POSITIVE ROLE OF POTASSIUM IN NEUTRALIZING THE NONSTOICHIOMETRY EFFECTS IN TL-2212 - SYNTHESIS AND MICROWAVE INVESTIGATIONS

Tl2Ba2CaCu2On, (designated as Tl-2212) has been synthesized using a pr ecursor matrix reaction method. The superconducting phase with T-c= 10 1 K was confirmed by microwave methods. A series of compounds Tl2Ba2-y CaCu2On, with barium deficiency (y = 0.0-0.75) were also prepared by t he same route. The value of T-c was found to monotonically decrease wi th deficiency of barium. Typically the value of T-c reduced by about 2 0 K when barium concentration per formula unit reduced from 2.0 to 1.2 5 (keeping the concentration of all other compounds constant). The who le series of compounds were found to exhibit a single superconducting transition by microwave investigations. It was found that potassium su bstitution in barium deficient compounds enhanced the T( )nearly to it s original value. Further it was shown that even in thallium deficient compounds the lowering of T-c was arrested and restored to its origin al value by substituting it with potassium. Thus it was shown that pre sence of potassium in T1-2212 has unique role of stabilizing the T-c a gainst the deteriorating effects due to possible Tl/Ba nonstoichiometr y. X-ray diffraction data suggests that thallium substitution occurs a t barium site in the former case, whereas potassium substitutes at bar ium and thallium sites and the extent of substitution is dependent on the relative concentrations of thallium and barium in the starting com position. (C) 1997 Elsevier Science Ltd.