Bonding heterogeneity and lone pair induced anharmonicity resulted in ultralow thermal conductivity and promising thermoelectric properties in n-type AgPbBiSe3AgPbBiSe_{3}

Efficiency in generation and utilization of energy is highly dependent on materials that have the ability to amplify or hinder thermal conduction processes. A comprehensive understanding of the relationship between chemical bonding and structure impacting lattice waves (phonons) is essential to furnish compounds with ultralow lattice thermal conductivity $(κ_{lat})$ for important applications such as thermoelectrics. Here, we demonstrate that the n-type rock-salt AgPbBiSe$_3$ exhibits an ultra-low $κ_{lat}$ of 0.5–0.4 W m$^{−1}$ K$^{−1}$ in the 290–820 K temperature range. We present detailed analysis to uncover the fundamental origin of such a low $κ_{lat}$. First-principles calculations augmented with low temperature heat capacity measurements and the experimentally determined synchrotron X-ray pair distribution function (PDF) reveal bonding heterogeneity within the lattice and lone pair induced lattice anharmonicity. Both of these factors enhance the phonon–phonon scattering, and are thereby responsible for the suppressed $κ_{lat}$. Further optimization of the thermoelectric properties was performed by aliovalent halide doping, and a thermoelectric figure of merit (zT) of 0.8 at 814 K was achieved for AgPbBiSe$_{2.97}$I$_{0.03}$ which is remarkable among n-type Te free thermoelectrics

Emphanisis in Cubic (SnSe)0.5(AgSbSe2)0.5(SnSe)_{0.5} (AgSbSe_{2})_{0.5} : Dynamical Off-Centering of Anion Leads to Low Thermal Conductivity and High Thermoelectric Performance

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)$_{0.5}$(AgSbSe$_2$)$_{0.5}$ 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)$_{0.5}$(AgSbSe$_2$)$_{0.5}$ 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)$_{0.5}$(AgSbSe$_2$)$_{0.5}$

Tuning of p–n–p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p–n–p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p–n–p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p–n–p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag_1–<i>x</i>CuS and AgCu_1–<i>x</i>S samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p–n–p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag⁺/Cu⁺) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag_1–<i>x</i>CuS and AgCu_1–<i>x</i>S samples exhibit ultralow thermal conductivity (∼0.3–0.5 W/m·K) in the 290–623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice

Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe$_2$, which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46–0.31 W m$^{−1}$ K$^{−1}$) in the 300–673 K. We established an intrinsic rattling nature in TlInTe$_2$ by studying the local structure and phonon vibrations using synchrotron X‐ray pair distribution function (PDF) (100–503 K) and inelastic neutron scattering (INS) (5–450 K), respectively. We showed that while 1D chain of covalently bonded [InTe$_2$]$^{-n}_n$ transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm$^{−1}$ inside distorted Thompson cage formed by [InTe$_2$]$^{-n}_n$. This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe$_2$

Phonon Localization and Entropy-Driven Point Defects Lead to Ultralow Thermal Conductivity and Enhanced Thermoelectric Performance in (SnTe)1–2x(SnSe)x(SnS)x\mathrm{(SnTe)_{1–2x}(SnSe)_{x}(SnS)_{x}}

Understanding of phase stability, chemical bonding, and phonon transport are essential to realize ultralow thermal conductivity in crystalline solids for designing high-performance thermoelectric (TE) materials. Pristine SnTe, a homologue of PbTe, exhibits poor TE performance primarily because of high lattice thermal conductivity, κlat. Herein, the amorphous limit of $κ_{lat}$ is achieved via engineering configurational and vibrational entropies in pseudoternary $\mathrm{(SnTe)_{1–2x}(SnSe)_{x}(SnS)_{x}}$. Density functional theory calculations and synchrotron X-ray pair distribution function analysis reveal that S atoms are locally off-centered in global cubic SnTe, resulting in a low-energy localized optical phonon which strongly couples with heat-carrying acoustic phonons. Additionally, substitution of Se and S in SnTe increases the configurational entropy and point defects, resulting in an ultralow $κ_{lat}$ of 0.52 W/mK. Finally, improvement of the Seebeck coefficient is achieved via the synergistic effect of resonant doping (In) and valence band convergence (Ag), which lead to a high TE figure of merit, zT, of ∼1.3 at 854 K

Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe$_2$, which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46–0.31 W m$^{−1}$ K$^{−1}$) in the 300–673 K. We established an intrinsic rattling nature in TlInTe$_2$ by studying the local structure and phonon vibrations using synchrotron X-ray pair distribution function (PDF) (100–503 K) and inelastic neutron scattering (INS) (5–450 K), respectively. We showed that while 1D chain of covalently bonded $[InTe_2]_{n}^{-n}$ transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm$^{−1}$ inside distorted Thompson cage formed by $[InTe_2]_{n}^{-n}$. This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe$_2$

Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe$_2$, which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46–0.31 W m$^{−1}$ K$^{−1}$) in the 300–673 K. We established an intrinsic rattling nature in TlInTe$_2$ by studying the local structure and phonon vibrations using synchrotron X‐ray pair distribution function (PDF) (100–503 K) and inelastic neutron scattering (INS) (5–450 K), respectively. We showed that while 1D chain of covalently bonded [InTe$_2$]$^{-n}_n$ transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm$^{−1}$ inside distorted Thompson cage formed by [InTe$_2$]$^{-n}_n$. This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe$_2$

Metavalent Bonding-Mediated Dual 6s26s^2 Lone Pair Expression Leads to Intrinsic Lattice Shearing in n-Type TlBiSe2TlBiSe_2

Metavalent bonding has attracted immense interest owing to its capacity to impart a distinct property portfolio to materials for advanced functionality. Coupling metavalent bonding to lone pair expression can be an innovative way to propagate lattice anharmonicity from lone pair-induced local symmetry-breaking via the soft p-bonding electrons to achieve long-range phonon dampening in crystalline solids. Motivated by the shared chemical design pool for topological quantum materials and thermoelectrics, we based our studies on a three-dimensional (3D) topological insulator TlBiSe$_2$ that held prospects for 6s$^2$ dual-cation lone pair expression and metavalent bonding to investigate if the proposed hypothesis can deliver a novel thermoelectric material. Herein, we trace the inherent phononic origin of low thermal conductivity in n-type TlBiSe$_2$ to dual 6s$^2$ lone pair-induced intrinsic lattice shearing that strongly suppresses the lattice thermal conductivity to a low value of 1.1–0.4 Wm$^{–1}$ K$^{–1}$ between 300 and 715 K. Through synchrotron X-ray pair distribution function and first-principles studies, we have established that TlBiSe$_2$ exists not in a monomorphous R-3m structure but as a distribution of distorted configurations. Via a cooperative movement of the constituent atoms akin to a transverse shearing mode facilitated by metavalent bonding in TlBiSe$_2$, the structure shuttles between various energetically accessible low-symmetry structures. The orbital interactions and ensuing multicentric bonding visualized through Wannier functions augment the long-range transmission of atomic displacement effects in TlBiSe$_2$. With additional point-defect scattering, a $κ_{latt}$ of 0.3 Wm$^{–1}$ K$^{–1}$ was achieved in TlBiSeS with a maximum n-type thermoelectric figure of merit (zT) of ∼0.8 at 715 K

Local ferroelectric polarization switching driven by nanoscale distortions in thermoelectric Sn0.7Ge0.3Te{\text {Sn}}_{0.7}{\text {Ge}}_{0.3}{\text {Te}}

A remarkable decrease in the lattice thermal conductivity and enhancement of thermoelectric figure of merit were recently observed in rock-salt cubic SnTe, when doped with germanium (Ge). Primarily, based on theoretical analysis, the decrease in lattice thermal conductivity was attributed to local ferroelectric fluctuations induced softening of the optical phonons which may strongly scatter the heat carrying acoustic phonons. Although the previous structural analysis indicated that the local ferroelectric transition temperature would be near room temperature in ${\text {Sn}}_{0.7}{\text {Ge}}_{0.3}{\text {Te}}$, a direct evidence of local ferroelectricity remained elusive. Here we report a direct evidence of local nanoscale ferroelectric domains and their switching in ${\text {Sn}}_{0.7}{\text {Ge}}_{0.3}{\text {Te}}$ using piezoeresponse force microscopy(PFM) and switching spectroscopy over a range of temperatures near the room temperature. From temperature dependent (250–300 K) synchrotron X-ray pair distribution function (PDF) analysis, we show the presence of local off-centering distortion of Ge along the rhombohedral direction in global cubic ${\text {Sn}}_{0.7}{\text {Ge}}_{0.3}{\text {Te}}$. The length scale of the Ge$^{2+}$ off-centering is 0.25–0.10 Å near the room temperatures (250–300 K). This local emphatic behaviour of cation is the cause for the observed local ferroelectric instability, thereby low lattice thermal conductivity in ${\text {Sn}}_{0.7}{\text {Ge}}_{0.3}{\text {Te}}$

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids

Understanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity $(κ_{lat})$ in crystalline materials. Herein, we demonstrate local cationic off-centring within the global rock-salt structure of AgSbSe$_2$ by using synchrotron X-ray pair distribution function analysis and unravel the origin of its ultralow $κ_{lat}$≈0.4 W mK$^{−1}$ at 300 K. The cations are locally off-centered along the crystallographic $\langle 100 \rangle$ 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 5s$_2$ 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