Dopant-site occupancies and superconducting properties of the Fe-substituted La2.5Nd0.5CaBa3(Cu1−xFex)7Oz perovskite

The structural and superconducting properties of single-phase Fe-substituted La 2.5 Nd 0.5 CaBa 3 (Cu 1− x Fe x ) 7 O z (LNCBCuFe) with 0.0⩽ x ⩽0.06 compounds having triple-perovskite structure are investigated using X-ray diffraction, a.c. susceptibility, d.c. magnetization, oxygen content and Mossbauer effect measurements. Mossbauer spectral analysis of x =0.03 sample displays unusual Fe-dopant site occupancies and the Cu(2) plane to Cu(1) chain site ratio in the LNCBCuFe are quite different from those of the usual Fe-doped YBa 2 Cu 3 O 7− δ . Specifically, we observe substantial occupation of a new chain-associated quasi-octahedral site, E, at 300 K which transforms into the well-known distorted tetrahedron chain site, A, on lowering the temperature to 78 K. The observed reduction of T c with increasing x in LNCBCuFe supports the view that the hole filling mechanism contributes predominantly to the suppression of superconductivity by Fe.