[Paper Review] 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+1 more|ArXiv.org|Feb 19, 2025

Advanced Thermoelectric Materials and Devices4 citations

TL;DR

This work uses first-principles calculations to predict ultra-low lattice thermal conductivity and high thermoelectric performance (ZT up to ~2.1 at 700 K) in alkali-metal Zintl phases Na2AgSb/Na2AgBi and K2AgSb/K2AgBi, driven by lattice anharmonicity, rattling behavior, and bonding heterogeneity.

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.

Motivation & Objective

• Identify thermoelectric potential of X2AgY (X = Na, K; Y = Sb, Bi) Zintl phases using comprehensive first-principles simulations.
• Quantify lattice thermal conductivity and its microscopic origins (phonon scattering, lifetimes, anharmonicity).
• Evaluate electronic transport and estimate thermoelectric figure of merit (ZT) at high temperatures.
• Compare performance with established thermoelectric materials and related Zintl phases.
• Provide design insights for achieving low lattice thermal conductivity while maintaining good electronic transport.

Proposed method

• Perform DFT calculations with VASP using PAW pseudopotentials and PBE exchange-correlation, including spin-orbit coupling and mBJ band gaps.
• Compute phonon properties with Phonopy and Phono3py to obtain dispersion, DOS, lifetimes, Grüneisen parameters, and lattice thermal conductivity via Boltzmann transport equation in RTA.
• Calculate three-phonon scattering phase space with ShengBTE and obtain force constants from finite displacement methods.
• Assess bonding characteristics with COHP analysis via TB-LMTO-ASA.
• Predict electronic transport properties using AMSET with acoustic deformation potential, ionized impurity, and polar optical phonon scattering mechanisms.
• Analyze structure, stability, and bonding to relate to heat conduction and thermoelectric performance.

Experimental results

Research questions

• RQ1 What are the lattice thermal conductivity values and their temperature dependence for Na2AgSb, Na2AgBi, K2AgSb, and K2AgBi?
• RQ2 What microscopic mechanisms (phonon velocities, lifetimes, phase space, anharmonicity) drive low kappa_L in these compounds?
• RQ3 What are the predicted thermoelectric figures of merit (ZT) at high temperature (up to 700 K) for p-type carriers in these materials?
• RQ4 How do electronic transport properties (Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor) correlate with the observed phonon transport behavior?

Key findings

• Na2AgSb, Na2AgBi, K2AgSb, and K2AgBi exhibit exceptionally low lattice thermal conductivities, e.g., 0.9–0.4 W m^-1 K^-1 (Na2AgSb) and 0.56–0.23 W m^-1 K^-1 (K2AgBi) between 300–700 K.
• Avoided crossing (rattling behavior) suppresses acoustic mode dispersion in all but Na2AgSb, contributing to low phonon velocities.
• In K-based materials, a gap in optical modes and flattened phonon modes further reduce phonon velocities and kappa_L; bonding heterogeneity also hinders heat conduction.
• Calculated ZT values at 700 K for p-type carriers reach ~2.1 (Na2AgSb), ~1.7 (Na2AgBi), ~0.9 (K2AgSb), and ~1.0 (K2AgBi), competitive with Mg3Sb2, PbTe, SnSe, and other Zintl phases.
• Phonon lifetimes are predominantly governed by three-phonon scattering with acoustic modes having longer lifetimes (~1–1.5 ps) than optical modes; average lifetimes are often <3 ps.
• Grüneisen parameters are generally >1, indicating strong anharmonicity; Na2AgBi and K2AgBi show particularly high acoustic-region anharmonicity (gamma up to ~4).