Growth, structural, and physical properties of superconducting Nd2-xCexCuO4 crystals (original) (raw)
Single crystals of the n-type Nd-Ce-Cu-0 superconducting materials have been grown via a Aux 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% Nd203-Ce02, 50 mol%%uo 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 Nd~Ce Cu04, 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 rnaximurn I', occurring at x =0.14. The oxygen uptake and loss in the Nd material is similar to that measured on La2Cu04 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 dificult to achieve. The resistivity temperature dependence above T, is metalliclike, and linear above 150 K, in contrast to bulk ceramics. Finally, no evidence for rnagnetic ordering over the temperature range 4-350 K was observed for the superconducting Ce-doped Nd&CuO4 phase, whereas signs of magnetic ordering were found at 340 K for the undoped material. pounds (e.g. , insulating LazCuO4 doped with Sr leads to a 40 K superconducting La2 Sr CuO&, Refs. 6-8). The formation of Cu with a valence greater than 2 (i.e. , oxidation of Cu) implies the introduction of a hole charge state. These holes, or missing electrons, are the charge carriers in all these cuprate superconductors and they are called, for this reason, p type. Recently Tokura et al. discovered a new class of materials Nd2 Ce Cu04 which are n-type superconductors at 23 K. They did this by chemically reducing the Cu (i.e. , doping the system with electrons) in two steps: (1) the replacement of trivalent (Nd) by tetravalent ions (Ce), and (2) the removal of oxygen. This new superconducting phase still contains CuOz planes, but the carriers are electrons (n type) and not holes (p type). The n-type charcharacter has been determined by Hall and Seebeck measurements , ' and x-ray absorption measurements" on polycrystalline samples indicate the presence of Cu+ scaling with Ce. Additional evidence supporting n-type conductivity in these new electron-doped superconductors is that the replacement of oxygen by fluorine (i.e. , again a reduction of the Cu) also induces superconductivity. ' By replacing the Nd in the parent compound Nd2, Ce"Cu04by other rare earths (Pr, Sm, Eu), and the Ce by Th, additional new n-type superconducting phases have been synthesized. ' '