Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Promising High Temperature Thermoelectric Performance of Alkali Metal-based Zintl phases X$_2$AgY (X = Na, K; Y = Sb, Bi): Insights from First-Principles Studies

Mohd Zeeshan, Indranil Mal, B K Mani

TL;DR

Problem: identify high-temperature thermoelectric materials with low lattice thermal conductivity and favorable electronic transport. Approach: perform first-principles calculations (DFT with SOC and mBJ, phonon plus Boltzmann transport with AMSET) on the alkali-metal Zintl phases $X_2$Ag$Y$ ($X= ext{Na}, ext{K}$; $Y= ext{Sb}, ext{Bi}$). Key results: dynamical stability and band gaps in the range $0.8$–$1.8$ eV; ultralow lattice thermal conductivities down to about $0.23$–$0.9$ W m$^{-1}$ K$^{-1}$ and high $ZT$ values at $700$ K (notably $ZT\sim 2.1$ for Na$_2$AgSb); underlying mechanisms include rattling-like avoided crossings, gap-induced phonon mode flattening in K-based compounds, and bonding heterogeneity. Significance: demonstrates that light alkali metals paired with Ag–Bi/Sb frameworks can achieve competitive high-temperature thermoelectrics and provides design guidelines for tuning heat and charge transport via composition and carrier engineering.

Abstract

In the quest for novel thermoelectric materials to harvest waste environmental heat, we investigate alkali metal-based Zintl phases X$_2$AgY (X = Na, K, and Y = Sb, Bi) utilizing first-principles methods. We obtain significantly low lattice thermal conductivity values ranging 0.9-0.5 W m$^{-1}$ K$^{-1}$ at 300~K, challenging established thermoelectric materials such as SnSe, PbTe, Bi$_2$Te$_3$ as well as other Zintl phases. We trace such astonishingly low values to lattice anharmonicity, large phonon scattering phase space, low phonon velocities, and lifetimes. In K-based materials, the low phonon velocities are further linked to flattened phonon modes arising from the gap in the optical spectrum. Furthermore, the existence of bonding heterogeneity could hamper heat conduction in these materials. In addition, an avoided crossing in the phonon dispersions suggesting rattling behavior, observed in all materials except Na$_2$AgSb, suppresses the dispersion of acoustic modes, further reducing the phonon velocities. When combined with electrical transport calculations, the materials exhibit high figure of merit values at 700~K, i.e., $ZT\sim2.1$ for Na$_2$AgSb, $1.7$ for Na$_2$AgBi, $0.9$ for K$_2$AgSb, and $1.0$ for K$_2$AgBi. Our predicted $ZT$ values are competitive with state-of-the-art thermoelectric materials such as Mg$_3$Sb$_2$, ZrCoBi, PbTe, SnSe, and as well as with contemporary Zintl phases. Our findings underscore the potential of light alkali metal atoms combined with Ag-Bi/Sb type frameworks to achieve superior thermoelectric performance, paving the way for material design for specific operating conditions.

Promising High Temperature Thermoelectric Performance of Alkali Metal-based Zintl phases X$_2$AgY (X = Na, K; Y = Sb, Bi): Insights from First-Principles Studies

TL;DR

Problem: identify high-temperature thermoelectric materials with low lattice thermal conductivity and favorable electronic transport. Approach: perform first-principles calculations (DFT with SOC and mBJ, phonon plus Boltzmann transport with AMSET) on the alkali-metal Zintl phases Ag (; ). Key results: dynamical stability and band gaps in the range – eV; ultralow lattice thermal conductivities down to about – W m K and high values at K (notably for NaAgSb); underlying mechanisms include rattling-like avoided crossings, gap-induced phonon mode flattening in K-based compounds, and bonding heterogeneity. Significance: demonstrates that light alkali metals paired with Ag–Bi/Sb frameworks can achieve competitive high-temperature thermoelectrics and provides design guidelines for tuning heat and charge transport via composition and carrier engineering.

Abstract

In the quest for novel thermoelectric materials to harvest waste environmental heat, we investigate alkali metal-based Zintl phases XAgY (X = Na, K, and Y = Sb, Bi) utilizing first-principles methods. We obtain significantly low lattice thermal conductivity values ranging 0.9-0.5 W m K at 300~K, challenging established thermoelectric materials such as SnSe, PbTe, BiTe as well as other Zintl phases. We trace such astonishingly low values to lattice anharmonicity, large phonon scattering phase space, low phonon velocities, and lifetimes. In K-based materials, the low phonon velocities are further linked to flattened phonon modes arising from the gap in the optical spectrum. Furthermore, the existence of bonding heterogeneity could hamper heat conduction in these materials. In addition, an avoided crossing in the phonon dispersions suggesting rattling behavior, observed in all materials except NaAgSb, suppresses the dispersion of acoustic modes, further reducing the phonon velocities. When combined with electrical transport calculations, the materials exhibit high figure of merit values at 700~K, i.e., for NaAgSb, for NaAgBi, for KAgSb, and for KAgBi. Our predicted values are competitive with state-of-the-art thermoelectric materials such as MgSb, ZrCoBi, PbTe, SnSe, and as well as with contemporary Zintl phases. Our findings underscore the potential of light alkali metal atoms combined with Ag-Bi/Sb type frameworks to achieve superior thermoelectric performance, paving the way for material design for specific operating conditions.

Paper Structure

This paper contains 10 sections, 5 equations, 9 figures, 1 table.

Table of Contents

  1. Introduction
  2. Computational Methods
  3. Results
  4. Structural Optimization
  5. Electronic Structure
  6. Phonons
  7. Lattice Thermal Conductivity
  8. Electrical Transport Properties
  9. Discussion
  10. Summary

Figures (9)

  • Figure 1: (a) Crystal structure of X$_2$AgY (X = Na/K, Y = Sb/Bi) in orthorhombic $Cmcm$ symmetry and the zigzag alignment of Ag-Sb/Bi, (b), (c), (d), and (e) are electronic structures of Na$_2$AgSb, Na$_2$AgBi, K$_2$AgSb, and K$_2$AgBi, respectively. The density of states is shown in arbitrary units.
  • Figure 2: Crystal orbital Hamiltonian population of (a) Na$_2$AgSb, (b) Na$_2$AgBi, (c) K$_2$AgSb, and K$_2$AgBi.
  • Figure 3: Phonon band structures and partial density of states of (a) Na$_2$AgSb, (b) Na$_2$AgBi, (c) K$_2$AgSb, and K$_2$AgBi. The density of states is shown in arbitrary units.
  • Figure 4: Average lattice thermal conductivity as a function of temperature for Na$_2$AgSb, Na$_2$AgBi, K$_2$AgSb, and K$_2$AgBi.
  • Figure 5: Cumulative lattice thermal conductivity in % as a function of phonon frequency at 300 K for (a) Na$_2$AgSb, (b) Na$_2$AgBi, (c) K$_2$AgSb, and (d) K$_2$AgBi. The phonon density of states in arbitrary units are overlaid.
  • ...and 4 more figures