Understanding the nature of chemical bonding and lattice dynamics together with their influence on phonon-transport is essential to explore and design crystalline solids with ultralow thermal conductivity for various applications including thermoelectrics. TlInTe, with interlocked rigid and weakly bound substructures, exhibits lattice thermal conductivity as low as ca. 0.5 W/mK near room temperature, owing to rattling dynamics of weakly bound Tl cations. Large displacements of Tl cations along the c-axis, driven by electrostatic repulsion between localized electron clouds on Tl and Te ions, are akin to those of rattling guests in caged-systems. Heat capacity of TlInTe exhibits a broad peak at low-temperatures due to contribution from Tl-induced low-frequency Einstein modes as also evidenced from THz time domain spectroscopy. First-principles calculations reveal a strong coupling between large-amplitude coherent optic vibrations of Tl-rattlers along the c-axis, and acoustic phonons that likely causes the low lattice thermal conductivity in TlInTe.

Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earth's crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi) sandwiched between two BiSe quintuple layers [Se-Bi-Se-Bi-Se], resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κ of ∼0.6 W/mK at 300 K), which is significantly lower than that of BiSe (κ of ∼1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi) (BiSe) . Phonon dispersion calculated from first-principles and the experimental low-temperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κ. These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κ. Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ∼ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature.

Understanding the origin of intrinsically low thermal conductivity is fundamentally important to the development of high-performance thermoelectric materials, which can convert waste-heat into electricity. Herein, we report an ultralow lattice thermal conductivity (ca. 0.4 W m(-1)  K(-1) ) in mixed valent InTe (that is, In(+) In(3+) Te2 ), which exhibits an intrinsic bonding asymmetry with coexistent covalent and ionic substructures. The phonon dispersion of InTe exhibits, along with low-energy flat branches, weak instabilities associated with the rattling vibrations of In(+) atoms along the columnar ionic substructure. These weakly unstable phonons originate from the 5s(2) lone pair of the In(+) atom and are strongly anharmonic, which scatter the heat-carrying acoustic phonons through strong anharmonic phonon-phonon interactions, as evident in anomalously high mode Grüneisen parameters. A maximum thermoelectric figure of merit (z T) of about 0.9 is achieved at 600 K for the 0.3 mol % In-deficient sample, making InTe a promising material for mid-temperature thermoelectric applications.

Fundamental understanding of the correlation between chemical bonding and lattice dynamics in intrinsically low thermal conductive crystalline solids is important to thermoelectrics, thermal barrier coating, and more recently to photovoltaics. Two-dimensional (2D) layered halide perovskites have recently attracted widespread attention in optoelectronics and solar cells. Here, we discover intrinsically ultralow lattice thermal conductivity (κ L ) in the single crystal of all-inorganic layered Ruddlesden−Popper (RP) perovskite, Cs 2 PbI 2 Cl 2 , synthesized by the Bridgman method. We have measured the anisotropic κ L value of the Cs 2 PbI 2 Cl 2 single crystal and observed an ultralow κ L value of ∼0.37−0.28 W/mK in the temperature range of 295−523 K when measured along the crystallographic c-axis. First-principles density functional theory (DFT) analysis of the phonon spectrum uncovers the presence of soft (frequency ∼18−55 cm −1 ) optical phonon modes that constitute relatively flat bands due to localized vibrations of Cs and I atoms. A further low energy optical mode exists at ∼12 cm −1 that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s 2 lone pair. We provide experimental evidence for such low energy optical phonon modes with low-temperature heat capacity and temperature-dependent Raman spectroscopic measurements. The strong anharmonic coupling of the low energy optical modes with acoustic modes causes damping of heat carrying acoustic phonons to ultrasoft frequency (maximum ∼37 cm −1 ). The combined effect of soft elastic layered structure, abundance of low energy optical phonons, and strong acoustic−optical phonon coupling results in an intrinsically ultralow κ L value in the all-inorganic layered RP perovskite Cs 2 PbI 2 Cl 2 .

The non-trivial electronic topology of a topological insulator is so far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E 2 g phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT)in Sb 2 Se 3 crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how non-adiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.