Pristine Compound Research Articles

Achieving enhanced power factor using dual substitution at Cu and S sites in tetrahedrites thermoelectric materials Cu12Sb4S13

Thermoelectric (TE) materials-based devices are valuable for translating heat into electricity, but the cost is largely driven by the use of purified metals required for efficient TE performance. This study focuses on tetrahedrite materials, which are cost-effective due to their abundance in copper ore mining waste and status as one of the most widespread sulfosalts on Earth. The research investigates the effects of co-doping and iso-valent doping on the TE properties of p-type tetrahedrite samples, specifically Cu10Mn2Sb4S11Se2 (Sample A) and Cu10MnZnSb₄S11Se2 (Sample B). These samples were synthesized using hot pressing at 773 K and compared with the existing pristine compound (Cu10Mn2Sb4S13). Doping with Mn and Zn (1:1) at the Cu site was found to optimize carrier concentration and enhance phonon scattering, leading to an improved power factor. Additionally, the Se substitution at the S site also introduced band degeneracy, further increasing the power factor. Further, sample B exhibited the highest Seebeck coefficient (∼300 μV/K) at 650 K, in comparison to Sample A (∼225 μV/K) and the pristine compound (∼250 μV/K). On the other hand, Sample A demonstrated significantly higher electrical conductivity (∼0.76 × 10⁴ S/m at 650 K), ∿ three times greater than that of the other prepared samples. Notably, the power factor of Sample A (∼0.389 mW/mK2 at 650 K) was more than twice that of Sample B (∼0.141 mW/mK2) and the pristine compound (∼0.160 mW/mK2). These findings suggest that dual-site substitution at the Cu and S positions presents a promising strategy to augment the TE performance of tetrahedrite compounds.

Impact of Fe-doping on magneto-optoelectronic properties of beryllium sulfide (BeS): A first principles insight

The investigation of electronic, magnetic, structural, and optical properties of Be1-xFexS alloys is carried out by utilizing FP-LAPW method based on density functional theory (DFT) calculations. Fe-doped BeS exhibits half-metallic ferromagnetic behaviour at all doping concentrations, transforming it as favourable contender for spintronic devices. Our findings reveal the presence of 100 % spin polarization of electronic states. The calculated values of magnetic moments are 4.0, 8.0, and 16.0 μB for Be0.9375Fe0.0625S, Be0.875Fe0.125S and Be0.75Fe0.25S, respectively. The pristine BeS compound adopts a cubic close packing structure, known as the zinc blende structure, with the space group F-43m. The static values of R(0) are observed to be 0.21, 0.56, and 0.72 for Fe doped BeS at Be0.9375Fe0.0625S, Be00875Fe0.125S and Be0.75Fe0.25S, respectively. The maximum values of σ(ω) are computed as 16375.9 (1/Ω cm) for 6.25 %, 14336.5 (1/Ω cm) for 12.5 %, and 11002.1 (1/Ω cm) for 25 % Fe doped BeS. The present study unveils maximum optical absorption and conductivity in the ultraviolet (UV) region, suggesting its potential application in UV-based photovoltaic devices.

(C3H7NH3)4Bi1-xSbxI9: 0D hybrid halide perovskite-like compounds with isolated triiodide units.

Antimony/bismuth-based organic-inorganic hybrid halide perovskite-like compounds have generated enormous research interest due to their excellent optical properties. Exploration of new compounds and understanding of their structural stability and optoelectronic properties is of utmost importance for practical applications of these materials. We report two new 0D perovskite-like compounds and their solid solution, (C3H7NH3)4Bi1-xSbxI9, having propyl amine as the spacer cation and iodine as the halide ion. All compounds crystallized in the space group C2/m at room temperature and undergo a phase transition from C2/m to P21/c at low temperature (90 K) as observed from the single-crystal study. A low-temperature (250 K, 180 K, 150 K and 90 K) single-crystal study shows that the (PA)4BiI9 compound retains the monoclinic space group C2/m until 150 K and undergoes a phase transition to the P21/c space group at 90 K. Further, it is observed that ordering, rearrangement and relaxation of the long-chain propyl amine group are primarily responsible for the structural transition. The structure contains [(Bi/Sb)I6]3- polyhedra along with linear I3- units, giving rise to the formula of (PA)3(Bi/Sb)I6·(PA)I3. The I3- units interact poorly while the [MI6]3- (M = Bi, Sb) octahedral units interact significantly with spacer cations via the H-bond, resulting in more distortion in these octahedral units. Theoretical calculations revealed that iodide ions have dual roles and contribute largely to both the valence band maxima and conduction band minima in these compounds. From both experimental and theoretical calculations, it is observed that the pristine compounds are of the indirect band gap-type and Sb substitution in (PA)4Bi1-xSbxI9 led to a gradual decrease in the band gap.

