Effect of substitution of Ca for Nd and Sr on the superconducting behaviour of NdBaSrCu 3O 7−δ

Abstract

The effect of substitution of Ca for Nd and Sr in NdBaSrCu 3O 7−δ has been studied. Substitution of 20 at.% Ca for Nd results in an increase in the superconducting transition temperature ( T c) from 64 K to 77 K indicating that the as-prepared NdBaSrCu 3O 7−δ is underdoped. Substitution of Ca for Sr, however, influences the transition temperature in a complex manner. T c increases initially up to 70 K upon substitution of Ca up to 10 at.% which is explained on the basis of the underdoped nature of the compound and internal charge-transfer phenomena. Substitution levels between 10–20 at.% of Ca for Sr, however, leave the transition temperature almost unaltered which is followed by a steady decrease of T c and then an upturn in it at 100 at.% i.e. for NdBaCaCu 3O 7−δ. This behaviour is explained on the basis of the probable site occupancy and solubility of the substituent ion.