DEFECT CHEMICAL STUDY OF ND2-XCEXCUO4-SIGMA

Oxygen nonstoichiometry-delta and electrical conductivity-sigma were measured of Nd2-xCexCuO4-delta (x = 0, 0.05 and 0.10) at partial oxygen pressures (P(O2)) between 10(-3) and 1 atm at temperatures from 873 to 1273 K. For the sample with x = 0, delta was proportional to P(O2)-1/6 between 973 and 1073 K. Sigma showed the same oxygen pressure dependence above 973 K, while sigma was independent of oxygen pressure at low delta-extent. The defect structure has been interpreted in terms of a model assuming doubly charged oxygen vacancies (V(O)..) predominate in Nd2CuO4-delta at large values of delta above 973 K and that the intrinsic ionization predominates at near stoichiomctric composition (delta = 0). At 1173 and 1273 K where deviation from -1/6 power of P(O2) was observed, the concentration dependent activity coefficient of V(O).. has been taken into account by using regular solution approximation, and the interaction energy-OMEGA(v) and standard Gibbs free energy change of the V(O).. formation has been calculated. In addition, standard Gibbs free energy change for the intrinsic ionization reaction and the mobility of electrons were estimated. The defect model has been applied to the Ce doped systems. and the calculated delta-values show good agreement with experimental data. 11 is suggested that the carrier concentration in Ce doped systems is mainly governed by the concentration of Ce and that delta-values tend to decrease compared with non-doped samples. The fact that it is also necessary to heat-treat the oxides doped with Ce to x = 0.15-0.20 under low P(O2) atmospheres, strongly suggests the important role of oxygen vacancies or reduction of the Cu oxidation state to promote superconductivity.