Emphanisis in Cubic (SnSe)_0.5(AgSbSe₂)_0.5: Dynamical Off-Centering of Anion Leads to Low Thermal Conductivity and High Thermoelectric Performance

Abstract

The structural transformation generally occurs from lower symmetric to higher symmetric structure on heating. However, the formation of locally broken asymmetric phases upon warming has been evidenced in PbQ (Q = S, Se, Te), a rare phenomenon called emphanisis, which has significant effect on their thermal transport and thermoelectric properties. (SnSe)0.5(AgSbSe2)0.5 crystallizes in rock-salt cubic average structure, with the three cations occupying the same Wycoff site (4a) and Se in the anion position (Wycoff site, 4b). Using synchrotron X-ray pair distribution function (X-PDF) analysis, herein, we show the gradual deviation of the local structure of (SnSe)0.5(AgSbSe2)0.5 from the overall cubic rock-salt structure with warming, resembling emphanisis. The local structural analysis indicates that Se atoms remain in off-centered position by a magnitude of ∼0.25 Å at 300 K along the [111] direction and the magnitude of this distortion is found to increase with temperature resulting in three short and three long M–Se bonds (M = Sn/Ag/Sb) within the average rock-salt lattice. This hinders phonon propagation and lowers the lattice thermal conductivity (κlat) to 0.49–0.39 W/(m·K) in the 295–725 K range. Analysis of phonons based on density functional theory (DFT) reveals significant soft modes with high anharmonicity which involve localized Ag and Se vibrations primarily. Emphanisis induced low κlat and favorable electronic structure with multiple valence band extrema within close energy concurrently give rise to a promising thermoelectric figure of merit (zT) of 1.05 at 706 K in p-type carrier optimized Ge doped new rock-salt phase of (SnSe)0.5(AgSbSe2)0.5.

Dynamical Bond Formation in KNi2Se2

“…[2] Termed emphanisis, this behavior is prima facie surprising since the normal expectation is that symmetry increases on warming (i. e. decreases on cooling as in, e. g. the Jahn-Teller effect, though there are exceptions [3] ), whereas the opposite is observed in PbTe -there is a local decrease in symmetry at elevated temperature. Follow up work has shown that the local offcentering displacements are not random, but highly correlated [4] , with similar behavior observed in other chalcogenides [5] and even halide perovskites [6] .…”

“…the Jahn-Teller effect, though there are exceptions 3 ), whereas the opposite is observed in PbTe -there is a local decrease in symmetry at elevated temperature. Follow up work has shown that the local off-centering displacements are not random, but highly correlated 4 , with similar behavior observed in other chalcogenides 5 and even halide perovskites 6 .…”

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

“…This is therefore not a standard phase transition and has been referred to as emphanisis. [33,34] The local distortion of Ag introduces a few extremely low frequency optical phonons at ≈20 cm −1 , resulting in a strong acousticoptical phonon scattering and an ultralow thermal conductivity of 0.26 W m −1 K −1 at 850 K in AgGaTe 2 . Our theoretical analysis suggests a large difference in energy between the s and d electron orbitals of the Ag leads to the weak sd 3 orbital hybridization, which is the driving force of the local distortion of Ag.…”

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.…”