Electrostatically tunable interaction of CO2 with MgO surfaces and chemical switching: first-principles theory.

Electric field-assisted CO2 capture using solid adsorbents based on basic oxides can immensely reduce the required energy consumption compared to the conventional processes of temperature or pressure swing adsorption. In this work, we present first-principles density functional theoretical calculations to investigate the effects of an applied external electric field (AEEF) within the range from -1 to 1 V Å-1 on the CO2 adsorption behavior on various high and low-index facets of MgO. When CO2 is strongly adsorbed on MgO surfaces to form carbonate species, the coupling of electric fields with the resulting intrinsic dipole moment induces a 'switch' from a strongly chemisorbed state to a weakly chemisorbed or physisorbed state at a critical value of AEEF. We demonstrate that such 'switching' enables access to different metastable states with variations in the AEEF. On polar MgO(111) surfaces, we find a distinct feature of the adsorptive dissociation of CO2 towards the formation of CO in contrast to that on the non-polar MgO(100) and MgO(110) surfaces. In some cases, we observe broken inversion symmetry because of the AEEF that results in induced polarity at the interaction site of CO2 on MgO surfaces. Our results provide fundamental insights into the possibility of using AEEFs in novel solid adsorbent systems for CO2 capture and reduction.

Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms.

An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8-N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first-principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out-of-plane expression of the lone pair versus in-plane bonding can be systematically explored. While their honeycomb lattices respecting the 8-N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in-plane, with cationic lone pair activating the out-of-plane structural puckering that controls their relative stability. Anomalies in Born-effective charges, dielectric constants, Grüneisen parameters occur only in their in-plane behaviour, confirming MVB is confined strictly to 2D and originates from p-p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.

Metavalent Bonding-Mediated Dual 6s2 Lone Pair Expression Leads to Intrinsic Lattice Shearing in n-Type TlBiSe2.

Metavalent bonding has attracted immense interest owing to its capacity to impart a distinct property portfolio to materials for advanced functionality. Coupling metavalent bonding to lone pair expression can be an innovative way to propagate lattice anharmonicity from lone pair-induced local symmetry-breaking via the soft p-bonding electrons to achieve long-range phonon dampening in crystalline solids. Motivated by the shared chemical design pool for topological quantum materials and thermoelectrics, we based our studies on a three-dimensional (3D) topological insulator TlBiSe2 that held prospects for 6s2 dual-cation lone pair expression and metavalent bonding to investigate if the proposed hypothesis can deliver a novel thermoelectric material. Herein, we trace the inherent phononic origin of low thermal conductivity in n-type TlBiSe2 to dual 6s2 lone pair-induced intrinsic lattice shearing that strongly suppresses the lattice thermal conductivity to a low value of 1.1-0.4 Wm-1 K-1 between 300 and 715 K. Through synchrotron X-ray pair distribution function and first-principles studies, we have established that TlBiSe2 exists not in a monomorphous R-3m structure but as a distribution of distorted configurations. Via a cooperative movement of the constituent atoms akin to a transverse shearing mode facilitated by metavalent bonding in TlBiSe2, the structure shuttles between various energetically accessible low-symmetry structures. The orbital interactions and ensuing multicentric bonding visualized through Wannier functions augment the long-range transmission of atomic displacement effects in TlBiSe2. With additional point-defect scattering, a κlatt of 0.3 Wm-1 K-1 was achieved in TlBiSeS with a maximum n-type thermoelectric figure of merit (zT) of ∼0.8 at 715 K.

Hg Doping Induced Reduction in Structural Disorder Enhances the Thermoelectric Performance in AgSbTe2.

Defect engineering, achieved by precise tuning of the atomic disorder within crystalline solids, forms a cornerstone of structural chemistry. This nuanced approach holds the potential to significantly augment thermoelectric performance by synergistically manipulating the interplay between the charge carrier and lattice dynamics. Here, the current study presents a distinctive investigation wherein the introduction of Hg doping into AgSbTe2 serves to partially curtail structural disorder. This strategic maneuver mitigates potential fluctuations originating from pronounced charge and size disparities between Ag+ and Sb3+, positioned in octahedral sites within the rock salt structure. Hg doping significantly improves the phase stability of AgSbTe2 by restricting the congenital emergence of the Ag2Te minor secondary phase and promotes partial atomic ordering in the cation sublattice. Reduction in atomic disorder coalesced with a complementary modification of electronic structure by Hg doping results in increased carrier mobility. The formation of nanoscale superstructure with sizes (2-5 nm) of the order of phonon mean free path in AgSbTe2 is further promoted by reduced partial disorder, causes enhanced scattering of heat-carrying phonons, and results in a glass-like ultralow lattice thermal conductivity (∼0.32 W m-1 K-1 at 297 K). Cumulatively, the multifaceted influence of Hg doping, in conjunction with the consequential reduction in disorder, allows achieving a high thermoelectric figure-of-merit, zT, of ∼2.4 at ∼570 K. This result defies conventional paradigms that prioritize increased disorder for optimizing zT.

Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4.

The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N2 and CO2 around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm-1). In contrast to a single vibrational mode generally reported for entrapped N2, we show three Raman modes of adsorbed N2 in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N2 adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.

Unusual CO2 Adsorption in ZIF-7: Insight from Raman Spectroscopy and Computational Studies.

Here, we use Raman spectroscopy to investigate temperature-dependent changes in the atomic-scale structure of the zeolitic imidazolate framework ZIF-7 in a CO2 atmosphere and uncover the mechanism of maximal CO2 adsorption at 206 K. At 301 K, the Raman spectra of ZIF-7 at various CO2 gas pressures reveal a narrow-pore (np) to large-pore (lp) phase transition commencing at 0.1 bar as a result of adsorption of CO2, as evident in the appearance of Fermi resonance bands of CO2 at 1272 and 1376 cm-1. Moreover, the Raman inactive bending mode of CO2 becomes active due to geometrical distortion of adsorbed CO2. It further splits into two peaks due to hydrogen bonding interactions between CO2 and the benzene ring of the benzimidazole linker ZIF-7, as supported by our computational studies. In addition, the interaction between CO2 molecules plays a key role. Upon reducing the temperature at 1 bar CO2 gas pressure, ZIF-7 exhibits softening of the imidazole puckering mode and the Fermi resonance CO2 band due to interactions between CO2 and the framework through hydrogen bonding. At 206 K, substantial modification in the lattice mode and disappearance of the Raman inactive CO2 bending mode confirm the changes in the size of the pore cavity through structural rearrangements of CO2.

Electron-Phonon Coupling Mediated Self-Trapped-Exciton Emission and Internal Quantum Confinement in Highly Luminescent Zero-Dimensional (Guanidinium)6Mn3X12 (X = Cl and Br).

We report a large Stokes shift and broad emission band in a Mn-based organic-inorganic hybrid halide, (Guanidinium)6Mn3Br12 [GuMBr], consisting of trimeric units of distorted MnBr6 octahedra representing a zero-dimensional compound with a liquid like crystalline lattice. Analysis of the photoluminescence (PL) line width and Raman spectra reveals the effects of electron-phonon coupling, suggestive of the formation of Frenkel-like bound excitons. These bound excitons, regarded as the self-trapped excitons (STEs), account for the large Stokes shift and broad emission band. The excited-state dynamics was studied using femtosecond transient absorption spectroscopy, which confirms the STE emission. Further, this compound is highly emissive with a PL quantum yield of ∼50%. With chloride ion incorporation, we observe enhancement of the emissive properties and attribute it to the effects of intrinsic quantum confinement. Localized electronic states in flat bands lining the gap and their strong coupling with phonons are confirmed with first-principles calculations.

Opportunities and challenges for 2D heterostructures in battery applications: a computational perspective.

With an increasing demand for large-scale energy storage systems, there is a need for novel electrode materials to store energy in batteries efficiently. 2D materials are promising as electrode materials for battery applications. Despite their excellent properties, none of the available single-phase 2D materials offers a combination of properties required for maximizing energy density, power density, and cycle life. This article discusses how stacking distinct 2D materials into a 2D heterostructure may open up new possibilities for battery electrodes, combining favourable characteristics and overcoming the drawbacks of constituent 2D layers. Computational studies are crucial to advancing this field rapidly with first-principles simulations of various 2D heterostructures forming the basis for such investigations that offer insights into processes that are hard to determine otherwise. We present a perspective on the current methodology, along with a review of the known 2D heterostructures as anodes and their potential for Li and Na-ion battery applications. 2D heterostructures showcase excellent tunability with different compositions. However, each of them has distinct properties, with its own set of challenges and opportunities for application in batteries. We highlight the current status and prospects to stimulate research into designing new 2D heterostructures for battery applications.

Activation of CO2 and CH4 on MgO surfaces: mechanistic insights from first-principles theory.

