Excess Bi Research Articles

Formation and characterization of Mn-Bi-Al ternary alloys of enhanced magnetic performance in MnBi/Al composites

Low-temperature phase manganese bismuth (LTP-MnBi) and a series of aluminum (Al) composites were synthesized using a potentially facile and scalable low-temperature liquid-phase sintering method in a vacuum at 300 °C. The incorporation of up to 10 at% Al led to a significant enhancement in coercivity (Hc), increasing from 2.32 ± 0.04 to 4.15 ± 0.07 kOe, while saturation magnetization showed a slight decrease of less than 3 %. However, beyond this concentration, a dramatic reduction in Hc was observed. The density of the freshly compacted powders, which included up to 10 at% Al, remained relatively constant at 7.47–7.58 g/cm³ but decreased with excess Al. A maximum energy product (BH)max of 1 MGOe was achieved in the fresh sample, with a 16 % enhancement in (BH)max in the MnBi composite containing 5 at% Al. Scanning electron microscopy revealed distinct MnBi and Bi-rich regions, while Al-rich areas became prominent at Al concentrations above 10 at%. Energy dispersive spectroscopy confirmed that only 3–5 at% Al could be effectively incorporated into Mn-Bi regions, with excess Al unevenly distributed on the surface. X-ray photoelectron spectroscopy indicated the formation of Al, Al₂O₃, and potential Mn-Bi-Al ternary alloys. Additionally, slab-DFT models, such as AlMn/MnBi, indicate that Al inclusion enhances the magnetization in MnBi composites, providing insights into its effects on the magnetic properties of Mn-Bi systems. These findings offer promising strategies to address the challenges posed by excess Bi in the MnBi structure, potentially optimizing the magnetic performance of similar non-magnetic or soft-magnetic composite systems.

Effect of A-site excess on the shape memory effect of sodium bismuth titanate ceramics

The impact of A-site excess on the shape memory effect of Na0.5Bi0.5TiO3 ceramics was investigated. We found that the recoverable plastic-like strain was increased by more than 10 % by A-site excess. The enhancement is attributed to the change in crystal structure from a rhombohedral phase to a monoclinic one, which further changes the phase transition behavior between the ferroelectric and nonpolar phases. Suppression of grain size and the generation of defects after the addition of excess Bi and Na were proposed for the appearance of monoclinic phase. This investigation proposes a viable approach to modulate the shape memory effect of Na0.5Bi0.5TiO3-based ceramics through the manipulation of the crystal structure and the associated phase transition processes, ultimately influencing the shape memory characteristics.

Effect of Bi on the Performance of Al-Ga-In Sacrificial Anodes.

Cathodic protection is widely used for metal corrosion protection. To improve their performance, it is necessary and urgent to study the influence of metal oxides on the microstructure and performance of aluminum alloy sacrificial anodes. Taking an Al-Ga-In sacrificial anode as the research object, the dissolution morphology and current efficiency characteristics were studied by means of electrochemical testing and microstructural observation, and the influence of varying Pb and Bi contents on the performance of an aluminum alloy sacrificial anode was investigated. The test results reveal that: (1) The Al-Ga-In sacrificial anode with 4% Pb and 1% Bi contents exhibits the best sacrificial anode performance. (2) The inclusion of an appropriate Bi element content shifts the open-circuit potential in a negative direction and promotes activation dissolution. Conversely, excessive Bi content leads to uneven dissolution, resulting in the shedding of anode grains and greatly reducing the current efficiency. (3) During the activation dissolution of the aluminum alloy, the second phase preferentially dissolves, and the activation point destroys the oxide film, resulting in the dissolution of the exposed aluminum matrix. Consequently, the concentration of dissolved metal ions is reduced and deposited back on the surface of the anode sample, promoting the continuous dissolution of the anode.

