The relation between c-axis lattice parameter and superconducting transition temperature of NdBa2Cu3Oy thin films

c-axis-oriented NdBa2Cu3Oy thin films on (100) SrTiO3 substrates were prepared by an off-axis rf sputtering method in different conditions. The superconducting property was measured by a standard DC four-probe technique, and the c-axis lattice parameter was determined from X-ray diffraction pattern. Under the optimum depositing temperature (740 degrees C < T-s < 780 degrees C) and pressure (1.2-1.8 x 10(-1) mbar, P-Ar/P-O2 = 4-2), thin films with T-c0 = 90 K in a deviation of 0.5 K were prepared reproducibly. A parabolic relation of T,, to the c-axis lattice parameter was observed, indicating an optimal oxygen content for the best superconductivity. The oxygen out-diffusion was investigated by in situ high-temperature X-ray diffraction in high-vacuum (similar to 10(-4) mbar). The result suggested that oxygen out-diffusion occurred below 770 K and resulted in an expansion of the c lattice parameter. The expansion coefficients of the c-axis for NdBa2Cu3Oy and the a-axis for SrTiO3 were determined from the result of the in situ high-temperature X-ray diffraction, (C) 1999 Elsevier Science B.V. All rights reserved.