Evidence of iso-structural phase transition in rhombohedral Bi-Sb alloy
01 Sep 2016-EPL (EDP Sciences, IOP Publishing and Società Italiana di Fisica)-Vol. 115, Iss: 5, pp 58001
TL;DR: In this article, temperature-dependent structural studies of polycrystalline Bi1−x Sb x alloys were carried out using synchrotron x-ray diffraction.
Abstract: Temperature-dependent structural studies of polycrystalline Bi1−x Sb x alloys were carried out using synchrotron x-ray diffraction. In-depth powder diffraction analysis reveals that an iso-structural phase transition takes place in the rhombohedral Bi-Sb alloy around 200 K, which is accompanied by anomalous behavior in the temperature-dependent linear thermal expansion data. In addition, the thermal variation of refined isotropic thermal parameters or Debye-Waller factor (B iso), ratio of the lattice parameters and Debye temperature indicates that anisotropy, arising due to local structural disorder, plays a significant role in this material.
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TL;DR: In this paper, the magneto-thermoelectric effects of this Bismuth-antimony (BiSb) alloy further improved the TE properties, leading to ZT of about 0.7.
Abstract: Thermoelectric (TE) materials research plays a vital role in heat-to-electrical energy conversion and refrigeration applications. Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures (100–150 K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples consisting of micron-size grains. The achieved ZT is nearly 50% higher than polycrystalline averaged single crystal ZT of ~0.4, and it is also significantly higher than ZT of less than ~0.3 measured below 150 K in Bi-Te alloys commonly used for cryogenic cooling applications. The improved thermoelectric properties can be attributed to the fine-grained microstructure achieved from rapid solidification, which not only significantly reduced the thermal conductivity but also mitigated a segregation effect. A record low thermal conductivity of ~1.5 W m−1 K−1 near 100 K was measured using the hot disk method. The thermoelectric properties for this intriguing semimetal-semiconductor alloy system were analyzed within a two-band effective mass model. The study revealed a gradual narrowing of the band gap at increasing temperature in Bi-Sb alloy for the first time. Magneto-thermoelectric effects of this Bi-Sb alloy further improved the TE properties, leading to ZT of about 0.7. The magneto-TE effect was further demonstrated in a combined NdFeB/BiSb/NdFeB system. The compactness of the BiSb-magnet system with high ZT enables the utilization of magneto-TE effect in thermoelectric cooling applications.
11 citations
TL;DR: Anharmonic lattice vibration in metallic and semiconducting Sb2Te3 was investigated using temperature dependent Raman spectroscopic studies, synchrotron powder diffraction, and heat capacity measurements.
Abstract: Anharmonic lattice vibration in metallic and semiconducting Sb2Te3 is investigated using temperature dependent Raman spectroscopic studies, synchrotron powder diffraction, and heat-capacity measurements. Thermal variation of structural parameters divulges temperature dependence of structural anisotropy in both the samples. It is revealed that semiconducting Sb2Te3, having higher defect density and larger phonon anharmonicity, possesses lower thermal conductivity. As compared to its metallic counterpart, significant increase in thermoelectric figure of merit, ZT is reported for the semiconducting Sb2Te3. Correlation among structural anisotropy, phonon anharmonicity and ZT is established.
5 citations
TL;DR: The high pressure phase behavior of the Bi-Sb system was investigated experimentally and modeled thermodynamically to study the effect of the transition to the incommensurate Bi-III type structure.
Abstract: The high-pressure phase behavior of the Bi–Sb system was investigated experimentally and modeled thermodynamically to study the effect of the transition to the incommensurate Bi-III type structure, which is adopted by the high-pressure phases Bi-III and Sb-II of the pure elements. The thermodynamic model of the Bi–Sb phase diagram including the effect of pressure in the isomorphous regime was extended to include solid–solid phase transitions of the pure elements. Consequently, the alloy phase diagram was found to transform to the eutectic form with pressure and is predicted here in the pressure range of 3–6 GPa. The alloy was found to transform from the ambient, rhombohedral A7 structure to the incommensurate phase. The transformation occurred at intermediate pressures between those observed in elemental Bi and Sb. The region of thermodynamic stability was determined by calculating the spinodal curves. To verify these predictions, the structure of a Sb-rich alloy was determined as a function of pressure by X-ray diffraction measurements in a diamond anvil cell, demonstrating the transition from A7 to the incommensurate Sb-II-like structure. It was found that the decomposition of the alloy under pressure is determined by the thermodynamic instability located at the spinodal curve. In contrast, alloy compositions located in the metastable region between the binodal phase line and the spinodal curve did not decompose under pressure, thus indicating a new region of phase space to be explored in high-pressure studies of alloys.
1 citations
TL;DR: In this paper , the authors report on the low-temperature (T <$ 120 K) thermal expansion of the bismuth-based topological semimetal and topological superconductor candidates.
Abstract: We report on the low-temperature ($T <$ 120 K) thermal expansion of the bismuth-based topological semimetal $\beta$-PtBi$_2$ and topological superconductor $\beta$-Bi$_2$Pd candidates. The linear thermal-expansion coefficient of tetragonal $\beta$-Bi$_2$Pd shows a pronounced anisotropy between the $a$- and $c$-axis while the volume thermal-expansion coefficient $\alpha_V$($\beta$-Bi$_2$Pd) is considerable larger than $\alpha_V$($\beta$-PtBi$_2$). The coefficient $\alpha_V$($\beta$-PtBi$_2$) nearly matches the experimental specific heat, from which a Debye temperature $\theta_D =$ 199 K is obtained. On the other hand, $\alpha_V$($\beta$-Bi$_2$Pd) reasonably fits the Debye model with $\theta_D =$ 138 K, extracted from the low-temperature specific heat. An almost constant Gr\"uneisen parameter $\Gamma \approx$ 2 is obtained for both compounds. No magnetostriction is observed in any of both compounds up to $\mu_0 H =$ 16 T. We compare our results with other Bi-based topological materials.