Quasi-FeCo-MOF/CoS amorphous Nanosheets: A stable and highly active catalyst for electrocatalytic water splitting and monosaccharide oxidation

Quasi-metal organic frameworks (quasi-MOFs) are intermediate materials between pristine MOFs and metal compounds, possessing unusual structures and properties, which makes them promising porous materials. However, their synthesis remains challenging due to the absence of reliable design strategies. Here, we propose a novel solvent-thermal synthesis strategy for preparing quasi-FeCo-MOF/CoS amorphous nanosheet composites. The strategy involves the use of high-coordination metal ions and simple organic ligands, followed by a simple solvent-thermal treatment to induce quasi-MOF formation. The formation of CoS not only transforms the original crystal structure of FeCo-MOF into amorphous nanosheets but also imparts structural stability to the composite material. Furthermore, density functional theory (DFT) calculations reveal the catalytic mechanism, demonstrating that the introduction of CoS can adjust the d-band center of the composite material, optimizing adsorption free energy and reducing its overpotential. This method significantly enhances the electrocatalytic activity of the composite material in alkaline and neutral solutions. Notably, the optimized composite material (FeCo-MOF/CoS-2/FF) exhibits low overpotentials of 142 mV and 55 mV (at 10 mA cm−2) for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively, and achieved a current density of 10 mA cm−2 with a low overpotential of 1.391 V for HER and monosaccharide oxidation (MOR) using FeCo-MOF/CoS-2/FF. This method provides new insights and perspectives for developing highly active and stable catalysts and is expected to advance the development of electrocatalytic water splitting technology.

Superconductivity of Co-Doped CaKFe4As4 Investigated via Point-Contact Spectroscopy and London Penetration Depth Measurements.

The iron-based superconductors (IBSs) of the recently discovered 1144 class, unlike many other IBSs, display superconductivity in their stoichiometric form and are intrinsically hole doped. The effects of chemical substitutions with electron donors are thus particularly interesting to investigate. Here, we study the effect of Co substitution in the Fe site of CaKFe4As4 single crystals on the critical temperature, on the energy gaps, and on the superfluid density by using transport, point-contact Andreev-reflection spectroscopy (PCARS), and London penetration depth measurements. The pristine compound (Tc≃36 K) shows two isotropic gaps whose amplitudes (Δ1 = 1.4-3.9 meV and Δ2 = 5.2-8.5 meV) are perfectly compatible with those reported in the literature. Upon Co doping (up to ≈7% Co), Tc decreases down to ≃20 K, the spin-vortex-crystal order appears, and the low-temperature superfluid density is gradually suppressed. PCARS and London penetration depth measurements perfectly agree in demonstrating that the nodeless multigap structure is robust upon Co doping, while the gap amplitudes decrease as a function of Tc in a linear way with almost constant values of the gap ratios 2Δi/kBTc.

