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 origin of the luminescence properties of CsPb2Br5 is ascribed to the presence of different amorphous lead bromide amine complexes.

Chemical bonding present in crystalline solids has a significant impact on how heat moves through a lattice, and with the right chemical tuning, one can achieve extremely low thermal conductivity. The desire for intrinsically low lattice thermal conductivity (κ lat ) has gained widespread attention in thermoelectrics, in refractories, and nowadays in photovoltaics and optoelectronics. Here we have synthesized a high-quality crystalline ingot of cubic metal halide CuBiI 4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κ lat of ∼0.34−0.28 W m −1 K −1 in the temperature range 4−423 K with an Umklapp crystalline peak of 1.82 W m −1 K −1 at 20 K, which is surprisingly lower than other copper-based halide or chalcogenide materials. The crystal orbital Hamilton population analysis shows that antibonding states generated just below the Fermi level (E f ), which arise from robust copper 3d and iodine 5p interactions, cause copper−iodide bond weakening, which leads to reduction of the elastic moduli and softens the lattice, finally to produce extremely low κ lat in CuBiI 4 . The chemical bonding hierarchy with mixed covalent and ionic interactions present in the complex crystal structure generates significant lattice anharmonicity and a low participation ratio in low-lying optical phonon modes originating mostly from localized copper−iodide bond vibrations. We have obtained experimental evidence of these low-lying modes by low-temperature specific heat capacity measurement as well as Raman spectroscopy. The presence of strong p−d antibonding interactions between copper and iodine leads to anharmonic soft crystal lattice which gives rise to low-energy localized optical phonon bands, suppressing the heat-carrying acoustic phonons to steer intrinsically ultralow κ lat in CuBiI 4 .

As the periodic atomic arrangement of a crystal is made to a disorder or glassy-amorphous system by destroying the long-range order, lattice thermal conductivity, κL, decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like κL in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (~0.20 W/m·K at room temperature) and glass-like temperature dependence (2–400 K) of κL in a single crystal of layered halide perovskite, Cs3Bi2I6Cl3. Acoustic phonons with low cut-off frequency (20 cm−1) are responsible for the low sound velocity in Cs3Bi2I6Cl3 and make the structure elastically soft. While a strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function evidence a local structural distortion in the Bi-halide octahedra and Cl vacancy. The hierarchical chemical bonding and soft vibrations from selective sublattice leading to low κL is intriguing from lattice dynamical perspective as well as have potential applications.

Thermoelectric (TE) energy conversion demands high performance crystalline inorganic solids that exhibit ultralow thermal conductivity, high mechanical stability, and good TE device properties. Pb-free germanium telluride (GeTe)-based material has recently attracted significant attention in TE power generation in mid temperatures, but pristine GeTe possesses significantly higher lattice thermal conductivity (κ latt ) compared to that of its theoretical minimum (κ min ) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κ l a t t of (Ge-Te) 1−2x (SnSe) x (SnS) x very near to its κ min . The (Ge-Te) 1−2x (SnSe) x (SnS) x system behaves as a coexistence of pointdefect rich solid solution and phase separation. Initially, the addition of equimolar SnSe and SnS in the GeTe reduces the κ latt by effective phonon scattering because of the excess point defects and rich microstructures. In the second step, introduction of Sb-doping leads to additional phonon scattering centers and optimizes the p-type carrier concentration. Notably, 10 mol % Sb-doped (GeTe) 0.95 (SnSe) 0.025 (SnS) 0.025 exhibits ultralow κ latt of ∼0.30 W/mK at 300 K. Subsequently, 10 mol % Sb-doped (GeTe) 0.95 (SnSe) 0.025 (SnS) 0.025 exhibits a high TE figure of merit (zT) of ∼1.9 at 710 K. The high-performance sample exhibits a Vickers microhardness (mechanical stability) value of ∼194 H V that is significantly higher compared to the pristine GeTe and other state-of-the-art thermoelectric materials. Further, we have achieved a high output power, ∼150 mW for the temperature difference of 462 K, in single leg TE device based on 10 mol % Sb-doped (GeTe) 0.95 (SnSe) 0.025 (SnS) 0.025 .