TL;DR: In this article , the authors report on the low-temperature (T < 120 K) thermal expansion of the bismuth-based topological semimetal β -PtBi 2 and topological superconductor β -Bi 2 Pd candidates.
Abstract: We report on the low-temperature ( T < 120 K) thermal expansion of the bismuth-based topological semimetal β -PtBi 2 and topological superconductor β -Bi 2 Pd candidates. The linear thermal-expansion coefficient of tetragonal β -Bi 2 Pd shows a pronounced anisotropy between the a - and c -axis while the volume thermal-expansion coefficient α V ( β -Bi 2 Pd) is considerable larger than α V ( β -PtBi 2 ). The coefficient α V ( β -PtBi 2 ) nearly matches the experimental specific heat, from which a Debye temperature θ D = 199 K is obtained. On the other hand, α V ( β -Bi 2 Pd) reasonably fits the Debye model with θ D = 138 K, extracted from the low-temperature specific heat. An almost constant Gr¨uneisen parameter Γ ≈ 2 is obtained for both compounds. No magnetostriction is observed in any of both compounds up to µ 0 H = 16 T. We compare our results with other Bi-based topological materials.
References
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TL;DR: In this paper, temperature-dependent electrical resistivity and thermoelectric power were studied over a wide range of Sb concentrations using molecular-beam epitaxy, and it was shown that the 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with the maximum band gap shifted to a lower concentration from 15% in bulk to 9%.
Abstract: We have grown ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ alloy thin films on $\mathrm{CdTe}(111)B$ over a wide range of Sb concentrations $(0l~xl~0.183)$ using molecular-beam epitaxy. Temperature-dependent electrical resistivity (\ensuremath{\rho}) and thermoelectric power (S) were studied. We have observed several differences over the bulk system. The 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with maximum band gap shifted to a lower Sb concentration from 15% in bulk to 9%. Based on a simple interpretation of the temperature-dependent resistivity the maximum gap would be 40 meV, which is larger than that observed in bulk alloys. In addition, we have observed that the power factor ${S}^{2}/\ensuremath{\rho}$ peaks at a significantly higher temperature (250 K) than previously reported for the bulk alloy (80 K). Differences between thin film grown on CdTe(111) and bulk alloy may arise from the effects of strain, which is supported by theoretical electronic band calculations. These results show that BiSb films may be useful as band-engineered materials in thermoelectric devices.
56 citations
TL;DR: In this article, a study of electronic transport in semimetallic Fermi liquid with an enhanced prefactor was conducted and the quantum limit for a field along the trigonal axis was found at a field as low as 3 T.
Abstract: We report on a study of electronic transport in semimetallic ${\text{Bi}}_{0.96}{\text{Sb}}_{0.04}$. At zero field, the system is a very dilute Fermi liquid displaying a ${T}^{2}$ resistivity with an enhanced prefactor. Quantum oscillations in resistivity as well as in Hall, Nernst, and Seebeck responses of the system are detectable and their period quantifies the shrinking of the Fermi surface with antimony doping. For a field along the trigonal axis, the quantum limit was found to occur at a field as low as 3 T. An ultraquantum anomaly at twice this field was detected in both charge transport and Nernst response. Its origin appears to lie beyond the one-particle picture and linked to unidentified many-body effects.
36 citations
TL;DR: In this paper, the effect of alloying on the incommensurate Bi-III type composite structure was investigated experimentally and by means of first-principles calculations in order to study the effect that alloying has on the high-pressure structural behavior of alloys.
Abstract: The high-pressure structural behavior of alloys ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ was investigated experimentally and by means of first-principles calculations in order to study the effect of alloying on the incommensurate Bi-III type composite structure, which is adopted by the high-pressure phases Bi-III and Sb-II of the pure elements. High-pressure experiments of ${\mathrm{Bi}}_{0.75}{\mathrm{Sb}}_{0.25}$ and ${\mathrm{Bi}}_{0.50}{\mathrm{Sb}}_{0.50}$ resulted in a decomposition into Sb-rich phases with the ambient A7 structure and Bi-rich phases with the Bi-III structure. For ${\mathrm{Bi}}_{0.50}{\mathrm{Sb}}_{0.50}$ two composite phases were observed with different compositions. ${\mathrm{Bi}}_{0.25}{\mathrm{Sb}}_{0.75}$ transformed from A7 to Bi-III, such as elemental Sb, and did not show a phase separation under pressure. For the pure elements, the structural parameters of the high-pressure composite phase are similar and vary only very slightly with pressure. The parameters of the alloy composite phases distribute rather smoothly in between those for the elements. The incommensurate composite phases in alloys ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ appear to be site disordered. Importantly, the incommensuration of the Bi-III structure is virtually not influenced by either composition or pressure. Total energy calculations as a function of pressure yielded endothermic enthalpies of formation for alloys ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ in both the ground-state A7 structure and the high-pressure Bi-III structure. The formation enthalpy takes a maximum value at a pressure corresponding to the alloy $\mathrm{A}\stackrel{\ensuremath{\rightarrow}}{7}\mathrm{Bi}\ensuremath{-}\mathrm{III}$ transition, which explains the experimentally observed phase separations.
18 citations