Enhanced thermoelectricity in Bi-sprayed bismuth sulphide particles

Bismuth sulphide (Bi2S3), an n-type semiconductor that critically demonstrates the Seebeck effect with Seebeck coefficients of about 300 μVK−1. However, its poor electrical conductivity makes it unsuitable for thermoelectric applications. In this study, we present a facile preparation method for fabricating Bi-sprayed Bi2S3 particles that alters their thermoelectric properties. Samples were created with differing Bi concentrations into the Bi2S3 compound to test for enhanced thermoelectric properties of the resulting Bi/Bi2S3 composites. The incorporation of excess Bi into Bi2S3 significantly improves the compound's electrical conductivity and optimises overall thermoelectric performance. The electrical conductivity of the Bi/Bi2S3 composites improved from 6.5 Scm−1 (for pristine Bi2S3) to 154 Scm−1 (for highest Bi added Bi2S3). Although the Seebeck coefficient of samples decreased with Bi incorporation, a high power factor (∼390 μWm−1K−2) has been achieved for an optimised composition of the composite. Incorporation of metallic Bi has led to an increase in the thermal conductivity of the samples, but the increase is not significant for the optimised composition of the composites where a high thermoelectric performance has been observed. Therefore, enhanced power factor and moderate thermal conductivity have resulted in a peak ZT value of 0.11 at room temperature. The strategy proposed here improves the thermoelectricity in Bi2S3 and shows excellent potential for developing better-performing thermoelectric compounds with excess elemental contents.

Shear Fatigue Analysis of SAC-Bi Solder Joint Exposed to Varying Stress Cycling Conditions

Sn–Ag–Cu (SAC) solder alloys are frequently used in electronic assemblies, and their fatigue resistance under various stress cycles is a significant factor that influences the reliability of electronics. Three distinct SAC-based solder joints (SAC305, SAC-Q, and SAC-R) were examined under various stress amplitude cycling and ambient temperature conditions. The stress on individual solder joints alternates between mild stress (MS) and harsh stress (HS) until complete failure. The hysteresis loop was investigated and compared under various stress amplitude cycles. In addition, the morphology and microstructure of the solder joints were analyzed. The results revealed that SAC-Q, with high Bi and Ag content, demonstrated superior fatigue resistance compared with SAC305 and SAC-R under varying stress amplitude conditions. The inelastic work increased with each change between the MS and HS cycles. SAC-Q exhibited solid solution and precipitate hardening mechanisms through the presence of bismuth (Bi) and silver (Ag) in the tin (Sn) lattice. At the same time, SAC305 showed Ag3Sn precipitates hardening, whereas SAC-R, with no Ag, demonstrated solid solution hardening exclusively. Different failure mechanisms were observed depending on the alloy composition and strain rate. SAC305 and SAC-R exhibited ductile failure. However, SAC-Q experienced an interfacial fracture. Excessive Bi may lead to the embrittlement of solder joints and a change in the failure mechanisms during mechanical testing.

Enhanced electromechanical response in (K, Na)NbO3-based ferroelectrics by phase boundary and nonstoichiometry engineering

Compositional fluctuation of the K and Na elements during the preparation process becomes a major issue for (K,Na)NbO3 (KNN) lead-free ceramics to maintain relatively stable electrical properties. The raw material Bi2O3 is more sensitive to high temperature. To improve electrical properties, it is more necessary to compensate the volatile Bi. In this work, the 0.96K0.48Na0.52Nb0.96Sb0.04O3-0.04Bi0.5*(1+x)Na0.5ZrO3 (x = −0.05, −0.02, 0.00, 0.02, 0.05, and 0.08) ceramics were selected as basis material. The excess Bi can increase the phase fraction of the T phase and decrease oxygen vacancy content, which effectively enhance the extrinsic contribution of the piezoelectric response. The electromechanical properties were optimized by excess Bi, and the optimal properties (d33 = 522 pC/N, kp = 58.9%, Tc = 235 °C) were obtained in the ceramic with x = 0.05. The present work provides new guidelines for enhancing the piezoelectric performance of KNN ceramics by a simple method, which can be applied to other lead-free piezoelectric ceramic systems.

