Disordering and Mixed Conductivity in the Solid Solution LaSr2Fe3-yCryO8+δ

Ceramic samples of LaSr 2 Fe 3 - y Cr y O 8+ δ ( y = 0, 0.3, 0.6, and 1.0) were characterized with X-ray powder diffraction, thermal gravimetry, dilatometry, and total conductivity measurements in the temperature range 750−950 °C at oxygen partial pressures between 10 - 22 and 0.5 atm. When prepared in air, the doped ferrites adopt rhombohedral R 3̄ c structure. The loss of oxygen from the crystal lattice results in formation of the vacancy-ordered orthorhombic structure LaSr 2 Fe 3 O 8 for the parent ferrite at temperatures above 750 °C and oxygen pressures below about 10 - 5 atm while chromium substituted samples retain the rhombohedral structure. The isothermal ion conductivity is found to decrease with the increase in chromium doping from y = 0 to y = 0.6. However, an increase in the chromium content to y = 1 results in an increase in the oxygen ion conductivity to about 0.2 S/cm. This behavior is explained with a model where the oxygen ions coordinated to chromium are not excluded entirely from the transport mechanism.