Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids

Abstract

Understanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity (κlat) in crystalline materials. Herein, we demonstrate local cationic off‐centring within the global rock‐salt structure of AgSbSe2 by using synchrotron X‐ray pair distribution function analysis and unravel the origin of its ultralow κlat≈0.4 W mK−1 at 300 K. The cations are locally off‐centered along the crystallographic ⟨100⟩ direction by about ≈0.2 Å, which averages out as the rock‐salt structure on the global scale. Phonon dispersion obtained by density functional theory (DFT) shows weak instabilities that cause local off‐centering distortions within an anharmonic double‐well potential. The local structural distortion arises from the stereochemically active 5s2 lone pairs of Sb. Our findings open an avenue for understanding how the local structure influences the phonon transport and facilitates the design of next‐generation crystalline materials with tailored thermal properties.

Hidden Local Symmetry Breaking in Silver Diamondoid Compounds is Root Cause of Ultralow Thermal Conductivity

“…It has been believed that the emphanisis phenomenon occurs only through the actions of the ns 2 lone pair of electrons associated with main-group metals and octahedral coordination environments such as Sn 2+ , Pb 2+ , and Sb 3+ , and has been observed in PbQ, SnQ (Q = S, Se, Te), rock-salt AgSbSe 2 , and cubic halide perovskites (CsSnBr 3 ). [33,[35][36][37] Significantly, in the diamondoid systems reported here, the mechanism does not involve ns 2 lone pairs, demonstrating that this phenomenon is more diverse than previously thought. Our findings expose the chimeric nature of Ag in these materials.…”

Advances and Challenges of AgSbSe2‐based Thermoelectric Materials

“…This local asymmetry is also supported by the presence of low-energy Raman active modes. [53] It is fascinating to figure out the precise positions of Ag and Sb atoms in AgSbSe 2 and to study the influence of ordering or disordering cations arrangements on its thermoelectric transport behaviors.…”

“…[56,57] Morelli et al clarified that the above mentioned "minimum thermal conductivity" in I-V-VI 2 semiconductors is limited solely by phonon-phonon interactions. [57] Many studies have been performed to rationalize such a high thermal resistance, [53,[57][58][59][60] and, as schematically shown in Figure 3, the underlying physical origins can be assigned to the following three reasons: 1) Highly disordered Ag/Sb positions give rise to effective scattering of heat carrying phonons. Ag and Sb atoms randomly and equally occupy the (0, 0, 0) site while Se atoms are exclusively located at the (1/2, 1/2, 1/2) site.…”

“…In AgSbSe 2 , the valence electron configuration of Sb is 5 s 2 5p 3 , wherein only 5p 3 electrons are sharing to form a polar covalent bond with Se valence electrons while the 5s 2 electrons of Sb becomes a lone pair. [53,60,62] Numerous studies [41,56,58,59,63,64] have unambiguously demonstrated that the strong hybridization and repulsion between the lone-pair 5s 2 electrons of the group V cation and the valence p orbitals of the group VI anion lead to soft phonon modes and therefore reduced thermal conductivity. 3) Strong phonon-phonon interactions resulting from the high degree of anharmonicity in SbÀ Se bonding arrangement.…”

Soft crystal lattice and large anharmonicity facilitate the self-trapped excitonic emission in ultrathin 2D nanoplates of RbPb2Br5

“…These results depict that the lattice parameters a and b are no longer equal in local scale for NPLs, as well as, for bulk samples and signify the presence of intrinsic local distortion in the RbPb 2 Br 5 lattice. 20 This local distortion in the average tetragonal structure may give rise to the strong lattice anharmonicity, 21,22 which will have impact on the luminescence properties. The selected area electron diffraction (SAED) pattern of the RbPb 2 Br 5 NPLs additionally confirms the single crystalline nature (Fig.…”