Engineering microstructure to improve coercivity of bulk MnBi magnet

MnBi is a candidate material for high-temperature magnets because of its increasing coercivity with increasing temperatures up to 255 °C. However, most efforts in fabricating bulk MnBi magnets have run into the problem of preserving the coercivity (Hcj) of its feedstock powders. About 70% of powder's Hcj would be lost during the densification process. Our micromagnetic modeling shows that the coercivity mechanism of the MnBi bulk magnet is controlled by nucleation of the reversal magnetization domains, and the large Hcj loss that occurred during the powder consolidation process can be attributed to the inter-grain magnetic coupling. To attain a high Hcj, the grains in the MnBi bulk magnet must be separated with a non-magnetic grain boundary phase (GBP). To validate this GBP hypothesis, we engineered MnBi bulk magnets with two different types of GBP. The first type of GBP was created in-situ by precipitating excessive Bi from the grains; the second type was created ex-situ by coating silicates on the feedstock powders before the consolidation. While both GBP work, the ex-situ approach resulted in a better Hcj due to a more uniform GBP distribution. The Hcj loss was reduced from 70% to 15%, and the (BH)max of a warm sintered bulk magnet reached 8.9 MGOe.

Composition and microstructure of Na0.5Bi0.5TiO3 ceramics with excess Bi

AbstractIn the present work, Na0.5Bi0.5TiO3 (NBT) ceramics with the addition of excess Bi in two different ways – before calcination and before sintering – are considered, revealing how the excess Bi affects their microstructure and chemical content. Average grain size is seen to decrease, with the grain size distribution becoming less diffused at higher excess Bi concentrations. The reason for such a feature is the shift of the sintering temperature region where the abnormal grain growth starts to contribute towards higher temperatures. The influence of excess Bi is more pronounced in the case if it is added before calcination. It was discovered that a small amount of excess Bi helps to prevent the formation of Bi‐deficient inclusions. While, high concentrations of excess Bi induce the formation of Bi‐rich inclusions – most probably Na0.5Bi4.5Ti4O15. Possible mechanisms of formation of both types of inclusions are discussed in detail. Instead of Bi over‐stoichiometry, elevated Na content and slightly lower O content were detected in the matrix grains of the sintered NBT ceramics prepared with excess Bi. These deviations increase upon increasing the added excess Bi concentration. The presence of another, Na‐rich phase, is assumed, which could not be detected by X‐ray diffraction or by energy‐dispersive X‐ray analysis.

Significantly reduced conductivity in strontium titanate-based lead-free ceramics by excess bismuth

Lead-free dielectric ceramics 0.75SrTiO3-0.25BiFeO3 (ST-BF) with an extra amount of bismuth (Bi) are synthesized by the conventional solid-state reaction method. The crystal structures of ST-BF retain pseudo-cubic phase as Bi content increases. The grain size is found to significantly increase from 1.8 to 4.9 µm and then fall to 3.3 µm in the presence of excess Bi, exhibiting a distinct heterogeneous core–shell microstructure. Impedance spectroscopy data indicates that with increasing excess Bi content to 2 mol%, the shell resistivity increases significantly to ∼ 9 MΩ cm with the largest volume fraction of the shell/core area, and both the core and shell activation energies are increased, resulting in reduced dielectric loss of ∼ 0.08 at 1 kHz and high breakdown strength of ∼ 140 kV cm−1. This work provides a potential strategy for further improvement of ST-BF for capacitor applications.

Optimizing Room-Temperature Thermoelectric Performance of n-Type Bi2Te2.7Se0.3.

