Synthesis and electrophysical study of cobaltites of composition LnMIICoO3.5 (MII — Mg, Ca, Sr, Ba)

Abstract

This work is devoted to study of electrical properties, phases of LnMIICoO3.5 (MII – Mg, Ca, Sr, Ba) which structure resembles to the structure of perovskite. The phases were obtained by solid-phase synthesis in accordance with the ceramic technology. The phase composition was determined by X-ray phase analysis. The diffractograms of the powders were indexed, the lattice parameters and its symmetry class were determined. It was found that the new compounds had a tetragonal symmetry, in the unit cell of which 16 structural units were located. Since complex oxides have high melting temperatures of about 1500–2000 ºC, it is impossible to obtain good quality single crystals suitable for measurements. For this reason, the study of their thermodynamic and electrophysical properties was carried out on samples in the form of sintered powders. In this study, the conductivity isotherms were measured by impedance spectroscopy at different temperatures. Resistance hodographs on the complex plane were constructed. Using the method of equivalent circuits, the grain boundaries and the bulk resistances of the sample grains were determined. The results showed that the new compounds had a small grain-boundary resistance. Using the temperature dependences, the activation energies of the conductivity of new cobaltites were calculated. The activation energy of conductivity for these compounds was 0.113–0.184 eV. The character of the dependence of the conductivity at moderate temperatures showed their thermal activation. In the Arrhenius coordinates, these sections of the graph were described by a straight line. An increase in the activation energy of the conductivity in the series of Mg2+–Ca2+– Sr2+–Ba2+cations was observed. The introduction of an alkaline earth ion with a large ionic radius led to a local distortion of the crystal lattice. This changed the value of the splitting of the d-levels of the transition metal- cobalt. This change in the activation energy as the alkaline earth metal cation changes can be explained by the change in the width of the forbidden band. Also, the replacement of rare-earth elements ions with alkaline- earth metal ions led to a structural disorder, to an increase in the formation of equilibrium charged point defects in the crystal structure, in which the strength of the bond with trapped charge carriers by electrons or holes in point defects in the crystal lattice with different alkaline-earth ions was different.