High Thermoelectric Figure Of Merit Research Articles

Temperature-induced first-order electronic topological transition in β -Ag2Se

β-Ag2Se is a promising material for room temperature thermoelectric applications and magneto-resistive sensors. However, no attention was paid earlier to the hysteresis in the temperature dependence of resistivity [ρ(T)]. Here, we show that a broad hysteresis above 35 K is observed not only in ρ(T), but also in other electronic properties such as Hall coefficient [RH(T)], Seebeck coefficient, thermal conductivity, and ultraviolet photoelectron spectra (UPS). We also show that the hysteresis is not associated with a structural transition. The ρ(T) and RH(T) show that β-Ag2Se is semiconducting above 300 K, but metallicity is retained below 300 K. While electronic states are absent in the energy range from the Fermi level (EF) to 0.4 eV below the EF at 300 K, a distinct Fermi edge is observed in the UPS at 15 K suggesting that the β-Ag2Se undergoes an electronic topological transition from a high-temperature semiconducting state to a low-temperature metallic state. Our study reveals that a constant and moderately high thermoelectric figure of merit in the range 300–395 K is observed due to the broad semiconductor to metal transition in β-Ag2Se.

Spin orbit coupling induced enhancement of thermoelectric performance of HfX2 (X = S, Se) and its Janus monolayer

Rashba spin-orbit coupling (SOC) plays a vital role in transporting carriers under external electric field or strain in two-dimensional transition metal di-chalcogenides in the absence of inversion symmetry. Here, the effect of SOC on the electronic and thermoelectric properties of two-dimensional monolayer HfS2, HfSe2 and their Janus monolayer HfSSe has been investigated using density functional theory (DFT) and Boltzmann transport equation (BTE). A considerable enhancement in thermoelectric power factor (PF) for n-type carriers has been observed due to the SOC in all the monolayers. This enhanced power factor for n-type carriers is mainly because of increased valley degeneracy at the conduction band minima (CBM), as there is no enhancement in electron relaxation time (τ) with SOC. The lattice thermal conductivity (κph) at room temperature is very low for these monolayers compared to extensively used TMDCs such as MoS2 and WS2. This is due to the strong coupling between acoustic and optical modes, which results in low group velocity and a short lifetime of phonons in HfS2, HfSe2 and HfSSe Janus monolayers. The ultralow value of κph results a very high thermoelectric figure of merit (ZT) at room temperature for n-type carriers and which increases significantly with SOC. The highest ZT values of 0.90 (HfSe2), 0.84 (HfS2) and 0.81 (HfSSe) for n-type at 600 K has been achieved with SOC. Our theoretical investigation predicts that there is a significant effect of SOC on the thermoelectric performance of HfX2 (X = S, Se) and its Janus monolayer HfSSe and they can be a revolutionary candidate for the fabrication of a highly efficient thermoelectric power generator.

2D Nb2SiTe4 and Nb2GeTe4: promising thermoelectric figure of merit and gate-tunable thermoelectric performance

Recently, the experimentally synthesized Nb2SiTe4 was found to be a stable layered narrow-gap semiconductor, and the fabricated field-effect transistors (FETs) based on few-layers Nb2SiTe4 are good candidates for ambipolar devices and mid-infrared detection (Zhao et al 2019 ACS Nano 13 10705–10). Here, we use first-principles combined with Boltzmann transport theory and non-equilibrium Green’s function method to investigate the thermoelectric transport coefficients of monolayer Nb2XTe4 (X = Si, Ge) and the gate voltage effect on the thermoelectric performance of the FET based on monolayer Nb2SiTe4. It is found that both monolayers have large p-type Seebeck coefficients due to the ‘pudding-mold-type’ valence band structure, and they both exhibit anisotropic thermoelectric behavior with optimal thermoelectric figure of merit of 1.4 (2.2) at 300 K and 2.8 (2.5) at 500 K for Nb2SiTe4 (Nb2GeTe4). The gate voltage can effectively increase the thermoelectric performance for the Nb2SiTe4-based FET. The high thermoelectric figure of merit can be maintained in a wide temperature range under a negative gate voltage. Furthermore, the FET exhibits a good gate-tunable Seebeck diode effect. The present work suggests that Nb2XTe4 monolayers are promising candidates for 2D thermoelectric materials and thermoelectric devices.