One of the most challenging topics in heterogeneous catalysis is conversion of CH4 to higher hydrocarbons. Direct conversion of CH4 to ethylene can be achieved via the oxidative coupling of methane (OCM) reaction. Despite studies which have shown MgO to activate CH4 and initiate the OCM reaction, its large-scale applications face a significant impediment due to formation of a byproduct, CO2, and poisoning of the catalyst due to carbonate formation. In the present work, we address two aspects of the OCM reaction on MgO surfaces: carbonate formation on the surface of the catalyst, and (dissociative) adsorption of CH4. We use first-principles density functional theoretical calculations to determine the energetics and underlying mechanisms of interaction of CO2 and CH4 with various surfaces of MgO: (100), (110), and (111) (both Mg- and O-terminations), and the seldom studied, hydroxylated (111) MgO surface with O-termination. We find that the strength of the interaction of CO2 with MgO surfaces depends on several factors: their surface energies, coordination number of surface O atoms, and ability to donate electrons. However, the O-terminated (111) surface of MgO bucks all aforementioned factors, with only oxygen richness affecting its reactivity towards CO2. The interaction of CH4 with MgO surfaces depends primarily on the coordination number of the surface O atoms and the orientation of the CH4 molecule with respect to the surface. Finally, we provide insights into (a) formation of surface carbonates, which is relevant to CO2 capture and conversion, and (b) C-H bond activation on MgO surfaces, which is crucial for direct conversion of CH4 to value-added chemicals.

Magneto-Optical Stark Effect in Fe-Doped CdS Nanocrystals.

Fe2+ doping in II-VI semiconductors, due to the absence of energetically accessible multiple spin state configurations, has not given rise to interesting spintronic applications. In this work, we demonstrate for the first time that the interaction of homogeneously doped Fe2+ ions with the host CdS nanocrystal with no clustering is different for the two spin states and produces two magnetically inequivalent excitonic states upon optical perturbation. We combine ultrafast transient absorption spectroscopy and density functional theoretical analysis within the ground and excited states to demonstrate the presence of the magneto-optical Stark effect (MOSE). The energy gap between the spin states arising due to MOSE does not decay within the time frame of observation, unlike optical and electrical Stark shifts. This demonstration provides a stepping-stone for spin-dependent applications.

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids.

Understanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity (κlat ) in crystalline materials. Herein, we demonstrate local cationic off-centring within the global rock-salt structure of AgSbSe2 by using synchrotron X-ray pair distribution function analysis and unravel the origin of its ultralow κlat ≈0.4 W mK-1 at 300 K. The cations are locally off-centered along the crystallographic ⟨ 100 ⟩ direction by about ≈0.2 Å, which averages out as the rock-salt structure on the global scale. Phonon dispersion obtained by density functional theory (DFT) shows weak instabilities that cause local off-centering distortions within an anharmonic double-well potential. The local structural distortion arises from the stereochemically active 5s2 lone pairs of Sb. Our findings open an avenue for understanding how the local structure influences the phonon transport and facilitates the design of next-generation crystalline materials with tailored thermal properties.

Emphanisis in Cubic (SnSe)0.5(AgSbSe2)0.5: Dynamical Off-Centering of Anion Leads to Low Thermal Conductivity and High Thermoelectric Performance.

The structural transformation generally occurs from lower symmetric to higher symmetric structure on heating. However, the formation of locally broken asymmetric phases upon warming has been evidenced in PbQ (Q = S, Se, Te), a rare phenomenon called emphanisis, which has significant effect on their thermal transport and thermoelectric properties. (SnSe)0.5(AgSbSe2)0.5 crystallizes in rock-salt cubic average structure, with the three cations occupying the same Wycoff site (4a) and Se in the anion position (Wycoff site, 4b). Using synchrotron X-ray pair distribution function (X-PDF) analysis, herein, we show the gradual deviation of the local structure of (SnSe)0.5(AgSbSe2)0.5 from the overall cubic rock-salt structure with warming, resembling emphanisis. The local structural analysis indicates that Se atoms remain in off-centered position by a magnitude of ∼0.25 Å at 300 K along the [111] direction and the magnitude of this distortion is found to increase with temperature resulting in three short and three long M-Se bonds (M = Sn/Ag/Sb) within the average rock-salt lattice. This hinders phonon propagation and lowers the lattice thermal conductivity (κlat) to 0.49-0.39 W/(m·K) in the 295-725 K range. Analysis of phonons based on density functional theory (DFT) reveals significant soft modes with high anharmonicity which involve localized Ag and Se vibrations primarily. Emphanisis induced low κlat and favorable electronic structure with multiple valence band extrema within close energy concurrently give rise to a promising thermoelectric figure of merit (zT) of 1.05 at 706 K in p-type carrier optimized Ge doped new rock-salt phase of (SnSe)0.5(AgSbSe2)0.5.

Predicting the DNA Conductance Using a Deep Feedforward Neural Network Model.

