Rotational Soft Modes and Octahedral Distortion as Design Principles for Ultralow Thermal Conductivity in Halide Materials

Abstract

We establish that ultralow lattice thermal conductivity in halide perovskites and related octahedral framework materials arises from two distinct and complementary mechanisms: (i) halogen-halogen-enabled rotational soft modes that reshape the low-frequency spectrum and intensify phonon scattering, and (ii) static octahedral distortions that further enhance anharmonicity and reduce phonon lifetimes. Using first-principles calculations on CsPbBr3, we demonstrate that Br-Br interactions induce rotational soft modes that decongest the phonon spectrum and enhance three- and four-phonon scattering, strongly suppressing particle-like thermal conductivity (kappa_p). Independently, static octahedral distortions further reduce kappa_p by amplifying anharmonicity while leaving wave-like conductivity (kappa_c) intact. Based on these mechanistic insights, we introduce a geometric distortion factor rho and perform a high-throughput screening that first selects materials with halogen-coordinated octahedral building blocks-ensuring the presence of rotational soft modes-and then identifies those with pronounced distortion. This strategy uncovers TaGaI8 with an ultralow kappa_L = 0.11 W/mK at room temperature. This work establishes halogen-halogen-enabled rotational soft modes and octahedral distortions as transferable design principles for octahedra-containing halides, spanning both extended frameworks and molecular-cluster motifs, for discovering ultralow-kappa_L materials.

Figures (5)

• Figure 1: (a) Phonon spectra of CsPbBr$_3$ comparing the pristine system with the case after weakening Br–Br interactions by half at 600 K. The low-frequency rotational soft modes exhibit significant hardening upon weakening the interaction. (b) Substitution analysis of particle-like thermal conductivity ($\kappa_p$) performed on the pristine system by individually replacing its heat capacity ($c$), group velocity ($v$), and phonon lifetime ($\tau$) with the corresponding values obtained from the weakened system. (c) Comparison of group velocities before and after weakening. (d) Weighted three-phonon phase space (WP$_3$) before and after weakening. (e) Weighted four-phonon phase space (WP$_4$) before and after weakening. (f) Grüneisen parameters before and after weakening.
• Figure 2: (a) Thermal conductivity components ($\kappa_p$, $\kappa_c$, and total $\kappa_L = \kappa_p + \kappa_c$) as functions of Pb displacement magnitude along the $x$ direction relative to the lattice constant. (b) Substitution analysis of $\kappa_p$ performed on the pristine system by individually replacing its heat capacity ($c$), group velocity ($v$), and phonon lifetime ($\tau$) with the corresponding values obtained from the system with 0.05 displacement. (c) Comparison of phonon spectra between the pristine system and the system with 0.05 Pb displacement. (d) Weighted three-phonon phase space (WP$_3$) before and after distortion. (e) Weighted four-phonon phase space (WP$_4$) before and after distortion. (f) Grüneisen parameters before and after distortion.
• Figure 3: Schematic illustration of the dual mechanisms governing ultralow lattice thermal conductivity in halide perovskites: halogen-atom interaction-driven rotational soft modes and octahedral distortions.
• Figure 4: (a) Crystal structure of TaGaI$_8$, featuring a zero-dimensional (0D) molecular motif composed of isolated clusters, each containing a distorted TaI$_6$ octahedron and a GaI$_4$ tetrahedron sharing two iodine atoms. (b) Phonon spectrum of TaGaI$_8$ projected by the Grüneisen parameter, revealing low-frequency soft modes with strong anharmonicity.
• Figure 5: (a) Temperature-dependent thermal conductivity components ($\kappa_p$, $\kappa_c$, and total $\kappa_L$) of TaGaI$_8$ calculated with three-phonon (3ph) and three-plus-four-phonon (3+4ph) scattering. (b) Phonon lifetimes at 300 K comparing 3ph and 3+4ph scattering. Dashed lines indicate the Ioffe-Regel ($\tau = \omega^{-1}$) and Wigner ($\tau = \Delta \omega_{\text{avg}}^{-1}$) limits separating particle-like and wave-like transport regimes.$^{22}$ (c) Four-phonon scattering phase space ($P_4$) decomposed into absorption, splitting, and redistribution processes. (d) Cumulative and differential contributions to $\kappa_p$ and $\kappa_c$ at 300 K (3+4ph).