EFFECT OF SUBSTITUTION OF CA FOR ND AND SR ON THE SUPERCONDUCTING BEHAVIOR OF NDBASRCU3O7-DELTA

The effect of substitution of Ca for Nd and Sr in NdBaSrCu3O7-delta ha s been studied. Substitution of 20 at.% Ca for Nd results in an increa se in the superconducting transition temperature (T-c) from 64 K to 77 K indicating that the as-prepared NdBaSrCu3O7-delta is underdoped. Su bstitution of Ca for Sr, however, influences the transition temperatur e in a complex manner. T-c increases initially up to 70 K upon substit ution of Ca up to 10 at.% which is explained on the basis of the under doped nature of the compound and internal charge-transfer phenomena. S ubstitution 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, and then an upturn in it at 100 at.% i.e. for NdBaCaCu 3O7-delta. This behaviour is explained on the basis of the probable si te occupancy and solubility of the substituent ion.