| 1. |
- Ji, Cheng, et al.
(författare)
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Crystallography of low Z material at ultrahigh pressure : Case study on solid hydrogen
- 2020
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Ingår i: Matter and Radiation at Extremes. - : American Institute of Physics (AIP). - 2468-2047 .- 2468-080X. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Diamond anvil cell techniques have been improved to allow access to the multimegabar ultrahigh-pressure region for exploring novel phenomena in condensed matter. However, the only way to determine crystal structures of materials above 100 GPa, namely, X-ray diffraction (XRD), especially for low Z materials, remains nontrivial in the ultrahigh-pressure region, even with the availability of brilliant synchrotron X-ray sources. In this work, we perform a systematic study, choosing hydrogen (the lowest X-ray scatterer) as the subject, to understand how to better perform XRD measurements of low Z materials at multimegabar pressures. The techniques that we have developed have been proved to be effective in measuring the crystal structure of solid hydrogen up to 254 GPa at room temperature [C. Ji et al., Nature 573, 558–562 (2019)]. We present our discoveries and experiences with regard to several aspects of this work, namely, diamond anvil selection, sample configuration for ultrahigh-pressure XRD studies, XRD diagnostics for low Z materials, and related issues in data interpretation and pressure calibration. We believe that these methods can be readily extended to other low Z materials and can pave the way for studying the crystal structure of hydrogen at higher pressures, eventually testing structural models of metallic hydrogen.
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| 2. |
- Majumdar, Arnab, et al.
(författare)
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Emerging piezochromism in lead free alkaline earth chalcogenide perovskite AZrS(3) (A = Mg, Ca, Sr and Ba) under pressure
- 2020
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 8:46, s. 16392-16403
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Tidskriftsartikel (refereegranskat)abstract
- Hydrostatic pressure is an effective tool that can give rise to novel crystal structures and physical properties. It has proven to be an alternative to chemical pressure. Therefore new functional materials with intriguing properties can be designed by exerting external pressure. Metal chalcogenide perovskites are a class of perovskites that have several advantages, namely high stability against moisture and light induced degradation along with nontoxic elemental composition. In this letter, we have used first principles methods to study the structural, electronic and optical properties of AZrS(3) where A = Mg, Ca, Sr and Ba upon compression. Upon being compressed, the direct band gaps decrease to desired values which can enable this class of zirconium based chalcogenide perovskites to be used in tandem solar cells. The mobility of the charge carriers increases with pressure as the effective masses decrease. Piezochromism is seen to exist upon compression which can be verified from the modifications in the optical absorption spectra. This work elucidates the effects of pressure on the sensitive tuning of properties of zirconium based chalcogenide perovskites, which can have significant photovoltaic applications.
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| 3. |
- Majumdar, Arnab, et al.
(författare)
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Emerging piezochromism in transparent lead free perovskite Rb3X2I9 (X=Sb, Bi) under compression : A comparative theoretical insight
- 2020
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 128:4, s. 045102-
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Tidskriftsartikel (refereegranskat)abstract
- Hydrostatic pressure has proven to be an alternative tool to chemical pressure to tune the crystal structure and physical and chemical properties, paving the way for a novel functional materials design. Halide perovskites consist of a large family of perovskites, with tremendous potential for energy applications. Their soft lattices and distinct behavior under pressure make them necessary to be studied under compressed conditions. Ternary antimony halides are one such class of perovskites that act as an alternative to lead (Pb) halide perovskites due to the lesser toxicity of Sb/Bi in comparison with Pb. In this letter, we have theoretically envisaged the effects of compression via hydrostatic pressure on the structural, electronic, and optical properties of Rb3X2I9, where X=Sb and Bi-. The bandgaps not only decrease but are also direct in the desired pressure range. The calculated effective masses decrease with pressure, thereby indicating higher mobility of the charge carriers in the compressed material. The presence of piezochromism has also been confirmed from the variation of optical properties under hydrostatic compression. Our investigations shed light on the pressure dependent sensitive tuning of electronic and optical properties of Rb3Bi2I9 and Rb3Sb2I9, rendering these materials to have significant photovoltaic applications.
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| 4. |
- Majumdar, Arnab, et al.
(författare)
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High exothermic dissociation in van der Waals like hexagonal two dimensional nitrogene from first-principles molecular dynamics
- 2020
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Ingår i: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 529
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Tidskriftsartikel (refereegranskat)abstract
- Mono and multilayered two dimensional (2D) nitrogene in which nitrogen atoms are single bonded are studied for energy applications. The structures are observed to be dynamically and thermally stable at room temperature but dissociate into triple bonded N-2 molecules at higher temperatures. From ab initio molecular dynamics simulations, the dissociation temperature is found to be decreasing with increasing number of layers (1500 K for single layer and 500 K for six layers). The energy released for the different layered cases is large due to the energy difference between single and triple-bonded nitrogen (similar to 0.88 - 1.8 eV/atom). Significantly high energy densities calculated for the multilayered structures (similar to 6 - 12.3 kJ/gm) can secure these materials an important position amongst non-nuclear and inorganic high energy density materials known today. We believe that this work will shed light on synthesizing next generation non-nuclear environmentally clean high energy density materials using multi-layer nitrogene that detonate at not very high temperatures.
