Kozlovskii A. Kinetic effects in cobaltites A'1-xА''xCoO3-d (А' = Ho, Er, La; А'' = Sr, Ca, Ag).

The thesis is devoted to investigation of the effect of alkaline earths and silver doping on the transport and thermoelectrical properties of the rare earth cobaltites. Conductivity and Seebeck effect in ceramics based on systems A'1-xА''xCoO3-d (А' = Ho, Er; А'' = Sr, Ca) and (La1-xSrx)1-yAgyCoO3-d having the perovskite structure have been studied. A'1-xА''xCoO3-d (А' = Ho, Er; А'' = Sr, Ca; х = 0,15; 0,25; 0,35; 0,45; 0,55; 0,65; 0,75; 0,85; 0,95) compounds are characterized by hopping conduction with the temperature dependence of resistivity R(T) = R0exp(Т0/T)1/n (n = 1; 2; 3; 4). Depending on the variance of A cationic radii, different types of the hopping conduction are realized. In Но1-xSrxCoO3-d compounds, the contribution into the conductivity from the thermally excited ions Co3+ are determined as the temperature rises up to 190 - 270 K. The temperature dependences of thermopower for A'1-xА''xCoO3-d (А' = Ho, Er; А'' = Sr, Ca) systems in the temperature range 100 - 600 К have been studied. It has been shown that the 3d level splitting and charge disproportionation of ions Co3+ is to be taken into account to explain the effect of alkaline earths concentrations and temperature on the thermopower value. It was determined that the small hole polarons are charge carriers in Ag-doped cobaltites La0,8Ag0,2CoO3-d and (La0,85Sr0,15)0,75Ag0,25СоО3-d. The conduction in La0,8Ag0,2CoO3-d has hopping nature at temperatures lower 370 K. At temperatures higher 370 К conductivity was described by two channel model which takes into account of ions Co3+ transition into high spin state and appearance of delocalized eg-electrons. The charge disproportionation reaction of the thermally excited ions Co3+ results in the thermopower value decrease as the temperature rises. In (La0,85Sr0,15)0,75Ag0,25СоО3-d a transition from a band polaron conduction to a hopping polaron conduction with temperature increase was revealed.