SYNTHESIS AND CHARACTERIZATION OF COMPOUNDS SR(2)RMCU(2)O(8-DELTA) (R=PR, ND, SM, EU, GD M=NB, TA)

The compounds Sr(2)RMCu(2)O(8-delta) with R = Pr, Nd, Sm, Eu, Gd and M = Nb, Ta have been synthesized and characterized. Rietveld structure refinements reveal that the title compounds crystallize in the so-call ed ''2112'' structural type of Ba(2)LaMCu(2)O(8-delta) (M = Nb,Ta). Wi th respect to the different crystal symmetries reported earlier for Ba 2LaNbCu2O8-delta, x-ray refinements of the crystal structure for all t he compounds were performed in comparison for the two crystal symmetri es: P4/mmm and I4/mcm. Neutron powder diffraction data for Sr2PrTaCu2O 8-delta confirmed the body centered tetragonal symmetry. An internally consistent set of the structural parameters is obtained for the whole series of the title compounds. Magnetic ordering of the Cu sublattice , as monitored on the (103)(m) reflection was observed below 190 K for Sr2PrTaCu2O8-delta. Although traces of superconductivity (< 0.2%) hav e been detected in Ba(2)LaM(1-x)W(x)Cu(2)O(8-delta) (x similar to 0.3, M =Nb,Ta) below 30 K, the superconducting impurity phase could not be resolved. The antiferromagnetic (AF) order of the rare-earth sublatti ce in this R-2112 system (e.g., T-N(Gd) = 2.18 K) appears to be simila r to that of the RBa(2)Cu(3)O(7-delta) series (e.g., T-N(Gd) = 2.29 K) ; however, the exceptional high AF order of Pr in Pr-123 (with 17 K) i s reduced to below 2.3 K for Sr(2)PrMCu(2)O(8-delta). The temperature and field dependence of the specific heat and the susceptibility is di scussed in terms of crystal held splitting derived from the RBa(2)Cu(3 )O(7-delta) compounds. Overall crystal held splitting in the title com pounds is comparable with that of the R-123 compounds.