Bi2Te3-based compounds are exclusive commercial thermoelectric materials around room temperature. For n-type compounds, optimal thermoelectric properties are normally obtained at temperatures higher than room temperature to suppress the bipolar effect through increased carrier concentration. We find that doping with trace amounts of Cd and the addition of excess Bi are effective ways to optimize carrier concentration and achieve enhanced room-temperature thermoelectric performance for the Bi2Te2.7Se0.3 alloy in this work. For the Cd-doped samples, the replacement of Cd with Bi leads to not only a significant decrease in electron concentration but also apparently reduces the total thermal conductivity. The addition of excess Bi in the samples creates a Bi-rich synthetic atmosphere during the synthesis process, leading to increased BiTe antisite defects, decreased electron concentration, and reduced total thermal conductivity. Doping a small amount of Cd or adding excess Bi causes optimal thermoelectric performance of the n-type Bi2Te2.7Se0.3 sample shifts obviously toward low temperatures, and the samples with 0.4 atom % Cd and 0.8 atom % excess Bi achieve maximum zT of ∼0.97 at 448 K and ∼0.88 at 348 K, respectively.

High-Quality Sputtered BiFeO3 for Ultrathin Epitaxial Films

BiFeO3 films were grown by RF magnetron sputtering with various O2 gas flow ratios and substrate temperatures. The optimal sputtering conditions for a slightly excess Bi content produced high-quality parameters: an atomically flat surface (Ra < 0.4 nm), low leakage current (Jc < 10–6 A/cm2), high ferroelectric polarization (72 μC/cm2//[001]pc), and large exchange bias (∼140 Oe). In addition to these typical characterizations, the following two advanced analyses were performed: (i) The lattice constant was identified by Bragg’s diffraction specific to a space group of R3c using X-ray diffraction; it was precisely determined as an expanded a-axis (abulk = 0.568 → aepi = 0.572 nm) and a shrunk c-axis (cbulk = 1.398 → cepi = 1.373 nm). (ii) The ferroelectricity was analyzed by first-order reversal curve (FORC) diagrams, which revealed that ferroelectric switching was packed in a narrow electric field area; an internal electric field in the film body was not observed despite the fact that the BiFeO3 films were as-grown samples. A 3 nm thick BiFeO3 film with a continuous and flat surface/interface was confirmed over a wide area. The crystal symmetry might be identified as a space group of R3c in the case of the 3 nm thick film by comparing the nanobeam selected area electron diffraction patterns with the patterns based on structural calculations. The ferroelectricity might be confirmed by the piezoresponse force microscopy of a 2 nm thick BiFeO3 epitaxial film, owing to the optimal condition of low Jc and uniform ferroelectric switching properties. Furthermore, a 0.4 nm thick ultrathin BiFeO3 film was confirmed to be a continuous one-unit cell perovskite (∼0.4 nm) layer, owing to the optimal condition of low Ra. This study provides a method for investigating the crystal symmetry that affects the multiferroic properties of ultrathin films, which can be used as barrier layers in multiferroic tunnel junctions for highly functional sensors.

Effect of non-stoichiometry on dielectric properties of BaTiO3-BiMg2/3Nb1/3O3 ceramics

One Stoichiometric and three non-stoichiometric compounds of 0.5BaTiO3-0.5BiMg2/3Nb1/3O3 (BT-BMN) were prepared using a conventional solid-state route. Two of the non-stoichiometric compounds had a 2 ​mol.% deficiency on the A-site where ‘A’ corresponds to Bi and Ba cations. Whereas the remaining one sample was fabricated with a 2 ​mol.% excess of Bi. X-ray diffraction revealed formation of a single-phase cubic perovskite structure except for 2 ​mol.% Bi excess sample where an additional phase of Bi2O3 was formed. Microstructural analysis of the sintered samples showed a dense microstructure comprised of hexagonal- and rectangle-like grains, consistent with the relative density >94%. The elemental mapping showed no noticeable non-uniformity in the chemical composition of the grains. From the analysis of the dielectric data, it was observed that deficiencies on A-site have helped in enhancing the dielectric loss window for BT-BMN while maintaining the relative permittivity. A relatively high maximum polarization Pmax ​= ​3.7 ​μC/cm2 was observed at 50 kV/cm-1 for the -2Bi sample. Moreover, -2Bi showed a low dielectric loss compared to stoichiometric sample and temperature stable relative permittivity (690 ​± ​15%) across 30–600 ​°C. While no material is defect free, it is important to note that the intentional removal of A-site cations can result in creation of additional oxygen vacancies according to (2VBi‴=3VO⋅⋅) and (VBa″=VO⋅⋅). It is suggested that the presence of A-O vacancy pairs may be enhancing the insulation resistance by trapping the free electrons within these materials.

