in the crystal, zero resistance is difficult to achieve. The resistivity temperature dependence above $T_c$ is metalliclike, and linear above 150 K, in contrast to bulk ceramics. Finally, no evidence for magnetic ordering over the temperature range 4–350 K was observed for the superconducting Ce-doped ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$ phase, whereas signs of magnetic ordering were found at 340 K for the undoped material.