Electrical conductivity of Ca-substituted lanthanum manganites

The physico-chemical properties of substituted perovskites materials have been analyzed with the aim of studying the relationships between structure and properties in this class of materials. Investigations were carried out into the effect of substitution in lanthanum manganites La1-xCaxMnO3 (x = 0; 0.05; 0.1; 0.15) materials obtained through sol-gel method followed by heat treatment at low temperatures and X-ray diffraction, into the surface area thereof, as well as into transmission electron microscopy for purposes of morpho-structural characterization. The results indicated a well-crystallized Pm-3m perovskite-type structure, and 20 nm average crystallite sizes for all samples. By means of complex impedance measurements in the 20 Hz-2 MHz frequency range, the electrical conductivity was determined at temperatures between 30 degrees C and 120 degrees C; the results showed that the conductivity obeys a Jonscher's universal law. It was found that below 10 kHz, the dc component of the conductivity increases with temperature for all samples, indicating that electrical conduction processes are activated thermally, in agreement with Mott's variable-range hopping (VRH) model; the model parameters (hopping distance and hopping energy) were also determined. In the high frequency range (f > 200 kHz) the ac conductivity is attributed to charge carriers hopping between the nearest neighboring states, in agreement with the correlated barrier hopping (CBH) model. Using this model, the energy band gap values: 0.364 eV, 0.372 eV and 0.424 eV of the substituted samples were found.