Pressure effects on the electrical transport and anharmonic lattice dynamics of r-GeTe: A first-principles study

Various strategies for thermoelectric material optimization have been widely studied and used for promoting electrical transport and suppressing thermal transport. As a nontraditional method, pressure has shown great potential, as it has been applied to obtain a high thermoelectric figure of merit, but the microscopic mechanisms involved have yet to be fully explored. In this study, we focus on r-GeTe, a low-temperature phase of GeTe, and investigate the pressure effects on the electronic structure, electrical transport properties and anharmonic lattice dynamics based on density functional theory (DFT), the Boltzmann transport equations (BTEs) and perturbation theory. Electronic relaxation times are obtained based on the electron-phonon interaction and the constant relaxation time approximation. The corresponding electrical transport properties are compared with those obtained from previous experiments. Hydrostatic pressure is shown to increase valley degeneracy, decrease the band effective mass and enhance the electrical transport property. At the same time, the increase in the low-frequency phonon lifetime and phonon group velocity leads to an increase in lattice thermal conductivity under pressure. This study provides insight into r-GeTe under hydrostatic pressure and paves the way for a high-pressure strategy to optimize transport properties.

Computationally Guided Synthesis of High Performance Thermoelectric Materials: Defect Engineering in AgGaTe2

AbstractThe rational synthesis of high‐performance thermoelectric (TE) materials guided by theoretical design is still in its infancy. Here by computationally exploiting the possibilities of materials’ dopability and hence the electron–phonon transport/scattering, a new defective compound, AgGaTe2, with simultaneous Ag deficiency and isoelectronic substitution of In on Ga‐site (InGa) is predicted, and its high performance is then confirmed via experiments. Using density functional theory and density functional perturbation theory calculations, it is identified that controlled defects viz. Ag vacancy and In substitution in AgGaTe2 system can lead to extremely low lattice thermal conductivity (κL) of around 0.13 WK−1 m−1 at 850 K. This ultralow κL results from both the Ag vacancy that serves as a better rattler and the extra phonon scattering due to the defect induced internal lattice distortion (ψ). The synthesized compounds Ag0.85Ga1−xInxTe2 (x = 0–0.3) indeed achieve the extremely low κL (0.08 WK−1 m−1 for x = 0.15). As a result, the highest TE figure of merit (ZT) of 1.44 is obtained, which is the highest recorded value for silver‐based ternary chalcopyrite semiconductors to date.

Improvement of the Self-Heating Performance of an Advanced SiGe HBT Transistor Through the Peltier Effect

This article focuses on the reduction of the self-heating (SH) effect in an advanced SiGe hetero-junction bipolar transistor (HBT) considering the Peltier effect. Bismuth Telluride is the working material for most Peltier cooling devices and thermoelectric (TE) generators. This is because Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> has the highest TE figure of merit (ZT), of any material around room temperature. An electro-thermal model is performed using the COMSOL multiphysics software. Accurate direct current (dc) and radio frequency (RF) electrical simulations, as well as the extraction of the device static and dynamic thermal parameters are performed on the proposed structure. The current gain ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> ), the cutoff frequencies ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{max}$ </tex-math></inline-formula> ), and the maximum network temperature <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{max}$ </tex-math></inline-formula> have been considered. We have extracted these parameters by taking into account the phenomenon of the SH which occurs in this device. We then have integrated around the structure, a cooling system considering elements with “Peltier effect.” So, in this case, the electrothermal model is essentially based on the Peltier effect. It allows generating a potential <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {V}$ </tex-math></inline-formula> difference from a temperature <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {T}$ </tex-math></inline-formula> gradient in the considered structure. Thus, we analyze carefully the heat distribution and the electrical potential on the surface of the component. By using this cooling method, we can observe clearly a decrease in the maximum temperature ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{max}$ </tex-math></inline-formula> ) of the HBT SiGe due to the SH phenomenon: from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{max} =467$ </tex-math></inline-formula> K to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{max}=330$ </tex-math></inline-formula> K. The proposed method shows a sensible improvement of dc and RF figures considering the thermal impact of Peltier effect upon the transistor.

A first principle study of structural, opto-electronic and thermoelectric properties of Sm filled skutterudite SmRu4Sb12

Structural, elastic, electronic, magnetic, thermodynamic, optical and thermoelectric properties of a filled skutterudite SmRu4Sb12 has been studied using full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). Local spin density approximation (LSDA) is used for the exchange–correlation potential. The material is found to be harder than the isostructural Ce filled skutterudites. Bulk modulus and Debye temperature are nearly constant up to 100 K. The spin-orbit interaction has significant influence on the band structures. It causes the splitting of the f-band and results in an effective magnetic moment of 4.88 μB indicating its ferromagnetic nature. Optical constants including the dielectric function, optical reflectivity, refractive index, electron energy loss and optical conductivity have also been calculated over a wide range of photon energy. The material has high reflectivity and low absorption near the Fermi energy and signature of surface plasmon effect is also visible at higher photon energy. Due to high concentration of flat bands near Fermi level the material shows high Seebeck coefficient and moderately high thermoelectric figure of merit (ZT).