Surface conversion of single-crystal Bi2Se3 to β-In2Se3

In this work, we demonstrate that the surface layers of single-crystal layered-2D Bi2Se3 can be converted to layered-2D rhombohedral β-In2Se3 by annealing under a trimethylindium (TMIn) flux. Samples were prepared in a metalorganic chemical vapor deposition (MOCVD) chamber, then transferred under vacuum to a surface analysis chamber for analysis with low-energy electron diffraction (LEED) and Auger electron spectroscopy. Additional ex situ characterization included x-ray diffraction, transmission-electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping, and Raman spectroscopy. The resulting single-crystal β-In2Se3 adopts the rhombohedral crystal structure (space group R-3m) and orientation of the underlying Bi2Se3, and the excess Bi atoms generated by this process create an underlying region of Bi-rich BixSey. Due to the difference in bandgap between Bi2Se3 and In2Se3, this conversion reaction presents a pathway to lateral heterojunctions if only selected regions are converted by masking the surface to spatially define the TMIn exposure. The conversion may also have implications for heteroepitaxy, because the in-plane lattice constants of Bi2Se3 and In2Se3 (0001) surfaces match those of InP and GaAs (111), respectively, and the natural cleavage planes of a layered-2D crystal facilitate substrate removal and reuse.

Nonstoichiometric effect of A-site complex ions on structural, dielectric, ferroelectric, and electrostrain properties of bismuth sodium titanate ceramics

To investigate the nonstoichiometric effect of (Bi0.5Na0.5)TiO3 (BNT) ceramics on their properties, we propose a novel chemical expression, (Bi0.5+xNa0.5−3x)TiO3. The nonstoichiometric effect of BNT can be explored in compounds with this composition without being hampered by the charge imbalance problem. With x ranging from −0.02 to 0.02, we find that the morphological, dielectric, ferroelectric, and electrostrain properties differ considerably between Na-rich and Bi-rich ceramic samples. The average grain size (AGS) increased significantly in Na-rich samples compared to that in stoichiometric BNT, while it decreased slightly in Bi-rich samples. The dielectric characteristics measured from 30 °C to 500 °C indicate that conductivity is activated in Na-rich nonstoichiometric samples but is effectively suppressed in Bi-rich nonstoichiometric samples. The ferroelectric properties also show the same trend. In Na-rich samples, elliptical polarization against electric field (P-E) hysteresis loops were detected, indicating a conductive character induced by high electric field loading. However, saturated P-E loops are observed in Bi-rich samples with well-inhibited conductivity. Furthermore, compared to stoichiometric BNT and nonstoichiometric x = 0.02 Bi-rich samples, (Bi0.5+xNa0.5−3x)TiO3 samples with x = 0.01 exhibit higher electrostrain from 30 °C to 150 °C. Based on the assumption of charge balance, our findings indicated that the presence of 1 mol% excess Bi would facilitate significant improvement in the dielectric, ferroelectric, and electrostrain properties of BNT and BNT-based systems.