Structure and dielectric spectroscopy of double perovskite La2-xPrxCoFe1-yMnyO6 system

Double perovskites have shown promise in the development of spin polarized conduction and ferromag semiconductors. In this category of double perovskite, we report on the lanthanum ferrocobaltite, co-doped with Pr and Mn. The polycrystalline ceramics were analysed for its structural properties by using Rietveld refinement of X-ray diffraction (XRD) and Raman spectroscopy measurements. The pristine and co-doped samples possess monoclinic structure with P21/n space group. In the pristine compound Co and Fe octahedra are ordered periodically which later gets distorted upon cationic substitution. Scanning electron microscopy (SEM) and energy dispersive spectra (EDS) reveals the grain morphology and elemental analysis. The samples were investigated for their dc and ac electrical responses. The two-probe dc resistance was measured from room temperature RT to 575 K. Dielectric spectroscopy in the RT to 650 K temperature range was performed in the frequency range of 50 kHz to 5 MHz. We observed that introducing Pr at La-site and Mn at Fe site creates structural disorder and tilt in octahedra which results in decreasing the dielectric constant at RT but increases significantly at higher temperature with low dielectric loss. ac conductivity was found to increase with increasing temperature indicating the thermally governed hopping mechanism. All these features validate their potential applications as a high temperature capacitor and can be widely used in automotive and aerospace domains for the storage device application.

Rational Design of Cu Vacancies and Antisite Defects for Boosting the Thermoelectric Properties of CuGaTe2-Based Compounds.

CuGaTe2-based compounds show great promise in the application for high-temperature thermoelectric power generation; however, its wide bandgap feature poses a great challenge for enhancing thermoelectric performance via structural defects modulation and doping the system. Herein, it is discovered that the presence of GaCu antisite defects in the CuGaTe2 compound promotes the formation of Cu vacancies, and vice versa, which tends to form the charge-neutral structure defects combination with one GaCu antisite defect and two Cu vacancies. The accumulation of Cu vacancies in the structure of the (Cu2Te)x(Ga2Te3)1-x compounds evolves into twins and stacking faults. This in conjunction with GaCu antisite defects intensify the point defects phonon scattering, yielding a dramatic reduction on lattice thermal conductivity from 6.95 W m-1 K-1 for the pristine CuGaTe2 sample to 2.98 W m-1 K-1 for the (Cu2Te)0.45(Ga2Te3)0.55 sample at room temperature. Furthermore, the high concentration of charge-neutral defects combination narrows the band gap and increases the carrier concentration, leading to an improved power factor of 1.58 mW/mK2 at 600 K for the (Cu2Te)0.49(Ga2Te3)0.51 sample, which is 41% higher than for the pristine CuGaTe2 sample. Consequently, the highest ZT value of 0.82 is achieved at 915 K for Cu0.015(Cu2Te)0.48(Ga2Te3)0.52, which represents an enhancement of about 22% over that of the pristine CuGaTe2 compound.

From wide band gap semiconductor to visible light responsive material: The role of Li in K2PdO2

The study used Density Functional Theory (DFT) first-principles calculations to analyze the impact of lithium insertion on K2PdO2, a wide band gap semiconductor with limited absorption in visible electromagnetic radiation. The results showed that Li insertion is a promising method to enhance the absorption behavior of these materials. Both pristine and doped compounds displayed negative formation energies, confirming their structural and thermodynamic stability. The electronic band structure and Density of States plots indicated the semiconducting nature of all three compounds, with the band gap decreasing from 2.29 eV to 1.69 eV upon doping. The rise in refractive index with increasing Li content indicates an increase in charge, leading to polarization and reduced light velocity. Optical conductivity increased beyond the band gap values, indicating the material’s responsiveness to visible electromagnetic radiation. Moreover, the thermoelectric response makes K2PdO2 as best candidate for different thermoelectric devices.

Review On Strategies For the Design and Synthesis of Flower‐Like Bi2WO6 and BiOX (X=F, Cl, Br, I) Composites for Photocatalytic Environmental Remediation

AbstractIn latest events, water pollution has posed an immense threat, and its sustainable treatment is a topic of concern. Among myriad of water treatment, research community has lensed towards sustainable heterogeneous photocatalysis especially, inorganic semiconductors. Bismuth tungstate a pristine Aurivillius compound with unique crystal structure, exceptional photochemical stability, and strong redox abilities but minimum visible light absorption and quick carrier recombination are its caveats, these caveats are circumvented by synthesizing BiOX (X=F, Cl, Br and I) composites with surface modifiers, doping with hetero atoms (metals and non‐metals), construction of the heterojunction and amalgamation of carbon materials. The heart of this review is strategic engineering of flower‐like morphology photocatalyst synthesis and optical properties, it also discusses the charge transfer kinetics, photocatalytic performance, and durability. The flower‐like morphology highlighted in this review is due to its ability to harvest light, providing larger specific surface area which introduces more active sites and efficient excitons generation. The comprehensive tabulated data in this review will help the actively engaged research community in Bismuth heterogenous photocatalysis. It also sheds light on the future view of environmental remediation via photocatalytic developments.

