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Synergistic effect of the electronic band delocalization and bond anharmonicity on the thermoelectric performance of Cs2TeX6(X=Cl, Br, I)

Heena, Vineet Kumar Pandey, Saanvi Marethiya, Ambesh Dixit, Ajay Singh Verma, K. C. Bhamu

TL;DR

The study demonstrates that Cs$_2$TeI$_6$, a vacancy-ordered double perovskite, simultaneously exhibits favorable electronic band dispersion and ultralow lattice thermal conductivity, yielding a high thermoelectric figure of merit of $ZT = 1.97$ at 800 K under $n$-type doping. Using first-principles calculations with SOC and HSE06 gaps, together with semiclassical Boltzmann transport, the authors attribute the superior performance to a coexistence of heavy and light bands and to weak Te–I bonding plus Cs rattling that strongly scatter phonons. The work highlights a viable path to high-performance, lead-free TE materials within the double perovskite family, with Cs$_2$TeI$_6$ emerging as a prime candidate due to its relatively low optimal carrier concentration and robust transport properties. These insights advance understanding of how band structure engineering and lattice dynamics synergistically boost TE efficiency in complex oxides and halide perovskites.

Abstract

We investigate the structural, mechanical, and thermoelectric properties of lead-free double halide perovskites Cs2TeX6 (X = Cl, Br, I) using first-principles calculations and semiclassical Boltzmann transport theory. The HSE06 band gap is incorporated using the scissor correction method along with PBE calculated electronic band structures including spin orbit coupling to accurately predict transport properties. The band gap values are 3.27, 2.50, and 1.55 eV for Cs2TeX6 (X = Cl, Br, I), respectively. The coexistence of heavy and light bands in the Cs2TeI6 band structure helps mitigate the trade-off between the Seebeck coefficient and electrical conductivity. Among these systems, Cs2TeI6 exhibits superior performance with a ZT of 1.97 at 800 K and an electronic concentration of 3.35 x 10^19 cm^-3. Such a high ZT at relatively low carrier concentration arises from high electrical conductivity combined with low lattice thermal conductivity. The lattice thermal conductivity of Cs2TeI6 is found to be 0.41 W m^-1 K^-1 at room temperature. This low lattice thermal conductivity is attributed to weak Te-I bonding and non-uniform out-of-phase displacement of Cs atoms. The presence of local TeX6 units together with weak bonds strongly resists heat conduction, leading to significant suppression of lattice thermal conductivity. In particular, transverse acoustic phonons and optical phonons play a key role in limiting lattice thermal conductivity. These results identify Cs2TeI6 as a promising candidate for high performance thermoelectric applications.

Synergistic effect of the electronic band delocalization and bond anharmonicity on the thermoelectric performance of Cs2TeX6(X=Cl, Br, I)

TL;DR

The study demonstrates that CsTeI, a vacancy-ordered double perovskite, simultaneously exhibits favorable electronic band dispersion and ultralow lattice thermal conductivity, yielding a high thermoelectric figure of merit of at 800 K under -type doping. Using first-principles calculations with SOC and HSE06 gaps, together with semiclassical Boltzmann transport, the authors attribute the superior performance to a coexistence of heavy and light bands and to weak Te–I bonding plus Cs rattling that strongly scatter phonons. The work highlights a viable path to high-performance, lead-free TE materials within the double perovskite family, with CsTeI emerging as a prime candidate due to its relatively low optimal carrier concentration and robust transport properties. These insights advance understanding of how band structure engineering and lattice dynamics synergistically boost TE efficiency in complex oxides and halide perovskites.

Abstract

We investigate the structural, mechanical, and thermoelectric properties of lead-free double halide perovskites Cs2TeX6 (X = Cl, Br, I) using first-principles calculations and semiclassical Boltzmann transport theory. The HSE06 band gap is incorporated using the scissor correction method along with PBE calculated electronic band structures including spin orbit coupling to accurately predict transport properties. The band gap values are 3.27, 2.50, and 1.55 eV for Cs2TeX6 (X = Cl, Br, I), respectively. The coexistence of heavy and light bands in the Cs2TeI6 band structure helps mitigate the trade-off between the Seebeck coefficient and electrical conductivity. Among these systems, Cs2TeI6 exhibits superior performance with a ZT of 1.97 at 800 K and an electronic concentration of 3.35 x 10^19 cm^-3. Such a high ZT at relatively low carrier concentration arises from high electrical conductivity combined with low lattice thermal conductivity. The lattice thermal conductivity of Cs2TeI6 is found to be 0.41 W m^-1 K^-1 at room temperature. This low lattice thermal conductivity is attributed to weak Te-I bonding and non-uniform out-of-phase displacement of Cs atoms. The presence of local TeX6 units together with weak bonds strongly resists heat conduction, leading to significant suppression of lattice thermal conductivity. In particular, transverse acoustic phonons and optical phonons play a key role in limiting lattice thermal conductivity. These results identify Cs2TeI6 as a promising candidate for high performance thermoelectric applications.
Paper Structure (13 sections, 3 equations, 12 figures, 3 tables)

This paper contains 13 sections, 3 equations, 12 figures, 3 tables.

Table of Contents

  1. Abstract
  2. Introduction
  3. Computational methodology
  4. Results and Discussion
  5. Crystal structure optimization
  6. Bonding analysis
  7. Electronic properties
  8. Vibrational properties and electron localization function (ELF)
  9. Mechanical properties
  10. Transport properties
  11. Conclusions
  12. Acknowledgements
  13. Declaration of competing interest

Figures (12)

  • Figure 1: Crystal structure of vacancy ordered double halide perovskite (a) Cs$_2$TeX$_6$ (X = Cl, Br, I), (b) Octahedra structure (TeX$_6$)
  • Figure 2: Projected crystal orbital Hamilton population of vacancy ordered perovskite derivatives for Cl-Te bond in (a) Cs$_2$TeCl$_6$ (b) Cs$_2$TeBr$_6$ (c) Cs$_2$TeI$_6$
  • Figure 3: Electronic band structure along with DOS of (a) Cs$_2$TeCl$_6$ , (b) Cs$_2$TeBr$_6$, and (c) Cs$_2$TeI$_6$.
  • Figure 4: Phonon band structure along with DOS of (a) Cs$_2$TeCl$_6$ , (b) Cs$_2$TeBr$_6$, and (c) Cs$_2$TeI$_6$. Electron localization function of (110) plane passing through constituent atoms in (d) Cs$_2$TeCl$_6$ , (e) Cs$_2$TeBr$_6$, and (f) Cs$_2$TeI$_6$. The attached ELF scale of 0, 0.5, and 1.0 indicate electron depletion, delocalized and localized electron-like features, respectively.
  • Figure 5: Transport properties of Cs$_2$TeI$_6$ (a) electrical conductivity (b) Seebeck coefficient (c) electronic thermal conductivity (d) thermoelectric power factor for electron doping.
  • ...and 7 more figures