Spontaneous weathering of natural minerals in charged water microdroplets forms nanomaterials.

In this work, we show that particles of common minerals break down spontaneously to form nanoparticles in charged water microdroplets within milliseconds. We transformed micron-sized natural minerals like quartz and ruby into 5- to 10-nanometer particles when integrated into aqueous microdroplets generated via electrospray. We deposited the droplets on a substrate, which allowed nanoparticle characterization. We determined through simulations that quartz undergoes proton-induced slip, especially when reduced in size and exposed to an electric field. This leads to particle scission and the formation of silicate fragments, which we confirmed with mass spectrometry. This rapid weathering process may be important for soil formation, given the prevalence of charged aerosols in the atmosphere.

A comprehensive analysis of the emerging modern trends in research on photovoltaic systems and desalination in the era of artificial intelligence and machine learning.

Integration of photovoltaic (PV) systems, desalination technologies, and Artificial Intelligence (AI) combined with Machine Learning (ML) has introduced a new era of remarkable research and innovation. This review article thoroughly examines the recent advancements in the field, focusing on the interplay between PV systems and water desalination within the framework of AI and ML applications, along with it analyses current research to identify significant patterns, obstacles, and prospects in this interdisciplinary field. Furthermore, review examines the incorporation of AI and ML methods in improving the performance of PV systems. This includes raising their efficiency, implementing predictive maintenance strategies, and enabling real-time monitoring. It also explores the transformative influence of intelligent algorithms on desalination techniques, specifically addressing concerns pertaining to energy usage, scalability, and environmental sustainability. This article provides a thorough analysis of the current literature, identifying areas where research is lacking and suggesting potential future avenues for investigation. These advancements have resulted in increased efficiency, decreased expenses, and improved sustainability of PV system. By utilizing artificial intelligence technologies, freshwater productivity can increase by 10 % and efficiency. This review offers significant and informative perspectives for researchers, engineers, and policymakers involved in renewable energy and water technology. It sheds light on the latest advancements in photovoltaic systems and desalination, which are facilitated by AI and ML. The review aims to guide towards a more sustainable and technologically advanced future.

Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting.

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

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.

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.

Ultrafast glimpses of the excitation energy-dependent exciton dynamics and charge carrier mobility in Cs2SnI6 nanocrystals.

Advancements in photovoltaic research suggest that tin-based perovskites are potential alternatives to traditional lead-based structures. Cs2SnI6, specifically, stands out as a notable candidate, exhibiting impressive performance. However, its complete potential remains untapped primarily owing to the limited understanding of its photophysics. In light of this, this study aims to bridge this knowledge gap. To commence our study, we first executed theoretical investigations to locate the energetically diverse excitons within the Brillouin zone. Building on this knowledge, we then utilized transient absorption spectroscopy to investigate their temporal evolution. Herein, we observed the formation of high-energy excitons even when the incident photon energy was below the necessary threshold, which is quite distinctive and intriguing. Of particular interest is the generation of ultraviolet (UV) domain exciton using visible photons, which implies that Cs2SnI6 has the potential for efficient solar light harvesting. Tracking the kinetics revealed that this unique finding arises due to the intertwined formation and decay pathways undertaken by the different excitons, aided by intervalley scattering and phonon absorption processes. In addition, we found that the decay of the UV exciton was unusually slow. Transient mobility investigations were undertaken to probe the carrier transport behavior that further established hot carriers (HCs) in Cs2SnI6 to be highly mobile and susceptible to polaron formation. Overall, our findings demonstrate that Cs2SnI6 is a strong candidate for HC-based photovoltaics because it possesses all the prerequisites desired for such applications.

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.

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.

Metavalent Bonding Origins of Unusual Properties of Group IV Chalcogenides.

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.

Dental Anxiety Assessment through the Drawings of Children and Influence of Art Therapy.

Background: Assessment of dental anxiety in children is difficult because of their immature cognitive and emotional development. Drawings are well well-established emotion assessment tool. These can be used as nonverbal methods of communication for expressing the children's anxiety and emotions. Art therapy utilizes creative therapy interventions to deal with children suffering from emotional problems. Aims and Objectives: The aim of the study is (1) To assess the drawings of children for the presence of dental anxiety. (2) To study the effectiveness of art therapy on dental anxiety in children. Materials and Methods: One hundred and twenty children within the age group of 6-12 years were part of this study. The pretest assessment of dental anxiety was done using Frankl and Five facial anxiety scales. All the participants received local anesthesia during their first treatment session. At the end of the treatment session, all the participants were asked to draw a picture of their experience. The drawn figures were assessed by a psychologist. The children were allotted randomly into the study group (n = 60) and control group (n = 60). Art therapy was given to 60 children in the study group for three consecutive appointments. The posttest assessment of dental anxiety was done using Frankl and Five facial anxiety scales after completion of the entire dental treatment. Results: The pretest scores revealed very high level of dental anxiety in 33 (55%) children in study group and 34 (56.67%) children in control group. The posttest scores of study group, who received the art therapy, revealed that 24 (40%) children had little anxiety and 32 (53.33%) children had some anxiety. Whereas in the control group, the posttest scores showed 20 (33.33%) children had high anxiety and 39 (65%) children had very high anxiety scores. There was a significant correlation between the objective score of dental anxiety and the subjective scores of CD: H. Conclusion: Drawings can be used as an assessment tool for the detection of dental anxiety and art therapy effectively reduces the anxiety in pediatric patients undergoing dental treatment.