Interpenetrated Lattices of Quaternary Chalcogenides Displaying Magnetic Frustration, High Na-Ion Conductivity, and Cation Redox in Na-Ion Batteries.

A series of quaternary selenides, NaxMGaSe4 (M = Mn, Fe, and mixed Zn/Fe), have been synthesized for the first time employing a high-temperature solid-state synthesis route through stochiometric or polychalcogenide flux reactions. Along with the selenides, a previously reported sulfide analogue, NaxFeGaS4, is also revisited with new findings. These compounds form an interpenetrated structure made up of a supertetrahedral unit. The electrochemical evaluations exhibit a reversible (de)intercalation of ∼0.6 and ∼0.45 Na-ions, respectively, from Na2.87FeGaS4 (1a) and Na2.5FeGaSe4 (2) involving Fe2+/Fe3+ redox when cycled between 1.5 and 2.5 V. Mössbauer spectroscopy of 1a shows the existence of a mixed oxidation state of Fe2+/3+ in the pristine compound and reversible oxidation of Fe2+ to Fe3+ during the electrochemical cycles. Na2.79Zn0.6Fe0.4GaSe4 possesses a reasonably high room temperature ionic conductivity of 0.077 ms/cm with an activation energy of 0.30 eV. The preliminary magnetic measurements show a bifurcation of FC-ZFC at 4.5 and 2.5 K, respectively, for 1a and Na3MnGaSe4 (4) arising most likely from a spin-glass like transition. The high negative values of the Weiss constants -368.15 and -308.43 K for 1a and 4, respectively, indicate strong antiferromagnetic interactions between the magnetic ions and also emphasize the presence of a high degree of magnetic frustration in these compounds.

Effect of Mn2+ Doping on the Photoluminescence of Hybrid One-Dimensional Lead Halide Post-Perovskites.

Although organic-inorganic hybrid one-dimensional (1D) lead halide postperovskites (LHPPs) have been reported to show white luminescence and tunable photoluminescence quantum yield (PLQY), their structure-property relationships are not fully understood. Here, we used Mn2+ to test the doping effect on the luminescence of two 1D-LHPPs compounds, namely, {TETA[Pb2Br6]}n 1 and {TETA[Pb2Cl6]}n 2, where TETA = triethylenetetrammonium. We found the pristine compounds show yellowish (551 nm) and bluish (447 nm) emission for 1 and 2, respectively, nanosecond excitation lifetimes (4.17 ns for 1 and 2.29 ns for 2) and low PLQYs (4.65 and 3.57% for 1 and 2, respectively). By fine-doping the Mn2+ ions to ca. 8% the PLQYs for 1 and 2 are maximized to 24 and 25% for 1 and 2, respectively. Upon the increasing Mn2+ dopant, the emission wavelengths can also vary gradually from 551 to 615 nm and from 447 to 660 nm for 1 and 2, respectively, covering almost the whole visible-light range, and the excitation lifetimes are enhanced to microseconds (0.77 μs for 1 and 0.39 μs for 2), owing to the more spin-forbidden d-d transition (4T1-6A1) component from the Mn2+ ions present in the photoluminescence spectra. Moreover, these Mn2+-doped 1D-LHPPs demonstrate high structural and optical stability in humid and high-temperature environments. Hence, such doped materials can be fabricated into a UV-pumped white light-emitting diode, rendering the potential application for solid-state lighting and display systems.

