Doping control and thermoelectric properties in $R_{1-x}{A}_x$ZnSbO ($R$ $=$ La, Ce; $A$ $=$ Ca, Sr)

As the two-dimensional analogues of a conventional thermoelectric semiconductor ZnSb, we investigated the thermoelectric properties of layer-structured $R_{1-x}{A}_x$ZnSbO ($R$ $=$ La, Ce; $A$ $=$ Ca, Sr) using the polycrystalline samples with compaction density around 55$\%$, combining with the electronic band-structure calculation. We found that the carrier concentration of $R$ZnSbO can be controlled by substituting $A^{2+}$ for $R^{3+}$ in the charge-reservoir layers without lowering the carrier mobility. The hole-doped materials exhibited relatively low thermal conductivity and a moderately high Seebeck coefficient, whose temperature and doping dependence were well reproduced by the calculation. $\mathit{ZT}$ values were found to increase without showing saturation up to 390 K, and even higher values would be expected along the conducting ZnSb layers for a single crystal. These results indicate the potential of the hole-doped $R$ZnSbO as a good thermoelectric material.

Figure 1

(a) Schematic crystal structure of $R_{1-x}{A}_x$ZnSbO. (b) The x-ray-diffraction patterns in the range of ${20}^{°}⩽2\theta ⩽{45}^{°}$. The $\mathit{hkl}$ indices in the figure are based on the ZrCuSiAs-type structure. (c) The in-plane lattice constant $a$ and (d) the out-of-plane lattice constant $c$ are plotted against the $A^{2+}$ concentration ($x$).

Figure 2

(a) Band structure of LaZnSbO and (b) DOS calculated using the PBE functional. (c) DOS calculated with the HSE06 functional. The HSE06 (PBE) calculation exhibits a 0.6-eV (zero) band gap at the $\Gamma$ point.

Figure 3

$(\alpha E){}^2$ vs $E$ for the LaZnSbO single crystal calculated from the transmittance spectrum, where $\alpha$ and $E$ denote absorption coefficient and photon energy, respectively. Propagation vector and electric field ($E^{\omega }$) of light are perpendicular and parallel to the $\mathit{ab}$ plane, respectively.

Figure 4

(a) Temperature dependence of resistivity ($\rho$) for $R_{1-x}$Sr${}_x$ZnSbO. $\rho$ for the LaZnSbO single crystal was measured with current ($I$) parallel to the conduction ZnSb layers. (b) Resistivity $\rho$ for the $R_{1-x}{A}_x$ZnSbO polycrystalline samples at 390 K as a function of the $A^{2+}$ concentration ($x$).

Figure 5

(a) Hall resistivity (${\rho }_{\mathit{yx}}$) of the LaZnSbO single crystal as a function of the magnetic field ($H$) at various temperatures. For the purpose of clarity, the data for each temperature are arbitrarily offset. The value of ${\rho }_{\mathit{yx}}$ was measured with current ($I$) parallel to the conducting ZnSb layers and $H$ perpendicular to the layers. Temperature ($T$) dependence of (b) the carrier concentration as estimated from the Hall coefficient ($R_H$) and (c) the Hall mobility (${\mu }_H$) for the LaZnSbO single crystal and La${}_{1-x}$Sr${}_x$ZnSbO polycrystalline samples ($x=0$ and 0.05).

Figure 6

(a) Temperature dependence of the experimentally observed Seebeck coefficient ($S$) for $R_{1-x}$Sr${}_x$ZnSbO. (b) The observed $S$ at 390 K for $R_{1-x}{A}_x$ZnSbO as a function of the $A^{2+}$ concentration ($x$). (c) The calculated $S$ along the $\mathit{ab}$ plane ($S_{\mathit{ab}}$, solid line) and $c$ axis ($S_c$, dashed line) for La${}_{1-x}{A}_x$ZnSbO using the PBE functional are plotted against temperature. (d) The calculated $S_{\mathit{ab}}$, $S_c$, and $S_{\mathrm{av}}$ at 400 K as a function of $x$.

Figure 7

(a) Temperature dependence of thermal conductivity ($\kappa$) for LaZnSbO and Ce${}_{1-x}$Sr${}_x$ZnSbO ($x$ $=$ 0, 0.05, and 0.1). The electronic thermal conductivity (${\kappa }_{\mathrm{el}}$) was evaluated using the Wiedemann-Franz law and the lattice thermal conductivity (${\kappa }_{\mathrm{lat}}$) was obtained via ${\kappa }_{\mathrm{lat}}=\kappa -{\kappa }_{\mathrm{el}}$ for Ce${}_{0.95}$Sr${}_{0.05}$ZnSbO. (b) $A^{2+}$ concentration ($x$) dependence of $\kappa$ for $R_{1-x}{A}_x$ZnSbO at 390 K.

Figure 8

(a) Temperature dependence of the figure of merit ($\mathit{ZT}$) for $R_{1-x}$Sr${}_x$ZnSbO. (b) The $\mathit{ZT}$ values for $R_{1-x}{A}_x$ZnSbO at 390 K are plotted as a function of $x$.