Synthesis and properties of NbM2RCu2O8and TaM2RCu2O8phases (MBA, Sr: RPr, Nd, Sm, Eu and Gd)

NbM2RCu2O8and TaM2RCu2O8(MBa, Sr; RPr, Nd, Sm, Eu and Gd) phases were synthesized. Electron diffraction study of TaSr2EuCu2O8showed the presence of a weak superstructure (a= 82a07), like that found for a number of similar phases. Rietveld refinement of this structure based on X-ray powder diffraction data was carried out in the space groupP4/mmm(R1= 0.038, Rp= 0.037) since no superstructure reflections were observed. Various types of cation replacement were performed in order to increase the copper valence. Monophasic Nb(Ta)1, Ti,Sr2EuCu2O8samples were synthesized atP(O2) = 1 bar andT= 1050°C up tox= 0.1 (Nb) and 0.3 (Ta), but no superconductivity was detected down to 12 K. High pressure syntheses were carried out to obtain Nb(Ta)Sr2Eu1, Ca, Cu2O8phases, but they were found to have a narrow homogeneity range that provides no opportunity of inducing superconductivity in such a way.