High thermoelectric performance of layered $\mathrm{LaAg}X\mathrm{O} (X=\mathrm{Se},\mathrm{Te})$ from electrical and thermal transport calculations

The present paper reports the electronic structure, thermal and electronic transport properties of layered oxychalcogenides $\mathrm{LaAg}X\mathrm{O}$ $(X=\mathrm{Se},\mathrm{Te})$ using density functional theory. Different scattering mechanisms, such as acoustic deformation scattering (ADP), ionized impurity scattering (IMP), and polar optical scattering (POP) are included to calculate the scattering rates at various doping concentrations and temperatures. The calculated scattering rates are used in the Boltzmann transport equation to get the absolute values of thermoelectric coefficients. The Seebeck coefficient of both the compounds is nearly $400 \mu \mathrm{V}/\mathrm{K}$ for optimal $p$-type doping. The lattice thermal conductivity of both $\mathrm{LaAg}X\mathrm{O}$ is ultralow with values around 0.20 W/mK along the “$c$” axis at 300 K due to low lifetime and low group velocity. This is lower than other well-known thermoelectric materials, such as PbTe and SnSe. Rattling motion observed in the Ag-Te tetrahedral layer might be the reason for the significant suppression of ${\kappa }_l$. We predict huge $ZT$ values of 1.63 for $p$-type and 2.8 for $n$-type LaAgTeO at 900 K, which are higher than that of promising thermoelectric materials, such as BiCuSeO (1.4) and LaCuSeO (2.71). There is a crossing in phonon band dispersion which forms a nodal line on the 001 plane that may lead to topological behavior. This study highlights $\mathrm{LaAg}X\mathrm{O}$ as potential thermoelectric materials for future device applications.

Figure 1

Crystal structure for $\mathrm{LaAg}X\mathrm{O}$ ($X$ = Se, Te) (a) layered form, (b) conventional unit cell and partial charge density for (c) the valence-band minimum (VBM) and (d) the conduction-band maximum (CBM). La, Ag, $X$, and O atoms are shown in green, blue, violet, and red spheres.

Figure 2

Electron localization function for (a) LaAgSeO and (b) LaAgTeO along the (100) plane.

Figure 3

Calculated potential-energy curves as a function of atom displacement from equilibrium configurations of (a) LaAgSeO and (b) LaAgTeO.

Figure 4

(a) and (c) Electronic band structure and density of states for LaAgSeO and for (b) and (d) LaAgTeO.

Figure 5

(a) and (c) Phonon dispersion and phonon density of states for LaAgSeO and for (b) and (d) LaAgTeO.

Figure 6

Calculated (a) lattice thermal conductivity $({\kappa }_l$) and (b) specific-heat conductivity $(c_v$) for $\mathrm{LaAg}X\mathrm{O}$.

Figure 7

The calculated group-velocity components (a) and (b) for LaAgSeO and (c) and (d) for LaAgTeO along the $a$ and $c$ directions.

Figure 8

The calculated lifetime for investigated compounds $\mathrm{LaAg}X\mathrm{O}$.

Figure 9

The calculated mean free path for investigated compounds $\mathrm{LaAg}X\mathrm{O}$.

Figure 10

The calculated Gruneisen parameter for investigated compounds $\mathrm{LaAg}X\mathrm{O}$.

Figure 11

The calculated cumulative lattice thermal conductivity as a function of (a) phonon mean free path and (b) frequency of investigated compounds $\mathrm{LaAg}X\mathrm{O}$.

Figure 12

(a) Crossing of phonon bands at the inversion points and schematic of nodal line (inside) (b) representation of nodal line in the Brillouin zone.

Figure 13

Scattering rates as a function of temperature for $p$-type and $n$-type carriers at fixed doping concentration ${10}^{19}{\mathrm{cm}}^{-3}$ in (a) and (c) for LaAgSeO and (b) and (d) for LaAgTeO.

Figure 14

The thermoelectric coefficients of $\mathrm{LaAg}X\mathrm{O}$, (a) and (b) Seebeck coefficient ($S$), and (c) and (d) electrical conductivity for LaAgSeO and LaAgTeO, respectively.

Figure 15

Mobility as a function of temperature for $p$-type and $n$-type carriers at fixed doping concentration ${10}^{19}{\mathrm{cm}}^{-3}$ in (a) and (c) for LaAgSeO and (b) and (d) for LaAgTeO.

Figure 16

Calculated electronic thermal conductivity of (a) $p$-type and (b) $n$-type $\mathrm{LaAg}X\mathrm{O}$.

Figure 17

The calculated power factor (a) and (b) and thermoelectric figure of merit (c) and (d) of $\mathrm{LaAg}X\mathrm{O}$ at $T$= 900 K for both $p$-type and $n$-type carriers.