Electrical conductivity of Sr2-xCaxFeMnO5 (x=0, 1, 2)

Variation of the electrical conductivity due to changes in crystal structure in a series of oxygen-deficient perovskites, Sr2-xCaxFeMnO5 (x = 0, 1, 2), has been investigated. The correlation between structural order and various aspects of electrical conductivity is demonstrated using X-ray and electron diffraction, transmission and scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and variable-temperature electrical conductivity studies. The increase in structural order from Sr2FeMnO5 to CaSrFeMnO5 and Ca2FeMnO5 affects the cation oxidation states. The XPS data indicate that trivalent Mn is stabilized as the structural order increases. The crystallite size is also correlated with structural order, where the increase in structural order leads to a systematic increase in crystallite size. Electrical conductivity studies were performed in both argon and air atmosphere for all three materials in a wide temperature range, from 298 K (25 A degrees C) to 1073 K (800 A degrees C). At room temperature, there is a direct correlation between ordering and electrical conductivity. Variable-temperature conductivity studies lead to a remarkable observation, where the highest conductivity at elevated temperature belongs to the CaSr compound that has an intermediate degree of structural order. This indicates that there is an optimum degree of ordering that can lead to the highest conductivity at high temperature. Comparison between conductivity studies in air and argon atmosphere indicates that the conductivity of the highly ordered Ca-2 compound remains almost unaffected by the change in atmosphere from argon to air. However, the less-ordered CaSr and Sr-2 compounds show an increase in conductivity in air compared to the argon atmosphere. Remarkably, the upturn in the conductivity of the less-ordered materials is interrupted and a decrease is observed at high temperature in air, whereas the ordered Ca-2 compound shows increase in conductivity in the entire temperature range.