The superconducting solid solutions Nd1.5−xBa1.5+x−yMyCu3Oz (M  Ca,Sr)

The cation stoichiometry ranges of the tetragonal Nd336:Ca,Sr solid solutions, Nd(1.5-x)Ba(1.5+x-y)M(y)Cu(3)O(z):M = Ca,Sr in air at 950 degrees C have been determined by means of a phase-diagram study. In both systems, -0.1 less than or equal to x less than or equal to 0.6 and 0 less than or equal to y less than or equal to 0.4, apart from compositions at high x/low y and low x/high y, which give phase mixtures, The distribution of Nd, Ba, and Sr/Ca over the A sites has been determined by Rietveld refinement of the 336 structure using X-ray powder diffraction data. In the Sr system, Sr substitutes directly for Ba; in the Ca system, Ca substitutes onto the Nd sites with a concomitant displacement of Nd onto the Ba sites. Samples were given a post-sinter anneal in O-2 at 400 degrees C, oxygen contents, determined by H-2 reduction TGA, vary from 6.73 to 7.03 depending on x and y. Maximum T-c values under these conditions in the two systems were 76 K (M = Ca for x = 0.5, y = 0.1) and 82 K (M = Sr for x = 0.5, y = 0.2 and x = 0.6, y = 0.4). The latter is the highest reported yet for any tetragonal structure in the 336 family. A comparison is made of the Nd and analogous La systems. In the La systems and in the Nd336:Ca system, there is an approximate correlation between T-c and average Cu valence, but none is apparent in the Nd336:Sr system.