Ultralow Thermal Conductivity, Enhanced Mechanical Stability, and High Thermoelectric Performance in (GeTe)1-2x(SnSe)x(SnS)x.

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 (κlatt) compared to that of its theoretical minimum (κmin) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κlatt of (GeTe)1-2x(SnSe)x(SnS)x very near to its κmin. The (GeTe)1-2x(SnSe)x(SnS)x system behaves as a coexistence of point-defect rich solid solution and phase separation. Initially, the addition of equimolar SnSe and SnS in the GeTe reduces the κlatt 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 κlatt 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 HV 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.

High thermoelectric figure of merit in monolayer Tl2O from first principles

The thermoelectric properties of monolayer Tl2O are studied using first-principles calculations with all involved electrical and thermal transport properties calculated in the parameter-free frameworks. It is found that monolayer Tl2O possesses remarkably high thermoelectric figure of merit, zT, due to its ultralow lattice thermal conductivity and fairly good power factor. The room temperature zT can be as high as 1.4 and 1.2 for n- and p-type systems, respectively, whereas the maximum zT values can reach up to 5.3 and 4.2 as the temperature increases to 800 K. In addition, it is clarified that the mobilities of monolayer Tl2O are orders of magnitude smaller than previous estimation from simplified semiempirical models. The room temperature electron and hole mobilities are only about 56 and 11cm2V−1s−1, respectively, due to the heavy effective mass along with strong polar optical phonons coupling scattering. Nonetheless, the intrinsically ultrahigh zT from entire first-principles calculations stimulate that the further experimental verification and exploration for practical application are worthwhile.

Thermoelectric Properties of CoCrFeNiNbx Eutectic High Entropy Alloys

Bulk CoCrFeNiNb0.45 eutectic high entropy alloy (EHEA) with ultrafine-lamellar microstructure shows outstanding thermal stability. The EHEA offers opportunities for the development of thermoelectric materials. In this paper, the thermoelectric properties of a CoCrFeNiNbx (x = 0, 0.25, and 0.45) EHEA system were investigated. The results indicated that the electrical conductivity decreased with a rise in Nb content in the CoCrFeNiNbx alloys, which resulted from the increased eutectic structure and phase interface. Moreover, the thermal conductivity increased with increased Nb content at low temperature (T ≤ 473 K), while thermal conductivity decreased at high temperature (T &gt; 573 K). The CoCrFeNiNb0.45 full eutectic high entropy alloy exhibited the lowest thermal conductivity and higher thermoelectric figure of merit (ZT) at a high temperature (T &gt; 573 K), which shows great promise for the thermoelectric application at high temperature.

Improvement of Thermoelectric Properties of AlSb by Incorporation of Mg as p-type Dopant

Due to the sizably larger direct band gap of 2.26 eV of AlSb, the thermoelectric properties of intrinsic AlSb is considerably low. Despite a promising Seebeck coefficient, the low electrical conductivity and high thermal conductivity has limited the improvement of thermoelectric figure of merit. Replacing small amounts of Al by Mg can increase the number of holes in the solid-state, effectively helping tune the carrier concentration. This work explores the effect of Mg doping in AlSb. The incorporation of Mg at Al sites has increased the electrical conductivity without decreasing the Seebeck coefficient detrimentally. The thermoelectric figure of merit has shown improvement despite the large thermal conductivity observed. The work additionally explores the possibility of improving thermoelectric properties further by forming a composite with β-Zn4Sb3. The highest thermoelectric figure of merit was found to be 0.075 at 873 K, which is 6 times higher than that in intrinsic AlSb.