Synthesis and Electrical Properties of a New Compound (BiSe)0.97(Bi2Se3)1.26(BiSe)0.97(MoSe2) Containing Metallic 1T-MoSe2

The synthesis and electrical properties of a new misfit compound containing BiSe, Bi2Se3, and MoSe2 constituent layers are reported. The reaction pathway involves competition between the formation of (BiSe)1+x(Bi2Se3)1+y(BiSe)1+x(MoSe2) and [(Bi2Se3)1+y]2(MoSe2). Excess Bi and Se are required in the precursor to synthesize (BiSe)1+x(Bi2Se3)1+y(BiSe)1+x(MoSe2). High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) confirm the stacking sequence of the heterostructure. Small grains of both 2H- and 1T-MoSe2 are observed in the MoSe2 layers. X-ray photoelectron spectroscopy (XPS) indicates that there is a significantly higher percentage of 1T-MoSe2 in (BiSe)1+x(Bi2Se3)1+y(BiSe)1+x(MoSe2) than in (BiSe)0.97(MoSe2), suggesting that more charge transfer to MoSe2 occurs due to the additional BiSe layer. The additional charge transfer results in (BiSe)1+x(Bi2Se3)1+y(BiSe)1+x(MoSe2) having a low resistivity (14–19 μΩ m) with metallic temperature dependence. The heterogeneous mix of MoSe2 polytypes observed in the XPS complicates the interpretation of the Hall data as two bands contribute to the electrical continuity.

Phase structure and electrical properties of (1-x)Bi1+yFeO3-xBaTiO3 lead-free ceramics with different Bi contents

In this work, (1-x)Bi1+yFeO3-xBaTiO3 (x = 0.25, 0.30, 0.35, 0.40; y = − 0.02, 0.00, 0.02, 0.04) ceramics were prepared via the solid-state reaction method. The effects of BaTiO3 doping and different Bi contents on the phase structure, microstructure, and electrical properties of the ceramics was investigated. Compared with pure BiFeO3, doping with BaTiO3 can significantly improve the electrical properties of the ceramics through affecting the phase structure. It is found that the phase structure undergoes a transition from a rhombohedral to a pseudo-cubic phase as x increases. The ceramics with x = 0.30 can be considered as a morphotropic phase boundary, where the rhombohedral and pseudo-cubic phases coexist; furthermore, their electrical performance was improved (Pr = 23.9 µC/cm2, EC = 28.5 kV/cm, $${d}_{33}^{*}$$ = 237 pm/V, TC = 419 °C). Bi excess can improve the dielectric performance of the ceramics at high temperature; however, a severe deficiency of Bi can induce a change in the structure from a mixed phase to a rhombohedral phase where an impurity phase is produced, thus reducing the ceramics performance.

Kinetic model for molecular beam epitaxy growth of InAsSbBi alloys

The growth of Bi-containing III-V alloys requires careful control over temperature and group-V fluxes due to the low equilibrium solubility of Bi and its tendency to surface segregate into Bi-rich droplet features. A model for molecular beam epitaxy growth based on the kinetics of atomic desorption, incorporation, surface accumulation, and droplet formation is applied to the bismide alloy InAsSbBi grown on GaSb substrates. A steady-state solution is derived for the Bi, Sb, and As mole fractions and surface layer coverages based on the Bi, Sb, and As fluxes. A nonlinear least-squares algorithm is used to fit the growth model parameters to experimentally measured Bi mole fractions in bulk and quantum well InAsSbBi samples grown at 400 °C and 420 °C. The Bi mole fraction ranges from 0.12% to 1.86% among 17 samples examined. The results indicate that as the growth temperature increases, the rate of Bi incorporation decreases and the rate of Bi self-desorption increases. A strong interaction is observed between Bi and As that plays a role in the desorption of excess Bi from the growth surface, thus reducing the likelihood of Bi-rich droplet formation when an excess As flux is present. Significantly, the model predicts that the incorporation of Bi is limited to mole fractions of 1.43% at 400 °C and 0.30% at 420 °C in lattice-matched bulk InAsSbBi grown on GaSb substrates.

