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

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 % Nd2O3"CeO2, 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 Nd2-xCexCuO4, 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 Tc occurring at x=0.14. The oxygen uptake and loss in the Nd material is similar to that measured on La2CuO4 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 Tc 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 Nd2CuO4 phase, whereas signs of magnetic ordering were found at 340 K for the undoped material.