| 1. |
- Ji, Fuxiang, 1991-, et al.
(author)
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Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2023
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In: Advanced Optical Materials. - : Wiley-Blackwell. - 2162-7568 .- 2195-1071.
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Journal article (peer-reviewed)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 2. |
- Ji, Fuxiang, 1991-, et al.
(author)
-
Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2024
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In: Advanced Optical Materials. - : John Wiley & Sons. - 2162-7568 .- 2195-1071. ; 12:8
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Journal article (peer-reviewed)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron-phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK(-1) based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III-V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 3. |
- Belonoshko, Anatoly, et al.
(author)
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Elastic properties of body-centered cubic iron in Earth's inner core
- 2022
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:18
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Journal article (peer-reviewed)abstract
- The solid Earth's inner core (IC) is a sphere with a radius of about 1300 km in the center of the Earth. The information about the IC comes mainly from seismic studies. The composition of the IC is obtained by matching the seismic data and properties of candidate phases subjected to high pressure (P) and temperature (T). The close match between the density of the IC and iron suggests that the main constituent of the IC is iron. However, the stable phase of iron is still a subject of debate. One such iron phase, the body-centered cubic phase (bcc), is dynamically unstable at pressures of the IC (330-364 GPa) and low T but gets stabilized at high T characteristic of the IC (5000-7000 K). So far, ab initio molecular dynamics (AIMD) studies attempted to compute the bcc elastic properties for a small (order of 102) number of atoms. The mechanism of the bcc stabilization cannot be enabled in such cells and that has led to erroneous results. Here we apply AIMD to compute elastic moduli and sound velocities of the Fe bcc phase for a 2000 Fe atom computational cell, which is a cell of unprecedented size for ab initio calculations of iron. Unlike in previous ab initio calculations, both the longitudinal and the shear sound velocities of the Fe bcc phase closely match the properties of the IC material at P = 360 GPa and T = 6600 K, likely the PT conditions in the IC. The calculated density of the bcc iron at these PT conditions is just 3% higher than the density of the IC material according to the Preliminary Earth Model. This suggests that the widely assumed amount of light elements in the IC may need a reconsideration. The anisotropy of the bcc phase is an exact match to the most recent seismic studies.
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| 4. |
- Burakovsky, Leonid, et al.
(author)
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Systematics of the Third Row Transition Metal Melting: The HCP Metals Rhenium and Osmium
- 2018
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In: Crystals. - : MDPI. - 2073-4352. ; 8:6
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Journal article (peer-reviewed)abstract
- The melting curves of rhenium and osmium to megabar pressures are obtained from an extensive suite of ab initio quantum molecular dynamics (QMD) simulations using the Z method. In addition, for Re, we combine QMD simulations with total free energy calculations to obtain its phase diagram. Our results indicate that Re, which generally assumes a hexagonal close-packed (hcp) structure, melts from a face-centered cubic (fcc) structure in the pressure range 20-240 GPa. We conclude that the recent DAC data on Re to 50 GPa in fact encompass both the true melting curve and the low-slope hcp-fcc phase boundary above a triple point at (20 GPa, 4240 K). A linear fit to the Re diamond anvil cell (DAC) data then results in a slope that is 2.3 times smaller than that of the actual melting curve. The phase diagram of Re is topologically equivalent to that of Pt calculated by us earlier on. Regularities in the melting curves of Re, Os, and five other 3rd-row transition metals (Ta, W, Ir, Pt, Au) form the 3rd-row transition metal melting systematics. We demonstrate how this systematics can be used to estimate the currently unknown melting curve of the eighth 3rd-row transition metal Hf.
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| 5. |
- Ghorai, Sagar, et al.
(author)
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Giant magnetocaloric effect in the (Mn,Fe)NiSi-system
- 2023
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Other publication (other academic/artistic)abstract
- The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we report on a new material, Mn1−xFexNiSi0.95Al0.05, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the Ni2In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.
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| 6. |
- Kerdsongpanya, Sit, et al.
