Realizing thermoelectric cooling and power generation in N-type PbS0.6Se0.4 via lattice plainification and interstitial doping.

Thermoelectrics have great potential for use in waste heat recovery to improve energy utilization. Moreover, serving as a solid-state heat pump, they have found practical application in cooling electronic products. Nevertheless, the scarcity of commercial Bi2Te3 raw materials has impeded the sustainable and widespread application of thermoelectric technology. In this study, we developed a low-cost and earth-abundant PbS compound with impressive thermoelectric performance. The optimized n-type PbS material achieved a record-high room temperature ZT of 0.64 in this system. Additionally, the first thermoelectric cooling device based on n-type PbS was fabricated, which exhibits a remarkable cooling temperature difference of ~36.9 K at room temperature. Meanwhile, the power generation efficiency of a single-leg device employing our n-type PbS material reaches ~8%, showing significant potential in harvesting waste heat into valuable electrical power. This study demonstrates the feasibility of sustainable n-type PbS as a viable alternative to commercial Bi2Te3, thereby extending the application of thermoelectrics.

Implications and Optimization of Domain Structures in IV-VI High-Entropy Thermoelectric Materials.

High-entropy semiconductors are now an important class of materials widely investigated for thermoelectric applications. Understanding the impact of chemical and structural heterogeneity on transport properties in these compositionally complex systems is essential for thermoelectric design. In this work, we uncover the polar domain structures in the high-entropy PbGeSnSe1.5Te1.5 system and assess their impact on thermoelectric properties. We found that polar domains induced by crystal symmetry breaking give rise to well-structured alternating strain fields. These fields effectively disrupt phonon propagation and suppress the thermal conductivity. We demonstrate that the polar domain structures can be modulated by tuning crystal symmetry through entropy engineering in PbGeSnAgxSbxSe1.5+xTe1.5+x. Incremental increases in the entropy enhance the crystal symmetry of the system, which suppresses domain formation and loses its efficacy in suppressing phonon propagation. As a result, the room-temperature lattice thermal conductivity increases from κL = 0.63 Wm-1 K-1 (x = 0) to 0.79 Wm-1 K-1 (x = 0.10). In the meantime, the increase in crystal symmetry, however, leads to enhanced valley degeneracy and improves the weighted mobility from µw = 29.6 cm2 V-1 s-1 (x = 0) to 35.8 cm2 V-1 s-1 (x = 0.10). As such, optimal thermoelectric performance can be achieved through entropy engineering by balancing weighted mobility and lattice thermal conductivity. This work, for the first time, studies the impact of polar domain structures on thermoelectric properties, and the developed understanding of the intricate interplay between crystal symmetry, polar domains, and transport properties, along with the impact of entropy control, provides valuable insights into designing GeTe-based high-entropy thermoelectrics.

High Carrier Mobility and Promising Thermoelectric Module Performance of N-Type PbSe Crystals.

The scarcity of Te hampers the widespread use of Bi2Te3-based thermoelectric modules. Here, the thermoelectric module potential of PbSe is investigated by improving its carrier mobility. Initially, large PbSe crystals are grown with the temperature gradient method to mitigate grain boundary effects on carrier transport. Subsequently, light doping with <1mole‰ halogens (Cl/Br/I) increases room-temperature carrier mobility to ~1600 cm2 V-1 s-1, achieved by reducing carrier concentration compared to traditional heavy doping. Crystal growth design and light doping enhance carrier mobility without affecting effective mass, resulting in a high power factor ~40 µW cm-1 K-2 in PbSe-Cl/Br/I crystals at 300 K. Additionally, Cl/Br/I doping reduces thermal conductivity and bipolar diffusion, leading to significantly lower thermal conductivity at high temperature. Enhanced carrier mobility and suppressed bipolar effect boost ZT values across the entire temperature range in n-type PbSe-Cl/Br/I crystals. Specifically, ZT values of PbSe-Br crystal reach ~0.6 at 300 K, ~1.2 at 773 K, and the average ZT (ZTave) reaches ~1.0 at 300-773 K. Ultimately, ~5.8% power generation efficiency in a PbSe single leg with a maximum temperature cooling difference of 40 K with 7-pair modules is achieved. These results indicate the potential for cost-effective and high-performance thermoelectric cooling modules based on PbSe.

