Disordering and mixed conductivity in the solid solution LaSr ₂Fe_3-yCr_yO_8+δ

Ceramic samples of LaSr₂Fe_3-yCr_yO _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 R3c structure. The loss of oxygen from the crystal lattice results in formation of the vacancy-ordered orthorhombic structure LaSr ₂Fe₃O₈ 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.