Authors: Wenjie Xiong | Hao Huang | Yu Wu | Xinji Xu | Geng Li | Zonglin Gu | Shuming Zeng
Source: Advanced Science (article, 2025-06-05)
Field: Materials Science — Materials Chemistry

Decoupling Ultralow Coherent and Particle‐Like Phonon Transport via Bonding Hierarchy in Soft Superionic Crystals Within the unified theoretical framework for thermal transport, the inherent interplay between coherent tunneling and propagative phonon mechanisms establishes an antagonistic relationship, thereby imposing fundamental limitations on suppressing lattice thermal conductivity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>L</mml:mi></mml:msub> <mml:annotation>$\rm \kappa _L$</mml:annotation></mml:semantics> </mml:math> . In this work, it is theoretically demonstrate that the superionic crystals X<sub>6</sub>Re<sub>6</sub>S<sub>8</sub>I<sub>8</sub> (X = Rb, Cs) exhibit ultralow glass-like and particle-like thermal conductivities. The weak interactions between free alkali metal ions X<sup>+</sup> (X = Rb, Cs) and I<sup>-</sup> anions induce pronounced lattice anharmonicity, which enhances phonon scattering and suppresses group velocities, thereby reducing the particle-like thermal conductivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>p</mml:mi></mml:msub> <mml:annotation>$\rm \kappa _p$</mml:annotation></mml:semantics> </mml:math> ). Concurrently, the significant bonding heterogeneity within and between the [Re<sub>6</sub>S<sub>8</sub>I<sub>6</sub>]<sup>4 -</sup> clusters promotes phonon band flattening and low-frequency phonon localization. The resulting discretized phonon flat bands substantially diminish the glass-like thermal conductivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>c</mml:mi></mml:msub> <mml:annotation>$\rm \kappa _c$</mml:annotation></mml:semantics> </mml:math> ). At room temperature, the total <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>L</mml:mi></mml:msub> <mml:annotation>$\rm \kappa _L$</mml:annotation></mml:semantics> </mml:math> of X<sub>6</sub>Re<sub>6</sub>S<sub>8</sub>I<sub>8</sub> (X = Rb, Cs) falls below 0.2 W m<sup>-1</sup> K<sup>-1</sup>. Furthermore, the bonding characteristics between X<sup>+</sup> and I<sup>-</sup> anions induce an anomalous cation mass-independent stiffening of low-frequency phonon branches in this system, resulting in counterintuitive thermal transport behavior. This work elucidates fundamental mechanisms governing heat transfer in ultralow <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>L</mml:mi></mml:msub> <mml:annotation>$\rm \kappa _L$</mml:annotation></mml:semantics> </mml:math> materials and establishes novel pathways for transcending conventional thermal conductivity limitations. Phonon ; Thermal conductivity ; Anharmonicity ; Condensed matter physics ; Materials science ; Quantum tunnelling ; Thermoelectric materials ; Lattice (music) ; Phonon scattering ; Chemistry ; Physics ; Thermodynamics Advanced Thermoelectric Materials and Devices ; Thermal Expansion and Ionic Conductivity ; Perovskite Materials and Applications