Realizing high-efficiency thermoelectric module by suppressing donor-like effect and improving preferred orientation in n-type Bi2(Te, Se)3.

Thermoelectric materials have a wide range of application because they can be directly used in refrigeration and power generation. And the Bi2Te3 stand out because of its excellent thermoelectric performance and are used in commercial thermoelectric devices. However, n-type Bi2Te3 has seriously hindered the development of Bi2Te3-based thermoelectric devices due to its weak mechanical properties and inferior thermoelectric performance. Therefore, it is urgent to develop a high-performance n-type Bi2Te3 polycrystalline. In this work, we employed interstitial Cu and the hot deformation process to optimize the thermoelectric properties of Bi2Te2.7Se0.3, and a high-performance thermoelectric module was fabricated based on this material. Our combined theoretical and experimental effort indicates that the interstitial Cu reduce the defect density in the matrix and suppresses the donor-like effect, leading to a lattice plainification effect in the material. In addition, the two-step hot deformation process significantly improves the preferred orientation of the material and boosts the mobility. As a result, a maximum ZT of 1.27 at 373 K and a remarkable high ZTave of 1.22 across the temperature range of 300-425 K are obtained. The thermoelectric generator (TEG, 7-pair) and thermoelectric cooling (TEC, 127-pair) modules were fabricated with our n-type textured Cu0.01Bi2Te2.7Se0.3 coupled with commercial p-type Bi2Te3. The TEC module demonstrates superior cooling efficiency compared with the commercial Bi2Te3 device, achieving a ΔT of 65 and 83.4 K when the hot end temperature at 300 and 350 K, respectively. In addition, the TEG module attains an impressive conversion efficiency of 6.5% at a ΔT of 225 K, which is almost the highest value among the reported Bi2Te3-based TEG modules.

Network Pharmacology Analysis of Liquid-Cultured Armillaria ostoyae Mycelial Metabolites and Their Molecular Mechanism of Action against Gastric Cancer.

Armillaria sp. are traditional edible medicinal mushrooms with various health functions; however, the relationship between their composition and efficacy has not yet been determined. Here, the ethanol extract of liquid-cultured Armillaria ostoyae mycelia (AOME), a pure wild Armillaria sp. strain, was analyzed using UHPLC-QTOF/MS, network pharmacology, and molecular docking techniques. The obtained extract affects various metabolic pathways, such as JAK/STAT and PI3K/AKT. The extract also contains important compounds such as 4-(dimethylamino)-N-[7-(hydroxyamino)-7-oxoheptyl] benzamide, isoliquiritigenin, and 7-hydroxycoumarin. Moreover, the extract targets key proteins, including EGFR, SCR, and IL6, to suppress the progression of gastric cancer, thereby synergistically inhibiting cancer development. The molecular docking analyses indicated that the main compounds stably bind to the target proteins. The final cell culture experimental data showed that the ethanol extract inhibited MGC-803 gastric cancer cells. In summary, our research revealed the beneficial components of AOME for treating gastric cancer and its associated molecular pathways. However, further research is needed to confirm its effectiveness and safety in gastric cancer patients.

Synthetic dual co-stimulation increases the potency of HIT and TCR-targeted cell therapies.

Chimeric antigen receptor T cells have dramatically improved the treatment of hematologic malignancies. T cell antigen receptor (TCR)-based cell therapies are yet to achieve comparable outcomes. Importantly, chimeric antigen receptors not only target selected antigens but also reprogram T cell functions through the co-stimulatory pathways that they engage upon antigen recognition. We show here that a fusion receptor comprising the CD80 ectodomain and the 4-1BB cytoplasmic domain, termed 80BB, acts as both a ligand and a receptor to engage the CD28 and 4-1BB pathways, thereby increasing the antitumor potency of human leukocyte antigen-independent TCR (HIT) receptor- or TCR-engineered T cells and tumor-infiltrating lymphocytes. Furthermore, 80BB serves as a switch receptor that provides agonistic 4-1BB co-stimulation upon its ligation by the inhibitory CTLA4 molecule. By combining multiple co-stimulatory features in a single antigen-agnostic synthetic receptor, 80BB is a promising tool to sustain CD3-dependent T cell responses in a wide range of targeted immunotherapies.

Unraveling the Role of Entropy in Thermoelectrics: Entropy-Stabilized Quintuple Rock Salt PbGeSnCdxTe3+x.

