Tailoring local structural distortions and the associated ferroelectric instability in SnTe via Ge alloying resulted in ultralow lattice thermal conductivity which boosts zT to 1.6 at 721 K.

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 .

Organic-inorganic halide perovskites are intrinsically unstable when exposed to moisture and/or light. Additionally, the presence of lead in many perovskites raises toxicity concerns. Herein is reported a thin film of BaZrS3, a lead-free chalcogenide perovskite.Photoluminescence and X-ray diffraction measurements show that BaZrS3 is far more stable than methylammonium lead iodide (MAPbI3) in moist environments. Moisture-and lightinduced degradations in BaZrS3 and MAPbI3 are compared by using simulations and calculations based on density functional theory. The simulations reveal drastically slower degradation in BaZrS3 due to two factorsweak interaction with water, and very low rates of ion migration. BaZrS3 photo-detecting devices with photo-responsivity of ~46.5 mA W -1 are also reported. The devices retain ~60% of their initial photo-response after 4 weeks in ambient conditions. Similar MAPbI3 devices degrade rapidly and show ~95% decrease in photoresponsivity in just 4 days. The findings establish the superior stability of BaZrS3 and strengthen the case for its use in optoelectronics. New possibilities for thermoelectric energy conversion using these materials are also demonstrated.

Understanding the mechanism that correlates phonon transport with chemical bonding and solid-state structure is the key to envisage and develop materials with ultralow thermal conductivity, which are essential for efficient thermoelectrics and thermal barrier coatings. We synthesized thallium selenide (TlSe), which is comprised of intertwined stiff and weakly bonded substructures and exhibits intrinsically ultralow lattice thermal conductivity (κL) of 0.62–0.4 W/mK in the range 295–525 K. Ultralow κL of TlSe is a result of its low energy optical phonon modes which strongly interact with the heat carrying acoustic phonons. Low energy optical phonons of TlSe are associated with the intrinsic rattler-like vibration of Tl+ cations in the cage constructed by the chains of (TlSe2) n n–, as evident in low temperature heat capacity, terahertz time-domain spectroscopy, and temperature dependent Raman spectroscopy. Density functional theoretical analysis reveals the bonding hierarchy in TlSe which involves ionic interaction in Tl+–Se while Tl3+–Se bonds are covalent, which causes significant lattice anharmonicity and intrinsic rattler-like low energy vibrations of Tl+, resulting in ultralow κL.

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The key challenge for superior thermoelectric performance of SnTe is optimization of very high hole concentration (∼1021 cm−3) arising from inherent Sn vacancies. Partial control of charge carriers can be achieved by self-compensation via careful filling of the vacancies using excess Sn, although high thermal conductivity remained a concern. In this context, with deliberate doping, an anharmonicity in phonon dispersion can be generated to obtain a poor thermal conductivity. We report on point defects and soft phonon mode driven poor thermal conductivity in self-compensated Sn1.03Te with Mn doping. The obvious modification in the electronic band structure has been demonstrated by four times enhancement in thermopower for Sn0.93Mn0.1Te from Sn1.03Te, and metallic behavior of temperature dependent resistivity. The observed soft phonon mode and impurity localized mode in Raman spectra have been explained based on the created anharmonicity in Sn1.03Te crystal with Mn doping.

Superionic Cu 2-x Te (CT) is an interesting and emerging p-type thermoelectric (TE) material due to the existence of various polymorphic phases and crystal structures, which undergo several structural phase transitions. On the basis of the stoichiometry of the CT compounds, the structure parameters, the carrier concentration (n p ), and the thermal conductivity (κ) can be modulated for optimum TE performance. Further, the understanding of the fundamental properties and their impact on TE parameters is not well understood because of their complex structures. We have investigated the vibrational properties of CT compounds such as Cu 1.25 Te (CT1.25), Cu 1.6 Te (CT1.6), and Cu 2 Te (CT2) using temperature dependent Raman studies in the temperature range of 300−773 K. Several structural phases are probed through remarkably distinct spectra for the CT compounds. The temperature transitions are complex such as (i) eutectic melting into CuTe and Te for both CT1.6 (above ∼593 K) and CT1.25 (above ∼613 K) and (ii) the structural transition from trigonal to orthorhombic and cubic phase for CT2 (above ∼553 K), which are strongly manifested in the Raman study. Further, the role of n p in the Raman spectra has also been investigated. The intensity of the Raman modes (>100 cm −1 ) showed strong n p dependence due to strong plasmon−phonon coupling. The analysis of full width at half-maximum (fwhm) of Raman peaks and qualitative estimation of phonon lifetime (τ i ) showed that CT2 has the minimum lattice thermal conductivity.

The charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides. We report on the observation of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition in single crystal 1T-VSe 2 using the spectroscopy technique in the temperature range of 50-120 K. The room temperature Raman spectra showed a sharp A 1g mode ∼206 cm −1 along with two additional modes associated to E g ∼ 257 cm −1 symmetry and two-phonon (2 ph) ∼ 332 cm −1 interactions. The onset temperature of I-CDW and C-CDW is estimated from resistance measurements supported by magnetic measurements. Remarkably, at the onset of I-CDW ∼ 115 K, a significant enhancement in the intensity of the weak E g mode is observed along with emergence of a doubly degenerate E g (2) mode ∼144 cm −1 , while below 70 K, the emergence of the A 1g mode ∼170 cm −1 signifies the onset of C-CDW. The observation of a sudden rise in the intensity of the E g mode ∼257 cm −1 mode, below I-CDW, showed the involvement of electron-phonon coupling in 1T-VSe 2 .