Thermal conductivity, thermopower, and figure of merit of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$

We present a study of the thermal conductivity $\kappa$ and the thermopower $S$ of single crystals of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ with $0⩽x⩽0.3$. For all Sr concentrations, ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ has rather low $\kappa$ values. For the insulators $(x<0.18)$ this arises from a suppression of the phonon thermal conductivity by lattice disorder due to temperature- and/or doping-induced spin-state transitions of the Co ions. For larger $x$, the heat transport by phonons remains low, but an additional contribution from mobile charge carriers causes a moderate increase of $\kappa$. The thermopower of the low-doped crystals is positive and shows a pronounced maximum as a function of temperature. With increasing $x$, this maximum broadens strongly and its magnitude decreases. For the highest Sr content $(x=0.3)$, $S$ becomes negative in the intermediate temperature range. From $S$, $\kappa$, and the electrical resistivity $\rho$, we derive the thermoelectric figure of merit $Z=S^2∕\kappa \rho$. For intermediate Sr concentrations we find notably large values of $Z$, indicating that Co-based materials could be promising candidates for thermoelectric cooling.

Figure 1

(Color online) Thermal conductivity of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ as a function of temperature for different doping $x$.

Figure 2

(Color online) Thermal conductivity $\kappa$ of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ with $x=0.25$ as a function of temperature measured in $15\mathrm{T}$ (엯) and in zero magnetic field (-∙-). The solid line is the phononic contribution ${\kappa }_{\mathit{ph}}$ of the $x=0.25$ crystal, which is obtained by ${\kappa }_{\mathit{ph}}=\kappa \left(B\right)-LT\sigma \left(B\right)$ using the electrical conductivity $\sigma$ and the Lorenz number $L$. For comparison, $\kappa$ of the $x=0.04$ sample (∇) is also depicted. The inset shows the magnetic-field dependencies $\Delta \kappa \left(B\right)=\kappa \left(B\right)-\kappa (B=0)$ (엯, left $y$ scale) and $T\Delta \sigma \left(B\right)=T[\sigma \left(B\right)-\sigma (B=0)]$ (—, right $y$ scale) for $B=5$, 10, and $15\mathrm{T}$, respectively. The value of the Lorenz number is obtained from the field- and temperature-independent scaling factor between both quantities, i.e., via $L=\Delta \kappa \left(B\right)∕\left[T\Delta \sigma \left(B\right)\right]=2.9\times {10}^{-8}{\mathrm{V}}^2∕{\mathrm{K}}^2$.

Figure 3

(Color online) Thermopower $S$ of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ as a function of temperature for different doping $x>0$. For $x=0.25$ we show $S\left(T\right)$ also for different magnetic fields.

Figure 4

(Color online) Top: Thermopower $S$ as a function of temperature for five different crystals of nominally undoped $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$. Bottom: Room-temperature values $S$ $\left(300\mathrm{K}\right)$ as a function of the charge carrier concentration $n=x+2\delta$ for Sr-doped (엯) and pure (●) $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$. The lines are calculated via Eq. (1) (see text).

Figure 5

(Color online) Figure of merit of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ as a function of temperature for different doping $x$.