Electronic conductivity, Seebeck coefficient, defect and electronic structure of nonstoichiometric La1−xSrxMnO3

In order to elucidate the relationship between the electrical properties and composition (d and x) of La1−xSrxMnO3+d, precise measurements were made on the conductivity, σ, and Seebeck coefficient, Q, for the oxide with 0≤x≤0.7 as a function of T and P(O2) up to 1273 K. Analysis was made for the high-temperature paramagnetic state using the nonstoichiometry data and defect and electronic structure models reported by the present authors. It was shown that σ and Q in the oxygen excess La1−xSrxMnO3+d (d>0) are fixed to the value at those of the stoichiometric oxygen content, d=0. In the oxygen deficient La1−xSrxMnO3+d, they are essentially determined by the mean Mn valence and temperature. The predominant electrical conduction was found to take place by the electron hopping on the eg↑ level of Mn. In La1−xSrxMnO3+d (d≤0) under the condition of z=x+2d≤1/3, σ is given by:σ=(2.8×106/T){2 (−z2−z+6)(6−18z)/(17−z)2+(−z2−z+6) (z2+18z+5)/(17−z)2} exp{(−Ea/(kT)}where the activation energy Ea=−0.59(3+z)+2.00 eV. For z≥1/3, it is given by:σ=(2.8×106/T) z(1−z) exp{(−Ea/(kT)}where Ea=−0.036(3+z)+0.16 eV. Q is also described essentially by this model. However, the effect of minority carrier conduction is clearly found in Q in addition to the major conduction on eg↑ level. The major carrier conduction is p-type and the minor carrier is n-type for z≤0.5 and vice versa for z≥0.5.