Entropy-engineered materials are garnering considerable attention owing to their excellent mechanical and transport properties, such as their high thermoelectric performance. However, understanding the effect of entropy on thermoelectrics remains a challenge. In this study, we used the PbGeSnCdxTe3+x family as a model system to systematically investigate the impact of entropy engineering on its crystal structure, microstructure evolution, and transport behavior. We observed that PbGeSnTe3 crystallizes in a rhombohedral structure at room temperature with complex domain structures and transforms into a high-temperature cubic structure at ∼373 K. By alloying CdTe with PbGeSnTe3, the increased configurational entropy lowers the phase-transition temperature and stabilizes PbGeSnCdxTe3+x in the cubic structure at room temperature, and the domain structures vanish accordingly. The high-entropy effect results in increased atomic disorder and consequently a low lattice thermal conductivity of 0.76 W m-1 K-1 in the material owing to enhanced phonon scattering. Notably, the increased crystal symmetry is conducive to band convergence, which results in a high-power factor of 22.4 µW cm-1 K-1. As a collective consequence of these factors, a maximum ZT of 1.63 at 875 K and an average ZT of 1.02 in the temperature range of 300-875 K were obtained for PbGeSnCd0.08Te3.08. This study highlights that the high-entropy effect can induce a complex microstructure and band structure evolution in materials, which offers a new route for the search for high-performance thermoelectrics in entropy-engineered materials.

Silver Atom Off-Centering in Diamondoid Solid Solutions Causes Crystallographic Distortion and Suppresses Lattice Thermal Conductivity.

The class I-III-VI2 diamondoid compounds with tetrahedral bonding are important semiconductors widely applied in optoelectronics. Understanding their heat transport properties and developing an effective method to predict the diamondoid solid solutions' thermal conductivity will help assess their impact as thermoelectrics. In this work, we investigated in detail the heat transport properties of CuGa1-xInxTe2 and Cu1-xAgxGaTe2 and found that in the Ag-alloyed solid solutions, the Ag atom off-centering effect results in crystallographic distortion and extra strong acoustic-optical phonon scattering and an extremely low lattice thermal conductivity. Moreover, we integrate the alloy scattering and the off-centering effect with the crystallographic distortion parameter to develop a modified Klemens model that predicts the thermal conductivity of diamondoid solid solutions. Finally, we demonstrate that Cu1-xAgxGaTe2 solid solutions are promising p-type thermoelectric materials, with a maximum ZT of 1.23 at 850 K for Cu0.58Ag0.4GaTe2.

Generation of T-cell-receptor-negative CD8αß-positive CAR T cells from T-cell-derived induced pluripotent stem cells.

The production of autologous T cells expressing a chimaeric antigen receptor (CAR) is time-consuming, costly and occasionally unsuccessful. T-cell-derived induced pluripotent stem cells (TiPS) are a promising source for the generation of 'off-the-shelf' CAR T cells, but the in vitro differentiation of TiPS often yields T cells with suboptimal features. Here we show that the premature expression of the T-cell receptor (TCR) or a constitutively expressed CAR in TiPS promotes the acquisition of an innate phenotype, which can be averted by disabling the TCR and relying on the CAR to drive differentiation. Delaying CAR expression and calibrating its signalling strength in TiPS enabled the generation of human TCR- CD8αß+ CAR T cells that perform similarly to CD8αß+ CAR T cells from peripheral blood, achieving effective tumour control on systemic administration in a mouse model of leukaemia and without causing graft-versus-host disease. Driving T-cell maturation in TiPS in the absence of a TCR by taking advantage of a CAR may facilitate the large-scale development of potent allogeneic CD8αß+ T cells for a broad range of immunotherapies.

High Thermoelectric Performance in Chalcopyrite Cu1-xAgxGaTe2-ZnTe: Nontrivial Band Structure and Dynamic Doping Effect.

