EMF STUDY ON THERMODYNAMIC INSTABILITY AND 1 8-ANOMALY OF (LA(1-X)M(X))(2)CUO4-DELTA (M=BA, SR AND CA) SOLID-SOLUTION/

The standard free energy, the standard enthalpy of formation, and the standard entropy, of a (La(1-x)M(x))(2)CuO4-delta (M = Ba, Sr and Ca) solid solution, were determined by the EMF (electromotive force) measu rement using a solid state cell. The standard free energy change of so lid solution formation from simple oxides was determined as a function of x. It was found that in the case of (La1-xBax)(2)CuO4-delta (LBCO) the solid solution is thermodynamically unstable in the compositional range 0.04<x<0.08 and the maximum instability energy is 3 kJ/mol. It was concluded that the thermodynamic instability results in the decrea ses in carrier concentration and T-c, and causes the so-called '1/8 an omaly'.