Random aliovalent cations induce synergistic bonding for high thermoelectric performance in NaSn₂SbSe₄

Abstract

The dimensionless figure of merit ZT is the key metric for quantifying thermoelectric performance; however, its optimization is inherently limited by the intrinsic coupling between electrical and thermal transport properties. Herein, we perform first-principles calculations using the temperature-dependent effective potential method to investigate cation alloying in SnSe, where Sn²⁺ ions were randomly substituted by aliovalent cations - specifically Na⁺ and Sb³⁺ - a strategy that induces chemical bond synergy in NaSn₂SbSe₄, effectively decoupling electronic and phonon transport behaviors. Random cation occupation induces a mixed covalent-ionic bonding character, generating local bond-strength fluctuations that act as phonon-scattering centers. Furthermore, localized Sb-Se and Sn-Se antibonding states below the Fermi level correlate with the softening of low-frequency optical phonon branches. Combined with pronounced lattice anharmonicity, this phonon softening significantly enhances four-phonon scattering rates and suppresses the lattice thermal conductivity. Concurrently, aliovalent cation incorporation promotes electron delocalization between Sn-5p and Se-4p orbitals and strengthens covalent bonding character. This modified orbital hybridization alters band dispersion, reducing the hole effective mass while preserving high electrical conductivity in NaSn₂SbSe₄. The optimal balance between low lattice thermal conductivity and high power factor yields superior thermoelectric performance for NaSn₂SbSe₄ relative to SnSe across the entire temperature range, with a peak ZT of 0.88 at 800 K. This study establishes chemical bonding engineering as a promising strategy for enhancing thermoelectric performance and provides guidance for exploring high-performance high-entropy thermoelectric materials.

Keywords

Thermoelectric / NaSn₂SbSe₄ / chemical bonding / anharmonicity / first-principles

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