Impact of doping and hydrostatic pressure on structural, electronic, optical, and mechanical properties of novel double halide perovskite Cs2LiGaBr6

The emergence of lead-free halide double perovskites exhibiting bandgaps within the visible spectrum represents a substantial advancement in engineering environmentally benign perovskite solar cells. In this work, we investigated the structural, optical, electronic, and mechanical properties of Cs-based lead-free Cs2LiGaBr6 double halide perovskites with Mn and Cr doping under hydrostatic pressure ranging from 2 to 80 GPa using density functional theory (DFT). The introduction of dopants consistently alters the lattice parameters because of the mismatch in atomic radii, whereas increasing the pressure leads to a reduction in these constants. All the studied Cs2LiGaBr6 compounds exhibited direct band gaps, which increased slightly with doping. This is attributed to the modulation of electronic states by dopant-related defect levels. The bandgap variation under pressure is primarily attributed to changes in the quantum confinement effects induced by compressive strain. Analysis of the density of states and optical properties revealed enhanced absorption in the visible spectrum for the doped compositions, and in the UV spectrum under pressure. The study of mechanical stability confirms the ductile nature of both the doped and pristine compounds under pressure, underscoring their suitability for thin film production. This study contributes to the understanding of sustainable alternatives for perovskite optoelectronic applications, emphasizing Cs2LiGaBr6's potential under diverse conditions and dopant influences.

Augmentation in the thermoelectric performance by integrating the natural minerals with synthetic tetrahedrite compounds

Thermoelectric (TE) materials-based devices are useful in converting heat into electricity. The cost of thermoelectric materials is mainly incurred from the purified pure metals used to make the efficient TE device. In the current study, tetrahedrite materials are considered due to their low cost, as these materials are abundant in mining waste of copper ores and also the most extensive sulfosalt on earth. Hence, to add value addition to mine waste materials, the composite approach has been employed to integrate the natural tetrahedrite (mine waste/tailings product) materials with synthetically developed tetrahedrite materials in the lab for the current investigation. Further, the optimum quantity of mine waste addition is optimized by the systematic investigation of TE performance with varying the mine waste percentage addition to the synthetic tetrahedrite. The p-type synthetic Cu10FeZnSb4S13 (Sample-A) tetrahedrite compound and 70 (Cu10FeZnSb4S13) + 30 (natural/mine waste) (Sample-B) have been synthesized employing hot-press at 773 K along with the pristine (Cu10Fe2Sb4S13) tetrahedrite compound. Moreover, the Seebeck coefficient is increased by 36 % (220 μV/K to 300 μV/K) for Sample-B, as compared to sample A and it is ∿ 2 times higher as compared to the pristine compound. The power factor is increased by ∿ 3 times that of the Sample-A (0.05 mW/mK2 to 0.15 mW/mK2). In addition, the thermal conductivity of Sample-B compound was further reduced by 30 % than Sample-A (0.98 W/mK to 0.69W/mK) and ∿ 44 % more than the pristine compound. Finally, the zT of Sample-B increased to almost 3 times than Sample-A (0.05–0.15).

Atomic Design of Copper Active Sites in Pristine Metal-Organic Coordination Compounds for Electrocatalytic Carbon Dioxide Reduction.

Electrocatalytic carbon dioxide reduction reaction (CO2RR) has emerged as a promising and sustainable approach to cut carbon emissions by converting greenhouse gas CO2 to value-added chemicals and fuels. Metal-organic coordination compounds, especially the copper (Cu)-based coordination compounds, which feature well-defined crystalline structures and designable metal active sites, have attracted much research attention in electrocatalytic CO2RR. Herein, the recent advances of electrochemical CO2RR on pristine Cu-based coordination compounds with different types of Cu active sites are reviewed. First, the general reaction pathways of electrocatalytic CO2RR on Cu-based coordination compounds are briefly introduced. Then the highly efficient conversion of CO2 on various kinds of Cu active sites (e.g., single-Cu site, dimeric-Cu site, multi-Cu site, and heterometallic site) is systematically discussed, along with the corresponding catalytic reaction mechanisms. Finally, some existing challenges and potential opportunities for this research direction are provided to guide the rational design of metal-organic coordination compounds for their practical application in electrochemical CO2RR.