Low-temperature thermoelectric behavior and impressive optoelectronic properties of two-dimensional XI2 (X = Sn, Si): A first principle study

The thermoelectric materials so far discovered have shown thermoelectric behavior at relatively higher temperatures. Low temperature thermoelectric materials are the need of the hour and in this work we have studied the thermoelectric behavior of 2D monolayer of XI2 (X = Sn, Si) at relatively low temperature by using Density Functional Theory (DFT) along with the Boltzmann Transport equation. We have found high thermoelectric figure of merit (ZT) for SnI2 and SiI2 at 600 K. At room temperature the maximum ZT is 0.66 (0.35), 0.78 (0.51) for p-type (n-type) SnI2 and SiI2 respectively and at 600 K it is 0.83 for SiI2. The study of the optical properties of both the materials shows that both of them have indirect band gap with SnI2 having a band gap of 2.06 eV where as SiI2 has a band gap of 1.63 eV. Both the materials have a very high absorption coefficient in the ultraviolet (UV) region hence these materials can be used as high sensitive UV photodetectors. Thus the SnI2 and SiI2 2D monolayers can have potential application in optoelectronic as well as thermoelectric device fabrication.

Thermoelectric figure of merit enhancement in cement composites with graphene and transition metal oxides

Having a high thermoelectric figure of merit is the essential prerequisites of a thermoelectric generator. In this study, a novel way to achieve a record high figure of merit for cement composites using graphene and metallic oxide nanoparticle inclusions is developed. This paper investigated the thermoelectric performances of graphene-metallic oxide cement composites fabricated by special dry mixing and pressing, followed by curing at ambient conditions. The experimental results indicate that the incorporation of oxide nanoparticles increases the Seebeck coefficient of the composites to 100 μV/K, which is factor ∼3 higher than that of graphene based cement composites reported in our previous study. Improved Seebeck coefficient would effectively enhance the thermoelectric figure of merit. A maximum figure of merit as 1.01 × 10−2 at 70 °C is obtained in this work, which is an order of magnitude higher than the other prior reported cement composites. The research findings provide an effective strategy to functionalize the cement based thermoelectric devices for the potential application in energy harvesting of smart buildings.

Ultralow thermal conductivity and high thermoelectric figure of merit in Cu2Te–Ag2Te composites

Incorporation of nanostructures into bulk materials is an important strategy to enhance the thermoelectric figure of merit (zT). Herein, nanostructured Ag2Te (prepared by wet chemical synthesis) was mechanically alloyed with bulk Cu2Te (synthesized via melting route) to form composites of (Cu2Te)100-x–(Ag2Te)x (where x varies from 0 to 50). The alloying increased the thermoelectric power factor from ∼0.18 mW m−1K−2 (for pristine Cu2Te) to ∼0.47 mW m−1K−2 for the composite at 588 K. The thermal conductivity reduced from ∼3.5 W m−1 K−1 in pristine Cu2Te to ∼0.26 W m−1 K−1 in the composites and reaches the theoretical minimum of Cu2Te (∼0.18 W m−1 K−1) at high temperatures due to the additional scattering of carriers as well as phonons by the Ag2Te nanostructures. The combined effect of moderate power factor and ultralow thermal conductivity led to the high zT value of ∼0.99 at 588 K in (Cu2Te)50.00–(Ag2Te)50.00 that represents a significant improvement for Cu2Te based alloys.

Preparation and Thermoelectric Properties of Microcrystalline Lead Telluride

We have worked out conditions for the preparation of microcrystalline n-type lead telluride-based materials doped with lead iodide and investigated their microstructure and thermoelectric properties. The materials were prepared by hot-pressing powders produced by grinding an ingot to a particle size on the order of hundreds of microns in a planetary mill and to a particle size under hundreds of nanometers (mechanical activation) and by melt spinning. Fracture surfaces of the hot-pressed samples were examined on an optical and a scanning electron microscope. All of the samples had a nonuniform microstructure, with both small and larger grains present. In the samples prepared from the powders produced by mechanical activation, nanograins were detected. We have measured the Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples at room temperature and in the range 300–800 K and evaluated their lattice thermal conductivity and thermoelectric figure of merit, ZT. Their lattice thermal conductivity was shown to decrease with decreasing grain size. The highest thermoelectric figure of merit, (ZT)max = 1.32 at 630 K, was offered by the materials produced from the mechanically activated powder.

Intrinsically high thermoelectric figure of merit of half-Heusler ZrRuTe

The electronic structure and thermoelectric properties of ZrRuTe-based half-Heusler compounds are studied using density functional theory and Boltzmann transport formalism. Based on rigorous computations of electron relaxation time τ considering electron–phonon interactions and lattice thermal conductivity κl considering phonon–phonon interactions, we find ZrRuTe to be an intrinsically good thermoelectric material. It has a high power factor of ∼2 × 10−3 W m−1 K−2 and low κl ∼ 10 W m−1 K−1 at 800 K. The thermoelectric figure of merit ZT ∼ 0.13 at 800 K is higher than similar other compounds. We have also studied the properties of the material as a function of doping and find the thermoelectric properties to be substantially enhanced for p-doped ZrRuTe with the ZT value raised to ∼0.2 at this temperature. The electronic, thermodynamic, and transport properties of the material are thoroughly studied and discussed.