The understanding of thermoelectric properties of ternary I-III-VI2 type (I = Cu, Ag; III = Ga, In; and VI = Te) chalcopyrites is less well developed. Although their thermal transport properties are relatively well studied, the relationship between the electronic band structure and charge transport properties of chalcopyrites has been rarely discussed. In this study, we reveal the unusual electronic band structure and the dynamic doping effect that could underpin the promising thermoelectric properties of Cu1-xAgxGaTe2 compounds. Density functional theory (DFT) calculations and electronic transport measurements suggest that the Cu1-xAgxGaTe2 compounds possess an unusual non-parabolic band structure, which is important for obtaining a high Seebeck coefficient. Moreover, a mid-gap impurity level was also observed in Cu1-xAgxGaTe2, which leads to a strong temperature-dependent carrier concentration and is able to regulate the carrier density at the optimized value for a wide temperature region and thus is beneficial to obtaining the high power factor and high average ZT of Cu1-xAgxGaTe2 compounds. We also demonstrate a great improvement in the thermoelectric performance of Cu1-xAgxGaTe2 by introducing Cu vacancies and ZnTe alloying. The Cu vacancies are effective in increasing the hole density and the electrical conductivity, while ZnTe alloying reduces the thermal conductivity. As a result, a maximum ZT of 1.43 at 850 K and a record-high average ZT of 0.81 for the Cu0.68Ag0.3GaTe2-0.5%ZnTe compound are achieved.

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

Typically, conventional structure transitions occur from a low symmetry state to a higher symmetry state upon warming. In this work, an unexpected local symmetry breaking in the tetragonal diamondoid compound AgGaTe2 is reported, which, upon warming, evolves continuously from an undistorted ground state to a locally distorted state while retaining average crystallographic symmetry. This is a rare phenomenon previously referred to as emphanisis. This distorted state, caused by the weak sd3 orbital hybridization of tetrahedral Ag atoms, causes their displacement off the tetrahedron center and promotes a global distortion of the crystal structure resulting in strong acoustic-optical phonon scattering and an ultralow lattice thermal conductivity of 0.26 W m-1 K-1 at 850 K in AgGaTe2 . The findings explain the underlying reason for the unexpectedly low thermal conductivities of silver-based compounds compared to copper-based analogs and provide a guideline to suppressing heat transport in diamondoid and other materials.

Valence Disproportionation of GeS in the PbS Matrix Forms Pb5Ge5S12 Inclusions with Conduction Band Alignment Leading to High n-Type Thermoelectric Performance.

Converting waste heat into useful electricity using solid-state thermoelectrics has a potential for enormous global energy savings. Lead chalcogenides are among the most prominent thermoelectric materials, whose performance decreases with an increase in chalcogen amounts (e.g., PbTe > PbSe > PbS). Herein, we demonstrate the simultaneous optimization of the electrical and thermal transport properties of PbS-based compounds by alloying with GeS. The addition of GeS triggers a complex cascade of beneficial events as follows: Ge2+ substitution in Pb2+ and discordant off-center behavior; formation of Pb5Ge5S12 as stable second-phase inclusions through valence disproportionation of Ge2+ to Ge0 and Ge4+. PbS and Pb5Ge5S12 exhibit good conduction band energy alignment that preserves the high electron mobility; the formation of Pb5Ge5S12 increases the electron carrier concentration by introducing S vacancies. Sb doping as the electron donor produces a large power factor and low lattice thermal conductivity (κlat) of ∼0.61 W m-1 K-1. The highest performance was obtained for the 14% GeS-alloyed samples, which exhibited an increased room-temperature electron mobility of ∼121 cm2 V-1 s-1 for 3 × 1019 cm-3 carrier density and a ZT of 1.32 at 923 K. This is ∼55% greater than the corresponding Sb-doped PbS sample and is one of the highest reported for the n-type PbS system. Moreover, the average ZT (ZTavg) of ∼0.76 from 400 to 923 K is the highest for PbS-based systems.

Low Thermal Conductivity in Heteroanionic Materials with Layers of Homoleptic Polyhedra.

Although BiAgOSe, an analogue of a well-studied thermoelectric material BiCuOSe, is thermodynamically stable, its synthesis is complicated by the low driving force of formation from the stable binary and ternary intermediates. Here we have developed a "subtraction strategy" to suppress byproducts and produce pure phase BiAgOSe using hydrothermal methods. Electronic structure calculations and optical characterization show that BiAgOSe is an indirect bandgap semiconductor with a bandgap of 0.95 eV. The prepared sample exhibits lower lattice thermal conductivities (0.61 W·m-1·K-1 at room temperature and 0.35 W·m-1·K-1 at 650 K) than BiCuOSe. Lattice dynamical simulations and variable temperature diffraction measurements demonstrate that the low lattice thermal conductivity arises from both the low sound velocity and high phonon-phonon scattering rates in BiAgOSe. These in turn result primarily from the soft Ag-Se bonds in the edge-sharing AgSe4 tetrahedra and large sublattice mismatch between the quasi-two-dimensional [Bi2O2]2+ and [Ag2Se2]2- layers. These results highlight the advantages of manipulating the chemistry of homoleptic polyhedra in heteroanionic compounds for electronic structure and phonon transport control.

