Synthesis and characterization of compounds ${\mathrm{Sr}}_2$RM${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$ (R=Pr, Nd, Sm, Eu, Gd; M=Nb, Ta)

The compounds ${\mathrm{Sr}}_2$RM${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$ with R=Pr,Nd,Sm,Eu,Gd and M=Nb,Ta have been synthesized and characterized. Rietveld structure refinements reveal that the title compounds crystallize in the so-called ‘‘2112’’ structural type of ${\mathrm{Ba}}_2$LaM${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$ (M=Nb,Ta). With respect to the different crystal symmetries reported earlier for ${\mathrm{Ba}}_2$${\mathrm{LaNbCu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$, x-ray refinements of the crystal structure for all the compounds were performed in comparison for the two crystal symmetries: P4/mmm and I4/mcm. Neutron powder diffraction data for ${\mathrm{Sr}}_2$${\mathrm{PrTaCu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\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${)}^{\mathit{m}}$ reflection was observed below 190 K for ${\mathrm{Sr}}_2$${\mathrm{PrTaCu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$.

Although traces of superconductivity (<0.2%) have been detected in ${\mathrm{Ba}}_2$La${\mathit{M}}_{1\mathrm{-}\mathit{x}}$${\mathrm{W}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$ (x∼0.3,M=Nb,Ta) below 30 K, the superconducting impurity phase could not be resolved. The antiferromagnetic (AF) order of the rare-earth sublattice in this R-2112 system (e.g., ${\mathit{T}}_{\mathit{N}}^{\mathrm{Gd}}$=2.18 K) appears to be similar to that of the R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ series (e.g., ${\mathit{T}}_{\mathit{N}}^{\mathrm{Gd}}$=2.29 K); however, the exceptional high AF order of Pr in Pr-123 (with 17 K) is reduced to below 2.3 K for ${\mathrm{Sr}}_2$PrM ${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$. The temperature and field dependence of the specific heat and the susceptibility is discussed in terms of crystal field splitting derived from the R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ compounds. Overall crystal field splitting in the title compounds is comparable with that of the R-123 compounds.