Thermal and Photo Sensing Capabilities of Mono- and Few-Layer Thick Transition Metal Dichalcogenides.

Two-dimensional (2D) materials have shown promise in various optical and electrical applications. Among these materials, semiconducting transition metal dichalcogenides (TMDs) have been heavily studied recently for their photodetection and thermoelectric properties. The recent progress in fabrication, defect engineering, doping, and heterostructure design has shown vast improvements in response time and sensitivity, which can be applied to both contact-based (thermocouple), and non-contact (photodetector) thermal sensing applications. These improvements have allowed the possibility of cost-effective and tunable thermal sensors for novel applications, such as broadband photodetectors, ultrafast detectors, and high thermoelectric figures of merit. In this review, we summarize the properties arisen in works that focus on the respective qualities of TMD-based photodetectors and thermocouples, with a focus on their optical, electrical, and thermoelectric capabilities for using them in sensing and detection.

Reliable Prediction of New Quantum Materials for Topological and Renewable-Energy Applications: A High-Throughput Screening.

Half-Heusler (HH) alloys provide a general platform for searching candidate materials for various energy applications. Here, we present a high-throughput first-principles calculation of a set of 960 eight valence-electron HH alloys to search potential candidates for thermoelectric (TE), solar harvesting (SH), topological insulator (TI), and transparent conductor (TC) applications. The initial screening parameters (such as stability, bandgap (Eg), band-inversion strength) followed by application specific descriptors are used to predict promising compounds. 121 out of 960 compounds were found to be dynamically and chemically stable. Of them, 31 compounds (with Eg < 1.5 eV) were studied for TE application, 30 (with 1 < Eg < 1.8 eV) for SH application, 21 for TI application, and 29 (with Eg > 2 eV) for TC applications. Some of the compounds show reasonably high thermoelectric figure of merit (ZT ∼ 1.6) and solar efficiency (SLME) > 20%, comparable to existing state-of-the-art materials. Surface band structure and topological Z2 index reconfirms the robustness of topological behavior. We strongly believe that our calculations should leverage useful insights to experimentalists.

A density functional theory study of the thermoelectric properties of K3AuO

We report for the first time the structural, electronic, lattice dynamics, mechanical and electronic transport properties of K3AuO calculated using first-principles concepts within the framework of density functional theory. Electronic transport coefficents are derived at the level of the Boltzmann’s transport theory with the electronic relaxation times of K3AuO estimated using the deformation potential method. The calculated lattice constants and equilibrium volumes of K3AuO are in excellent agreement with the available experimental data. Moreover, the obtained elastic constants satisfy the elastic stability criteria and thus infer that K3AuO is mechanically stable. The calculated band gap of K3AuO is in good agreement with the experimental measured value. Furthermore, our obtained phonon spectrum do not have negative frequencies and thus indicate that K3AuO is dynamically stable. Our results also show that K3AuO possesses a low lattice thermal conductivity (κL) value of 0.54 W/mK at 300 K, a value which is even lower than the measured κL of Bi2Te3 (1.28 W/mK at 300 K). Moreover, p-doped K3AuO is found to produce a power factor which is 1 order of magnitude larger than that of practical Bi2Te3 at 300 K. A high thermoelectric figure of merit is revealed in p-type K3AuO (ZT = 2.41 at 750 K) which indicate that K3AuO is a promising candidate for thermoelectric applications.

Ferroelectric Instability Induced Ultralow Thermal Conductivity and High Thermoelectric Performance in Rhombohedral p-Type GeSe Crystal.

The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: Pnma), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: R3m), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferroelectric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe2. We show ultralow lattice thermal conductivity (κL) of 0.74-0.47 W/mK in the 300-723 K range and high zT ≈ 1.25 at 723 K in the p-type rhombohedral (GeSe)0.9(AgBiSe2)0.1 crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferroelectric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)0.9(AgBiSe2)0.1 are directly evident in the low temperature heat capacity (Cp) and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferroelectric instability induced soft transverse optical phonons significantly reduces the κL and enhances the thermoelectric performance in rhombohedral (GeSe)0.9(AgBiSe2)0.1 crystals.