Electrical resistivity and thermopower measurements of the hole- and electron-doped cobaltites ${\mathit{Ln}\text{CoO}}_3$

Two perovskite cobaltites, ${\text{LaCoO}}_3$ and ${\text{DyCoO}}_3$, which are border compounds with respect to the Ln size, were investigated by the electric resistivity and thermopower measurements up to 800–1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by ${\text{Mg}}^{2+}$ or ${\text{Ti}}^{4+}$ ions. The experiments on the La-based compounds were complemented by magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding ${\text{Co}}^{3+}$ sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low-temperature resistivity, which is of Arrhenius type $\rho \sim \text{exp}\left(E_A/kT\right)$ for the hole $\left({\text{Co}}^{4+}\right)$ -doped samples, while an unusual dependence $\rho \sim 1/T^{\nu }$ $\left(n=8–10\right)$ is observed for the electron $\left({\text{Co}}^{2+}\right)$ -doped samples. At higher temperatures, additional hole carriers are massively populated in the ${\text{Co}}^{3+}$ background, leading to a resistivity drop. This transition become evident at $\sim 300\text{K}$ and 450 K and culminates at $T_{I-M}=540$ and 780 K for the La- and Dy-based samples, respectively. The electronic behaviors of the cobaltites in dependence on temperature are explained considering local excitations from the diamagnetic low-spin (LS) ${\text{Co}}^{3+}$ to close-lying paramagnetic high-spin (HS) ${\text{Co}}^{3+}$ states and subsequent formation of a metallic phase of the IS ${\text{Co}}^{3+}$ character through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such intermediate (IS) phase.

Figure 1

The inverse susceptibility in LaCoO3, demonstrating two regions of Curie-Weiss behavior. The squares refer to the measured susceptibility while circles are obtained by subtraction of the Pauli temperature-independent term ${\chi }_o\sim 0.0004\text{emu}/\text{mol}$. Open symbols are data taken from Ref. 15

Figure 2

(Color online) Electric resistivity and thermopower for $p$-type $\text{LaCoO3}$, ${\text{LaCo}}_{0.95}{\text{Ga}}_{0.05}{\text{O}}_3$, ${\text{LaCo}}_{0.98}{\text{Mg}}_{0.02}{\text{O}}_3$, and ${\text{LaCo}}_{0.95}{\text{Mg}}_{0.05}{\text{O}}_3$. The lanthanum site substituted systems ${\text{La}}_{0.99}{\text{Sr}}_{0.01}{\text{CoO}}_3$ and ${\text{La}}_{0.98}{\text{Sr}}_{0.02}{\text{CoO}}_3$ are added for comparison. The inset of upper panel shows an apparent activation energy defined as $E_A=k.d\left(\text{ln}\rho \right)/d\left(1/T\right)$.

Figure 3

(Color online) The measured magnetic susceptibility on ${\text{LaCoO}}_3$ and some Co site substituted samples. Lower panel shows a separation into two components, one of which is taken as a simple Curie term, ${\chi }_C=C/T$ (plotted here for ${\text{LaCo}}_{0.95}{\text{Gd}}_{0.05}{\text{O}}_3$ and ${\text{LaCo}}_{0.95}{\text{Mg}}_{0.05}{\text{O}}_3$ only). It appears that the susceptibility data after subtraction of this term merge together and show the same anomalies as undoped ${\text{LaCoO}}_3$.

Figure 4

(Color online) Electric resistivity and thermopower for $n$-type ${\text{LaCoO}}_3$, ${\text{LaCo}}_{0.98}{\text{Ti}}_{0.02}{\text{O}}_3$, ${\text{LaCo}}_{0.95}{\text{Ti}}_{0.05}{\text{O}}_3$. The data for $p$-type ${\text{LaCoO}}_3$ are added for comparison

Figure 5

(Color online) Electric resistivity and thermopower for ${\text{DyCo}}_{1-x}{M}_x{\text{O}}_3$, $M={\text{Mg}}^{2+}$, ${\text{Ti}}^{4+}$ systems.

Figure 6

(Color online) The resistivity in ${\text{LaCo}}_{0.98}{M}_{0.02}{\text{O}}_3$ $\left(T<300\text{K}\right)$ and ${\text{DyCo}}_{0.98}{M}_{0.02}{\text{O}}_3$ $\left(T<450\text{K}\right)$, where $M={\text{Mg}}^{2+}$, ${\text{Ti}}^{4+}$. The upper panel shows the linear dependence of an apparent activation energy in ${\text{Ti}}^{4+}$-doped systems $E_A=\nu kT$, compared to distinct dependence for the ${\text{Mg}}^{2+}$ doped systems. The lower plot of $\rho$ vs $1/T$ demonstrates the simple Arrhenius-type behavior for the ${\text{Mg}}^{2+}$ doping, while the data for the ${\text{Ti}}^{4+}$ doping are fitted by a power law dependence $\rho \sim 1/T^{\nu }$ ($\nu =8$ and 10). The distribution of hopping barriers that may explain such dependence is schematically shown in the inset (for details see the text).