Enhanced Thermoelectric Properties of Cu2SnSe3-Based Materials with Ag2Se Addition.

In this work, (Ag, In)-co-doped Cu2SnSe3-based compounds are prepared using a self-propagating high-temperature synthesis process. Ag2Se and as-synthesized (Ag, In)-co-doped Cu2SnSe3-based powders are mixed in a proportion according to the formula of Cu1.85Ag0.15Sn0.91In0.09Se3/x Ag2Se (x = 0, 3, 4, and 5%), which is followed by a subsequent plasma-activated sintering (PAS) to obtain consolidated composite bulks. A sandwich experiment is designed to reveal the evolution of the microstructure and phase composition of the composite samples during the PAS process. We investigate the reaction mechanism between Ag2Se and Cu2SnSe3-based matrix as well as the influence of Ag2Se on the phase composition, microstructure, and thermoelectric transport properties of the composites. Ag2Se addition is proven to be effective to improve Ag solubility in the Cu1.85Ag0.15Sn0.91In0.09Se3 matrix and introduce a CuSe secondary phase and an Ag-rich phase at grain boundaries. The electrical conductivity of Cu1.85Ag0.15Sn0.91In0.09Se3/x Ag2Se (x = 0, 3, 4, and 5%) composites decreases while the Seebeck coefficient increases with increasing Ag2Se addition, resulting in an optimized power factor. Moreover, benefiting from the collective phonon scattering at various defects induced by Ag2Se addition, the composite samples exhibit significantly suppressed lattice thermal conductivity, which reaches as low as 0.11 W m-1 K-1 at 700 K for the x = 5% sample. A peak figure-of-merit (ZT) of 1.26 at 750 K and an average ZT of 0.75 at 300-800 K are obtained for Cu1.85Ag0.15Sn0.91In0.09Se3/5% Ag2Se. This work provides an efficient way to improve average ZT values of Cu2SnSe3-based compounds for promising power generation at intermediate temperatures.

Synergistically Enhanced Thermoelectric Performance of Cu2SnSe3-Based Composites via Ag Doping Balance.

As an ecofriendly and low-cost thermoelectric material, Cu2SnSe3 has recently drawn much attention. In this work, the thermoelectric properties of Cu2SnSe3-based materials have been synergistically optimized. Ag doping at the Cu site enables strong phonon scattering via large strain field fluctuations and increases the effective mass of carriers through band engineering, which results in a reduced lattice thermal conductivity and enhanced Seebeck coefficient. Thus, a peak ZT value of 1.04 at 800 K is obtained for Cu1.85Ag0.15SnSe3. Then, In is further doped at the Sn site to further increase the carrier concentration and power factor; the ZT values of Cu1.85Ag0.15Sn1-yInySe3 samples are highly improved in the temperature range of 300-800 K, and a peak ZT value of 1.12 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3 sample. Considering that Ag2S decomposes to Ag and S completely under vacuum and high current field, the sulfur vapor would be pumped away by a vacuum pump, and the generated Ag not only enriches at the grain boundary of the Cu1.85Ag0.15Sn0.91In0.09Se3 bulk material but also enters the matrix and occupies the Cu site, leading to the extruding Cu and Se forming the second phase of CuSe nanoparticles. Thus, the power factor greatly improves to 13.8 µW cm-1 K-2 at 700 K, and the lattice thermal conductivity is as low as 0.12 W m-1 K-1 at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite. Finally, a high ZT value of 1.58 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite, which is nearly an increase of 204% compared to that of Cu2SnSe3. This work provides an effective solution to optimize the conflicting material properties for Cu2SnSe3-based thermoelectric materials.

Ultralow Thermal Conductivity in Diamondoid Structures and High Thermoelectric Performance in (Cu1-xAgx)(In1-yGay)Te2.