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| 5. |
- Majumdar, Arnab, et al.
(författare)
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Interplay of charge density wave and multiband superconductivity in layered quasi-two-dimensional materials : The case of 2H-NbS2 and 2H-NbSe2
- 2020
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 4:8
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Tidskriftsartikel (refereegranskat)abstract
- Despite intense efforts on all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings, particularly charge density waves and superconductivity, are still attractive problems with several open questions. Here, in order to reveal how the superconducting gap evolves, we report on high quality complementary measurements of magneto-optical imaging, specific heat, magnetic susceptibility, resis-tivity measurements, Andreev spectroscopy, and London penetration depth lambda(ab)(T) measurements supplemented with theoretical calculations for 2H-NbSe2 and 2H-NbS2 single crystals. The temperature dependence of lambda(ab)(T) calculated from the lower critical field and Andreev spectroscopy can be well described by using a two-band model with s-wave-like gaps. The effect of pressure on the superconducting gap of both systems illustrates that both bands are practically affected. Upon compression, the Fermi surfaces do not change significantly, and the nesting remains almost unaffected compared to that at ambient condition. However, a strong bending in the upper critical fields (H-c2) curves is obtained under pressure and support the presence of a strong Pauli paramagnetic effect. In NbSe2, using a two-band model with s-wave-like gaps, the temperature dependence H-c2(T) can be properly described. In contrast to that, the behavior of H-c2 for NbS2 is ruled by the spin paramagnetic effect. The estimated values of the penetration depth at T = 0 K confirm that NbSe2 and NbS2 superconductors depart from a Uemura-style relationship between Tc with lambda(-2)(ab) (T ), the in-plane superconducting penetration depth.
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| 6. |
- Majumdar, Arnab, et al.
(författare)
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Ultralow Thermal Conductivity and High Thermoelectric Figure of Merit in Two-Dimensional Thallium Selenide
- 2020
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 3:9, s. 9315-9325
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Tidskriftsartikel (refereegranskat)abstract
- Several selenides have been studied as potential thermoelectric materials in the past. Thermoelectric performance improves on reducing the dimension of the system. Following these notions, we studied three hexagonal two-dimensional phases of thallium selenide for promising thermoelectric performance by using first-principles techniques. Their dynamical and mechanical stabilities have been shown. The band gaps using different approximations of calculations have been reported and are in the range that can render these materials to show optimal thermoelectric performance. The lattice thermal conductivity is ultralow between 0.088 and 0.429 W m(-1) K-1 at 900 K. High thermoelectric performance has been predicted in the systems with the figure of merit reaching magnitudes of similar to 0.5-1.94 between 300 and 900 K. The high thermoelectric performance results from an ultralow thermal conductivity arising due to the strong lattice anharmonicity. These results can have significant impact on the synthesis of high performance thermoelectric materials based on thallium selenide.
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| 7. |
- Zidane, Mustapha, et al.
(författare)
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Electrical and thermal transport properties of Fe-Ni based ternary alloys in the earth's inner core : An ab initio study
- 2020
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Ingår i: Physics of the Earth and Planetary Interiors. - : ELSEVIER. - 0031-9201 .- 1872-7395. ; 301
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Tidskriftsartikel (refereegranskat)abstract
- Besides iron, the Earth's core also contains 5-10% of nickel and several light elements such as H, C, N, O, Si and S. Fe-Ni alloys have been considered as the host material with 10% Ni. The impurity atoms which are comprised of the light elements have been incorporated at different concentrations in the host alloy, giving rise to ternary alloys. All these systems have been subjected to compression at pressures similar to that in the inner core of the Earth. The atomic concentration of the impurities has been varied from 2.5% to 50%. The formation enthalpies suggest that these ternary alloys are energetically favorable. The impurity resistivity is calculated using the Kubo-Greenwood formula. At approximately similar to 30% of atomic concentration of impurities, the electrical resistivities start to saturate. For lesser percentage of hydrogen, the impurity resistivity is in the same range as that of the rest. But once the concentration of hydrogen increases, the impurity resistivity becomes very high. Generally, the resistivities are seen to plummet with increasing pressure. Using the Wiedemann-Franz law, the thermal conductivities were also determined from the calculated electrical resistivities for both varying concentration and pressure. On compression, the thermal conductivities increased by approximately 5% in the inner core pressure range.
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