EFFECT OF MO AND MO-CA SUBSTITUTION ON THE SUPERCONDUCTIVITY OF GDBA2CU3O7-DELTA

The structural and superconducting properties of (Gd1-x-yCayMox)Ba2Cu3 Oz samples are investigated using X-ray diffraction, ac susceptibility , electrical resistivity and oxygen content measurements. The effect o f increasing the Mo concentration in (Gd1-xMox)Ba2Cu3Oz changes the st ructure from orthorhombic to tetragonal accompanied by an increase in the normal state resistivity and decreases both the oxygen content and T-c which is attributed to hole filling by Mo. The substitution of Ca for Gd in (Gd0.85-yCayMo0.15)Ba2Cu3Oz provides proper matching betwee n the ionic radius and valence of Gd3+ (0.94 Angstrom) and the average ionic radius and valence of Mo4+ (0.70 Angstrom) and Ca2+ (0.99 Angst rom). As the Ca content increases, the T-c increases from 58.5 K (y = 0) to 86 K (y = 0.15, compensated oxide), closer to the value of 91 K for pure GdBa2Cu3O7-delta due to the balance between the hole filling by Mo and hole doping by Ca. (C) 1997 Elsevier Science B.V.