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.

Glassy thermal conductivity in Cs3Bi2I6Cl3 single crystal.

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.

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.

CO2 Utilization Through its Reduction to Methanol: Design of Catalysts Using Quantum Mechanics and Machine Learning.

Reducing levels of CO2, a greenhouse gas, in the earth's atmosphere is crucial to addressing the problem of climate change. An effective strategy to achieve this without compromising the scale of industrial activity involves use of renewable energy and waste heat in conversion of CO2 to useful products. In this perspective, we present quantum mechanical and machine learning approaches to tackle various aspects of thermocatalytic reduction of CO2 to methanol, using H2 as a reducing agent. Waste heat can be utilized effectively in the thermocatalytic process, and H2 can be generated using solar energy in electrolytic, photocatalytic and photoelectrocatalytic processes. Methanol being a readily usable fuel in automobiles, this technology achieves (a) carbon recycling process, (b) use of renewable energy, and (c) portable storage of H2 for applications in automobiles, alleviating the problem of rising CO2 emissions and levels in atmosphere.

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.

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.

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.

Radiation Therapy Treatment Outcomes for Patients with Head and Neck Cancer: a Tertiary Care Centre Experience in a Tier Three Town.

To report the outcomes of patients with head and neck squamous cell cancers (HNSCC) who received radiotherapy as a part of their first line treatment. All patients treated with radiotherapy for HNSCC between 2013 and 2017 were retrospectively identified from the department records. Patients with follow-up of less than 3 months following primary treatment were excluded. The demographic, disease and outcome details were retrieved from the patients' hospital records. The disease free survival (DFS) was estimated with Kaplan-Meier analysis, and effect of different variables on DFS were compared using log-rank test. The late toxicities of treatment were also recorded and reported. A total of 311 patients were found to be eligible. Most patients were males (82%), and oral cavity cancers predominated (45.3%). 237 patients (76.2%) had locally advanced disease at presentation. Radiotherapy was used as definitive treatment for 152 patients (48.9%). At a median follow up of 14.8 months (range: 3-85.2 months), the estimated Disease-Free Survival (DFS) was 52.7 months (95%CI: 37.7-67.7 months). Locally advanced disease correlated with a significantly lower median DFS (61.2 vs. 32.8 months; p = 0.01), but other factors, including gender, use of concurrent chemotherapy and adjuvant vs definitive radiotherapy appeared to have no statistical association with DFS. 181 (58.2%) patients had chronic RT-related toxicities at last follow-up, but most were generally mild, most frequently subcutaneous fibrosis (48.6%, which occurred significantly more frequently in patients receiving adjuvant RT following surgery- 40.8% versus 57.2%; p = 0.03) and xerostomia (32.4%). Radiation induced osteoradionecrosis was rare, occurring in only three (1%).Most HNSCC patients present with locally advanced disease, where disease control remains poor. Though there is substantial toxicity following treatment, the risk of severe toxicities with modern radiotherapy is low.

Modulation of the electronic structure and thermoelectric properties of orthorhombic and cubic SnSe by AgBiSe2 alloying.

Recently, single-crystals of tin selenide (SnSe) have drawn immense attention in the field of thermoelectrics due to their anisotropic layered crystal structure and ultra-low lattice thermal conductivity. Layered SnSe has an orthorhombic crystal structure (Pnma) at ambient conditions. However, the cubic rock-salt phase (Fm3̄m) of SnSe can only be stabilized at very high pressure and thus, the experimental realization of the cubic phase remains elusive. Herein, we have successfully stabilized the high-pressure cubic rock-salt phase of SnSe by alloying with AgBiSe2 (0.30 ≤ x ≤ 0.80) at ambient temperature and pressure. The orthorhombic polycrystalline phase is stable in (SnSe)1-x (AgBiSe2) x in the composition range of 0.00 ≤ x < 0.28, which corresponds to narrow band gap semiconductors, whereas the band gap closes upon increasing the concentration of AgBiSe2 (0.30 ≤ x < 0.70) leading to the cubic rock-salt structure. We confirmed the stabilization of the cubic structure at x = 0.30 and associated changes in the electronic structure using first-principles theoretical calculations. The pristine cubic SnSe exhibited the topological crystalline insulator (TCI) quantum phase, but the cubic (SnSe)1-x (AgBiSe2) x (x = 0.33) showed a semi-metallic electronic structure with overlapping conduction and valence bands. The cubic polycrystalline (SnSe)1-x (AgBiSe2) x (x = 0.30) sample showed n-type conduction at room temperature, while the orthorhombic (SnSe)1-x (AgBiSe2) x (0.00 ≤ x < 0.28) samples retained p-type character. Thus, by optimizing the electronic structure and the thermoelectric properties of polycrystalline SnSe, a high zT of 1.3 at 823 K has been achieved in (SnSe)0.78(AgBiSe2)0.22.

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.