Owing to the diversity of composition and excellent transport properties, the ternary I-III-VI2 type diamond-like chalcopyrite compounds are attractive functional semiconductors, including as thermoelectric materials. In this family, CuInTe2 and CuGaTe2 are well investigated and achieve maximum ZT values of ∼1.4 at 950 K and an average ZT of 0.43. However, both compounds have poor electrical conductivity at low temperature, resulting in low ZT below 450 K. In this work, we have greatly improved the thermoelectric performance in the quinary diamondoid compound (Cu0.8Ag0.2)(In0.2Ga0.8)Te2 by understanding and controlling the effects of different constituent elements on the thermoelectric transport properties. Our combined theoretical and experimental effort indicates that Ga in the In site of the lattice decreases the carrier effective mass and improves the electrical conductivity and power factor of Cu0.8Ag0.2In1-xGaxTe2. Furthermore, Ag in the Cu site strongly suppresses the heat transport via the enhanced acoustic phonon-optical phonon coupling effects, leading to the ultralow thermal conductivity of ∼0.49 W m-1 K-1 at 850 K in Cu0.8Ag0.2In0.2Ga0.8Te2. Defect formation energy calculations suggest intrinsic Cu vacancies introduce defect levels that are important to the temperature-dependent hole density and electrical conductivity. Therefore, we introduced extra Cu vacancies to optimize the hole carrier density and improve the power factor of Cu0.8Ag0.2In0.2Ga0.8Te2. As a result, a maximum ZT of ∼1.5 at 850 K and an average ZT of 0.78 in the temperature range of 400-850 K are obtained, which is among the highest in the diamond-like compound family.

Unraveling the Critical Role of Melt-Spinning Atmosphere in Enhancing the Thermoelectric Performance of p-Type Bi0.52Sb1.48Te3 Alloys.

Melt spinning has proven effective in maintaining chemical homogeneity and introducing multiscale microstructures that can reduce the lattice thermal conductivity and consequently enhance the thermoelectric performance of consolidated bulk materials. In this work, p-type Bi0.52Sb1.48Te3 bulk alloys are fabricated by melt spinning (MS) followed by subsequent plasma activated sintering (PAS). The influence of different MS atmospheres (air, Ar, N2, and He) on the morphologies of MS ribbons and the thermoelectric properties of MS-PAS bulk materials has been investigated systematically. Because of the relatively high thermal conductivity, a He atmosphere expedites the heat dissipation in the MS process and results in severe sublimation of tellurium and thus inferior thermoelectric performance. In contrast, an Ar atmosphere can essentially prevent heat loss of the fusant and suppress the sublimation of tellurium. Consequently, the corresponding Bi0.52Sb1.48Te3 sample (MS in Ar atmosphere) presents the highest peak ZT and average ZT values of 1.09 (at 340 K) and 0.81 (in 300-500 K), respectively. The average ZT of the sample prepared using an Ar atmosphere is almost three times the one prepared using a He atmosphere. This reflects the importance of using the appropriate atmosphere during the melt-spinning process. This result, which indicates that melt spinning in an Ar atmosphere is preferable to avoid heat loss, can also be extended to other materials.

All-Inorganic Halide Perovskites as Potential Thermoelectric Materials: Dynamic Cation off-Centering Induces Ultralow Thermal Conductivity.

Halide perovskites are anticipated to impact next generation high performance solar cells because of their extraordinary charge transport and optoelectronic properties. However, their thermal transport behavior has received limited attention. In this work, we studied the thermal transport and thermoelectric properties of the CsSnBr3-xIx perovskites. We find a strong correlation between lattice dynamics and an ultralow thermal conductivity for series CsSnBr3-xIx reaching 0.32 Wm-1K-1 at 550 K. The CsSnBr3-xIx also possess a decent Seebeck coefficient and controllable electrical transport properties. The crystallography data and theoretical calculations suggest the Cs atom deviates from its ideal cuboctahedral geometry imposed by the perovskite cage and behaves as a heavy atom rattling oscillator. This off-center tendency of Cs, together with the distortion of SnX6 (X = Br or I) octahedra, produces a highly dynamic and disordered structure in CsSnBr3-xIx, which gives rise to a very low Debye temperature and phonon velocity. Moreover, the low temperature heat capacity data suggests strong coupling between the low frequency optical phonons and heat carrying acoustical phonons. This induces strong phonon resonance scattering that induces the ultralow lattice thermal conductivity of CsSnBr3-xIx.