Epoxy modified styrene butadiene rubber (SBR) in green tire application

Styrene butadiene rubber (SBR) is an important elastomer in tire application. The tire industries are facing challenges in optimizing the magic triangle, which balances three contradicting properties: abrasion resistance, wet traction, and rolling resistance (RR). There are numerous approaches foroptimizingtire compound characteristics, which eventually impact tire performance. Choosing an appropriate kind of reinforcing filler is one of the techniques that may be used. Silica based tires give better traction properties and low rolling resistance, but silica has hydroxyl groups on the surface, and it is not compatible with non-polar elastomers such as Emulsion-based Styrene Butadiene Rubber (ESBR). Hence, it is necessary to incorporate some polar moieties into ESBR chains to enhance the compatibility with silica. In this study, non-polar Emulsion-based Styrene Butadiene Rubber (ESBR) is transformed into polar ESBR by modifying with epoxidizing reagent at room temperature. The epoxidized-ESBR (E-ESBR) was characterized by 1H NMR, FT-IR, and DSC analyses. Synthesized E-ESBR was compounded with ESBR and silica filler for the ‘green tire’ tread compounding (ESBR-Silica/E-ESBR). The incorporation of E-ESBR improved silica dispersion in the ESBR matrix. Hence, the tensile strength of ESBR-Silica/E-ESBR compound improved. The rolling resistance decreased compared to the ESBR-Silica compound. Furthermore, dry, wet, and snow traction properties of the compounded system were improved by 9 %, 25 %, and 37 %, respectively, in comparison to the pristine EBSR-Silica compound. Thus, the incorporation of the E-ESBR system could generate a new pave for better dispersion of silica in the non-polar elastomer matrix.

Potential improvement in power factor of (Bi0.98Ge0.02)2Te2.7Se0.3 compound due to defect engineering

In recent years, thermoelectricity has gained popularity as a renewable energy source, with applications including Peltier coolers and thermoelectric generators, particularly focusing on materials, like bismuth telluride and its doped derivatives. This study investigates Bi2Te3, (Bi0.98Ge0.02)2Te2.7Se0.3, and (Bi0.98Ge0.02)2Te2.7Se0.3/Bi2Te3 synthesized via solid-state reaction, revealing a rhombohedral structure in the XRD pattern and confirming chemical composition and composite homogeneity through EDS and porosity, density, and selenium integration via FESEM. Electrical resistivity decreases with rising temperature, while the Seebeck coefficient shows a linear increase, indicating n-type semiconductor behaviour. The highest power factor of 108 μW/mK2 is achieved by (Bi0.98Ge0.02)2Te2.7Se0.3, contrasting with the lowest of 20 μW/mK2 observed for the pristine sample at 250 °C. Ge atoms enhance the power factor of (Bi0.98Ge0.02)2Te2.7Se0.3 by 5.4 times compared to the pristine compound, making it ideal for thermoelectric applications through acceptor behaviour and defect engineering.

Recent Advances in Catalytic Compounds Developed by Thermal Treatment of (Zr-Based) Metal–Organic Frameworks

Metal–organic frameworks (MOFs) have been the subject of extensive scientific investigation in the last three decades and, currently, they make up one of the types of compounds most studied for their potential application in a wide range of distinct catalytic processes. Pristine MOF compounds provide several intriguing benefits for catalytic applications, including large interior surface areas and high densities of active sites; high catalytic reaction rates per volume; post-synthesis modifications with complementary catalytic groups; and the ability for multiple functional groups to catalyze the reaction. For most large-scale catalytic applications, including those in fuel processing, gas emission reduction, and chemical synthesis, pristine MOFs often show limited stabilities and opportunities for regeneration at high temperatures. As a result, the real applications of MOFs in these technologies are likely to be constrained, and a controlled thermal modification to prepare MOF-derivative compounds has been applied to induce crystalline structural changes and increase the structural stability of the MOFs, enhancing their potential applicability in more severe catalytic processes. Recent advances concerning the use of this strategy to boost the catalytic potential of MOF-derivative compounds, particularly for stable Zr-based MOFs, are outlined in this short review article.