Effect of Bi addition on the shear strength and failure mechanism of low-Ag lead-free solder joints

To optimize the mechanical properties of low-Ag lead-free solder, different amount of Bi element was added into the Sn–1.0Ag–0.5Cu (SAC105) to form SAC105-xBi (0 ≤ x ≤ 4) solder alloy. The microstructural evolution and shear fracture of SAC105-xBi solder joints were systematically investigated to clarify the failure mechanism after reflow and after 175 °C aging compared to SAC305. It was found that Bi element could provide solution strengthening and second-phase strengthening in the low-Ag SAC solder joints depending on the amount of added Bi element. The shear strength after reflow increased from 34.79 to 70.16 MPa with the increase of Bi addition from x = 0.0 to x = 4.0, while that after aging at 175 °C for 168 h increased from 28.97 MPa (x = 0.0) to 55.02 MPa (x = 4.0). The thickness of IMC decreased with the addition of Bi, and excessive Bi would precipitate in the matrix when the Bi content reached 2 wt% or even form brittle Bi barrier layer after 4 wt% addition. During shear tests, three kinds of fracture mode including ductile fracture, quasi-cleavage fracture, and cleavage brittle fracture were observed, and three kinds of failure position including solder matrix, Cu6Sn5/solder interface, and Cu3Sn/Cu interface were revealed. The failure mechanism of different solder joints was closely related to the coupling effects among the Bi amount in the matrix, the thicknesses of IMC, and the status of Kirkendall voids, which significantly affected the reliability of SAC solder joints. Finally, SAC105-1.0Bi solder alloy was considered having the most suitable Bi addition with anticipated comprehensive properties compared with SAC305.

Enhancement of thermoelectric performance of n-type AgBi1+xSe2 via improvement of the carrier mobility by modulation doping

High charge carrier mobility with low lattice thermal conductivity is one of the key factors for the design of a good thermoelectric material. Recent studies show that n-type Te-free AgBiSe2 is promising compound for thermoelectric energy conversion due to intrinsically low lattice thermal conductivity. However, low charge carrier mobility in AgBiSe2 is the constraint for enhancement of its power factor. In the present study, we use a chemical modification way to realize modulation doping in AgBiSe2. The addition of 2–6 mol% excess Bi in AgBiSe2 results in the formation of Bi-rich modulation-doped microstructures of topological semimetal, Bi4Se3 in AgBiSe2 matrix. We show that due to facile carrier transport via semi-metallic Bi4Se3 microstructure results in overall improvement of carrier mobility without compromising Seebeck coefficient in AgBiSe2 system, which in turn results in a remarkable improvement in the power factor (σS2) value. A highest σS2 value of ~6.35 μW cm−1 K−2 at 800 K has been achieved in AgBiSe2-3% Bi excess sample, which is higher than previously reported metal ion and halogen-doped AgBiSe2.

Adsorption-controlled growth of MnTe(Bi2Te3)n by molecular beam epitaxy exhibiting stoichiometry-controlled magnetism

We report the growth of the intrinsic magnetic topological system MnTe(Bi2Te3)n by molecular beam epitaxy. By mapping the temperature and the Bi:Mn flux ratio, it is shown that there is a narrow growth window for the n=1 phase MnBi2Te4 with 2.0<Bi:Mn<2.6 at 225 {\deg}C. Here the films are stoichiometric and excess Bi and Te is not incorporated. At higher flux ratios (Bi:Mn>4.5) it is found that the n = 2 MnBi4Te7 phase is stabilized. Transport measurements indicate that the MnBi2Te4 and MnBi4Te7 undergo magnetic transitions around 25 K, and 10 K, respectively, consistent with antiferromagnetic phases found in the bulk. Further, for Mn-rich conditions (Bi:Mn<2), ferromagnetism emerges that exhibits a clear hysteretic state in the Hall effect, which likely indicates Mn-doped MnBi2Te4. Understanding how to grow ternary chalcogenide phases is the key to synthesizing new materials and to interface magnetism and topology, which together are routes to realize and control exotic quantum phenomena.