Double-stranded DNA (dsDNA) has been established as an efficient medium for charge migration, bringing it to the forefront of the field of molecular electronics and biological research. The charge migration rate is controlled by the electronic couplings between the two nucleobases of DNA/RNA. These electronic couplings strongly depend on the intermolecular geometry and orientation. Estimating these electronic couplings for all the possible relative geometries of molecules using the computationally demanding first-principles calculations requires a lot of time and computational resources. In this article, we present a machine learning (ML)-based model to calculate the electronic coupling between any two bases of dsDNA/dsRNA and bypass the computationally expensive first-principles calculations. Using the Coulomb matrix representation which encodes the atomic identities and coordinates of the DNA base pairs to prepare the input dataset, we train a feedforward neural network model. Our neural network (NN) model can predict the electronic couplings between dsDNA base pairs with any structural orientation with a mean absolute error (MAE) of less than 0.014 eV. We further use the NN-predicted electronic coupling values to compute the dsDNA/dsRNA conductance.

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance.

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe)0.9 (AgBiTe2 )0.1 , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe)1-x (AgBiTe2 )x . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)1-x (AgBiTe2 )x . (GeSe)0.9 (AgBiTe2 )0.1 has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 µW cm-1 K-2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm-1 K-1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)0.9 (AgBiTe2 )0.1 . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.

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.

Intrinsically Ultralow Thermal Conductivity in Ruddlesden-Popper 2D Perovskite Cs2PbI2Cl2: Localized Anharmonic Vibrations and Dynamic Octahedral Distortions.

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 (κL) in the single crystal of all-inorganic layered Ruddlesden-Popper (RP) perovskite, Cs2PbI2Cl2, synthesized by the Bridgman method. We have measured the anisotropic κL value of the Cs2PbI2Cl2 single crystal and observed an ultralow κL 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 3s2 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 κL value in the all-inorganic layered RP perovskite Cs2PbI2Cl2.

Chemical Route to Twisted Graphene, Graphene Oxide and Boron Nitride.

The recently discovered twisted graphene has attracted considerable interest. A simple chemical route was found to prepare twisted graphene by covalently linking layers of exfoliated graphene containing surface carboxyl groups with an amine-containing linker (trans-1,4-diaminocyclohexane). The twisted graphene shows the expected selected area electron diffraction pattern with sets of diffraction spots out with different angular spacings, unlike graphene, which shows a hexagonal pattern. Twisted multilayer graphene oxide could be prepared by the above procedure. Twisted boron nitride, prepared by cross-linking layers of boron nitride (BN) containing surface amino groups with oxalic acid linker, exhibited a diffraction pattern comparable to that of twisted graphene. First-principles DFT calculations threw light on the structures and the nature of interactions associated with twisted graphene/BN obtained by covalent linking of layers.

Role of van der Waals interaction in enhancing the photon absorption capability of the MoS2/2D heterostructure.

van der Waals (vdW) interaction-based heterostructures are known for enhanced photon absorption. However, the origin of these phenomena is not yet completely understood. In this work, using first-principles calculations, we provide a comprehensive study to show the effect of vdW interactions on the optical and electrical characteristics of the device and its origin. Herein, MoS2/2D (where 2D varies as graphene, black and blue phosphorene, and InSe) vdW heterojunctions are considered as model structures. The change in the band gap of the heterostructures is because of hybridisation and the non-linearity of the exchange-correlation functional. Hybridisation is correlated with strain and the difference in interstitial potential between layers of the heterostructure and the vacuum level. Significantly, the estimated values of energy conversion efficiency are high in the case of MoS2/InSe and MoS2/BlackP vdW heterostructures as compared to MoS2/GR and MoS2/BlueP, suggesting their potential application in efficient and atomically thick excitonic solar cell devices.

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n-Type BiTe.

A challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (µ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high µ. Weak topological insulators (WTI) are of interest because of their layered hetero-structural nature which has a low lattice thermal conductivity (κlat ). BiTe, a unique member of the (Bi2 )m (Bi2 Te3 )n homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi2 Te3 (where m:n=0:1). Herein, we report intrinsically low κlat of 0.47-0.8 W m-1 K-1 in the 300-650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high µ≈516 cm2 V-1 s-1 and 707 cm2 V-1 s-1 along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature.

Realization of Both n- and p-Type GeTe Thermoelectrics: Electronic Structure Modulation by AgBiSe2 Alloying.

Successful applications of a thermoelectric material require simultaneous development of compatible n- and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe2 with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe)100-x(AgBiSe2)x for x ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around x ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe2 concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe)100-x(AgBiSe2)x. This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe)100-x(AgBiSe2)x has promising thermoelectric performance in the mid-temperature range (400-500 K) with maximum thermoelectric figure of merit, zT, reaching ∼1.3 in p-type (GeTe)80(AgBiSe2)20 at 467 K and ∼0.6 in n-type (GeTe)50(AgBiSe2)50 at 500 K.