SILVER RAISES THE T(C) OF PB-1212 TO 78-K

We report the first case of a substantial T(c) enhancement in a cuprat e superconductor by silver addition. The T(c) enhancement has been obs erved in several Pb-I 212 superconductors with the general formula (Pb 0.5Cu0.5)Sr2(R1-xCax)Cu2O7 where R=Y, Gd, Dy, Ho, Er or Tm. The result s are: (1) The highest T(c) was 78 K for (Pb0.5Cu0.5)Sr2(Y0.5Ca0.5)Cu2 O7. (2) T(c)'s were enhanced up to 15 K above those of the correspondi ng Ag-free samples. (3) The compounds with larger rare earths (and low er T(c)) show the greatest T(c) enhancement. (4) T(c) enhancement incr eases with Ca content and is highest at x= 0.5. (5) The T(c) enhanceme nt is larger with slower cooling rates. In fact, quenched samples (wit h Ag) showed a T(c) depressed compared to similarly prepared silver-fr ee samples. (6) EDX indicates that Ag enters the structure; the lattic e parameters change with silver addition; the a-axis decreases by 0.15 %, the c-axis increases by 0. 5%. (7) Silver helps in oxygen transport thereby enabling an optimal oxygen content to be achieved.