Doping effects on the physicochemical and electrochemical properties of lanthanum manganite

The structural, thermal, electrical, and electrochemical properties of La0. 7Ca0.3Mn1-x(Co,Ni)(x)O-3 and La0.6Sr0.4Mn1-x(Co,Ni)(x)O-3 (x = 0, 0.02, 0.0 5, 0.1) electrode materials were studied. Doping of the cubic perovskites w ith Co or Ni increases the fraction of Mn4+ ions, up to 49% in La0.7Ca0.3Mn 0.9Co0.1O3 and 57% in La0.6Sr0.4Mn0.95Co0.05O3. The 300-K conductivity of L a0.7Ca0.3Mn1-xNixO3 passes through a maximum at x = 0.05, while that of La0 .6Sr0.4Mn1-xCoxO3 decreases steadily with increasing x. In the range 300-11 00 K, the conductivity of the Ca-containing manganites exhibits semiconduct ing behavior, whereas that of the Sr-containing materials shows metallic be havior. No phase transformations were detected in this temperature range. I n the four systems, the thermal-expansion coefficients are virtually indepe ndent of x. In both undoped and doped electrode materials, the resistance p arameter pld of electrode layers on solid-electrolyte substrates shows semi conducting behavior at temperatures from 300 to 1100 K and oxygen partial p ressures from 10(2) to 10(5) Pa. With increasing oxygen partial pressure or electrode-layer thickness (d = 15-100 mg/cm(2)), rho /d decreases. The opt imal electrode-layer thickness is found to be about 50 mg/cm(2). The introd uction of Co or Ni into the electrode materials decreases the polarization resistance of the electrode layer in gas/electrode/electrolyte systems. Com positions ensuring the lowest polarization resistance were found.