INTERNAL CHARGE-TRANSFER IN TETRAGONAL SUPERCONDUCTOR (CAXLA1-X)(BA1.75-XLA0.25+X)CU3OY

We report (CaxLa1-x) (Ba1.75-xLa0.25+x) Cu3Oy, that is a tetragonal su perconductor family within the 1:2:3 system existing for 0 less-than-o r-equal-to x less-than-or-equal-to 0.5. With constant Q (= 7.25, Q den oting the charge on the non copper cations) and constant y (y congruen t-to 7.1 ) the average and mobile hole densities do not change with x. Nevertheless, there is a big increase in T(c) with x. Besides, T(c) i s very sensitive to small changes in Q or in y. In addition, the resis tivity of low x compositions increases by an order of magnitude and ch anges from metallic to semiconductorlike behaviour upon small decrease in y. We attribute this unusual behaviour to La that substitutes for Ba. It attracts more oxygens in the plane of the ''chains'' that induc e the orthorhombic to tetragonal phase transition. These oxygens then promote additional internal electron transfer from the planes to the ' 'chains''.