Boosting the thermoelectric properties of layered SnSb2Te4 compound by microstructure regulation combined with heterovalent halogen substitution

The layered IVV2Te4-based compounds have recently received significant attention due to their intrinsically low lattice thermal conductivities. However, the influence of microstructure regulation and halogen doping on their thermoelectric properties has been scarcely studied. In this work, we systematically investigated the anisotropic thermoelectric transport properties of pristine SnSb2Te4 compounds with varied microstructures manipulated by ball milling. With the help of thermoelectric transport models, the optimal ball milling time is estimated based on the calculation of quality factor. Furthermore, the remarkably improved Seebeck coefficient is achieved by the introduction of halogen elements into Te sites, which is attributed to reduced carrier concentration and converged carrier pockets. As a result, a maximal zT ∼0.4 at 725 K is achieved in SnSb2(Te0·94I0.06)4 sample aligned with the SPS pressure direction with a ball-milling time of 1 h, which demonstrates a 35 % enhancement over the pristine sample. This work provides an insightful guide to elevate the thermoelectric properties of layered IVVI2Te4-based compounds.

Optoelectronic and mechanical properties of gallium arsenide alloys: Based on density functional theory

First principles calculations based on density functional theory (DFT) were performed to investigate the structural, electronic, optical and mechanical properties of pristine GaAs compound and its alloy; Ga0.75Al0.25As, Ga0.75In0.25As, Ga0.75Sn0.25As, Ga0.75Ti0.25As. WIEN2K and Quantum expresso (QE) codes were adopted for calculations using generalized gradient approximation (GGA) in Perdew-Burke Erzenhoff (PBE) as exchange correlation function for both codes. Full potential linear augmented plane wave (FPLAPW) with the local orbital method was adopted as implement in WIEN2K code. In QE code, norm-conserving pseudopotentials were employed on a plane-wave expansion of the wave functions. Structural and electronic properties were elaborated since their result gives information about the optical and mechanical performance. Electronic band structure and optical parameters were performed using WIEN2K code. Underestimation of band gap observed from DFT calculations were corrected by using Modified Becke and Johnson (mBJ). Mechanical components were determined using QE with thermo_pw package. Lattice constant, volume, bulk modulus and other physical parameters were calculated for structural properties. Discrepancy in these parameters as observed in crystal structure is associated to difference in ionic radius of host and substituted atom. The results of band structure and density of states were calculated for electronic properties. All the studied compounds were semiconductors in nature except Ga0.75Sn0.25As which displayed metallic character. Optical parameters including extinction coefficient, absorption coefficient, refractive index, optical conductivity, optical reflectivity and energy loss function have been computed from the dielectric function at energy range of 0 to 25 eV using the Kramers-Kronig transformations. Calculated elastic function were used to compute the mechanical properties such as anisotropic, brittle characteristics, stiffness and many others. All the results were compared with available theoretical and experimental records.

Influence of High-Pressure Induced Lattice Dislocations and Distortions on Thermoelectric Performance of Pristine SnTe

As a sister compound of PbTe, SnTe possesses the environmentally friendly elements. However, the pristine SnTe compounds suffer from the high carrier concentration, the large valence band offset between the L and Σ positions and high thermal conductivity. Using high-pressure and high-temperature technology, we synthesized the pristine SnTe samples at different pressures and systemically investigated their thermoelectric properties. High pressure induces rich microstructures, including the high-density dislocations and lattice distortions, which serve as the strong phonon scattering centers, thereby reducing the lattice thermal conductivity. For the electrical properties, pressure reduces the harmful high carrier concentration, due to the depression of Sn vacancies. Moreover, pressure induces the valence band convergence, reducing the energy separation between the L and Σ positions. The band convergence and suppressed carrier concentration increase the Seebeck coefficient. Thus, the power factors of pressure-sintered compounds do not deteriorate significantly under the condition of decreasing electrical conductivity. Ultimately, for a pristine SnTe compound synthesized at 5 GPa, a higher ZT value of 0.51 is achieved at 750 K, representing a 140% improvement compared to the value of 0.21 obtained using SPS. Therefore, the high-pressure and high-temperature technology is demonstrated as an effectively approach to optimize thermoelectric performance.