High thermoelectric figure of merit and thermopower in layered perovskite oxides

We predict high thermoelectric efficiency in the layered perovskite ${\mathrm{La}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, based on calculations (mostly ab initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit $ZT$ computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around ${10}^{20}{\mathrm{cm}}^{-3}$. The Seebeck thermopower coefficient is between 200 and $300\mu \mathrm{V}$/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the potential of this material is due to its lattice thermal conductivity of order 1 W/(K m); using a model based on ab initio anharmonic calculations, we interpret this low value as due to effective phonon confinement within the layered-structure blocks.

Figure 1

Sketch of the structure of LTO. The $a$ axis points into the page, the $b$ axis vertically, and the $c$ axis from left to right. The primitive cell is outlined.

Figure 2

Relaxation time $\tau (T,E)$ vs $T$ and $E$.

Figure 3

Energy-averaged scattering time $\tau \left(T\right)$ vs $T$.

Figure 4

Lattice thermal conductivity vs $T$ for ${\mathrm{BaTiO}}_3$ in bulk and film form.

Figure 5

The $a$ component of (top left to bottom right) the $ZT$, Seebeck, electrical conductivity, and thermal conductivity tensors vs $n$-type carrier density for $T$ between 300 and 1250 K (thick to thin lines) in steps of 50 K. Squares indicate the value at optimal doping (i.e., the one maximizing $ZT)$.

Figure 6

Components of the $ZT$, Seebeck (absolute value), electrical conductivity, and electronic thermal conductivity tensors vs $T$ at optimal $n$ doping for each $T$.

Figure 7

$ZT$ calculated from the inverse average of $\sigma$ and $\kappa$ tensors, and the trace of the Seebeck tensor (display conventions as in Fig. 5).

Figure 8

The $a$ component of $ZT$ calculated with $T$-dependent and constant relaxation time.

Figure 9

Components of $ZT$ and Seebeck vs $T$ at optimal $p$ doping for each $T$.