Effect of the spin and valence states of cobalt ions on the kinetic properties of Ho$_{1-x}$Sr$_x$CoO$_{3-\delta}$ and Er$_{1-x}$Sr$_x$CoO$_{3-\delta}$ compounds
Abstract:
The electrical conductivity and Seebeck effect in ceramics based on cobaltites Ho$_{1-x}$Sr$_x$CoO$_{3-\delta}$ ($x$ = 0.65, 0.75, 0.85, 0.95) and Er$_{1-x}$Sr$_x$CoO$_{3-\delta}$ ($x$ = 0.75, 0.85, 0.95) with a perovskite-like structure have been investigated in the temperature range $T >$ 77 K. All the compounds under study are characterized by the variable-range-hopping conductivity with the temperature dependence of the electrical resistivity corresponding to the Mott law. It has been found that, in the Ho$_{0.35}$Sr$_{0.65}$CoO$_{3-\delta}$ compound, thermally excited Co$^{3+}$ ions contribute to the electrical conductivity with an increase in temperature to 250 K. The Seebeck coefficient of the systems studied decreases as the strontium concentration and temperature increase. It has been shown that, for an adequate explanation of this behavior, proper allowance must be made for the splitting of the 3$d$ levels, as well as for the charge disproportionation of the cobalt ions.
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