(author)
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Phonon thermal conductivity of scandium nitride for thermoelectrics from first-principles calculations and thin-film growth
- 2017
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:19
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Journal article (peer-reviewed)abstract
- The knowledge of lattice thermal conductivity of materials under realistic conditions is vitally important since many modern technologies require either high or low thermal conductivity. Here, we propose a theoretical model for determining lattice thermal conductivity, which takes into account the effect of microstructure. It is based on ab initio description that includes the temperature dependence of the interatomic force constants and treats anharmonic lattice vibrations. We choose ScN as a model system, comparing the computational predictions to the experimental data by time-domain thermoreflectance. Our experimental results show a trend of reduction in lattice thermal conductivity with decreasing domain size predicted by the theoretical model. These results suggest a possibility to control thermal conductivity by microstructural tailoring and provide a predictive tool for the effect of the microstructure on the lattice thermal conductivity of materials based on ab initio calculations.
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| 7. |
- Klarbring, Johan, et al.
(author)
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Electronic structure of the magnetic halide double perovskites Cs-2(Ag, Na)FeCl6 from first principles
- 2023
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In: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 7:4
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Journal article (peer-reviewed)abstract
- A family of magnetic halide double perovskites (HDPs) have recently attracted attention due to their potential to broaden application areas of halide double perovskites into, e.g., spintronics. Up to date the theoretical modeling of these systems have relied on primitive approximations to the density functional theory (DFT). In this paper, we study structural, electronic and magnetic properties of the Fe3+-containing HDPs Cs2AgFeCl6 and Cs2NaFeCl6 using a combination of more advanced DFT-based methods, including DFT + U, hybrid-DFT, and treatments of various magnetic states. We examine the effect of varying the effective Hubbard parameter, U-eff, in DFT + U and the mixing-parameter, alpha, in hybrid DFT on the electronic structure and structural properties. Our results reveal a set of localized Fe(d) states that are highly sensitive to these parameters. Cs2AgFeCl6 and Cs2NaFeCl6 are both antiferromagnets with Neel temperatures well below room temperature and are thus in their paramagnetic (PM) state at the external conditions relevant to most applications. Therefore, we have examined the effect of disordered magnetism on the electronic structure of these systems and find that while Cs2NaFeCl6 is largely unaffected, Cs2AgFeCl6 shows significant renormalization of its electronic band structure.
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| 8. |
- Knoop, Florian, et al.
(author)
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TDEP:Temperature Dependent Effective Potentials
- 2024
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In: Journal of Open Source Software. - : Open journals. - 2475-9066. ; 9:94
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Journal article (peer-reviewed)abstract
- The Temperature Dependent Effective Potential (TDEP) method is a versatile and efficient approach to include temperature in a binitio materials simulations based on phonon theory. TDEP can be used to describe thermodynamic properties in classical and quantum ensembles, and several response properties ranging from thermal transport to Neutron and Raman spectroscopy. A stable and fast reference implementation is given in the software package of the same name described here. The underlying theoretical framework and foundation is briefly sketched with an emphasis on discerning the conceptual difference between bare and effective phonon theory, in both self-consistent and non-self-consistent formulations. References to numerous applications and more in-depth discussions of the theory are given.
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| 9. |
- Pilemalm, Robert, 1983-
(author)
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Theoretical and experimental studies of ternary and quaternary nitrides for machining and thermoelectric materials
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Nitrides are used as coatings and thin films for a wide range of applications. The study and use of nitrides in the recent decades have shifted towards ternary, quaternary or even higher order (complex) nitrides. There is an interest to use ternary and quaternary nitrides for machining and thermoelectric materials, because it gives the possibility to choose composition and thereby design the materials properties. This thesis presents research results on TiAlN and and TiAlN-based coatings that are used as hard coatings for machining and on ternary scandium nitrides that are of interest for thin films for thermoelectric applications. The high-pressure high-temperature behavior of cubic TiAlN deposited on cubic boron nitride has been experimentally studied. It has been shown that the spinodal decomposition, which means decomposition into cubic domains enriched in TiN and AlN, is delayed as a result of high pressure compared to ambient pressure. No chemical interaction between coating and substrate occurs. TiZrAlN has been theoretically and experimentally studied at high temperature. The results show that the when Zr-content is decreased and the Al-content is increased the decomposition route changes from nucleation and growth to spinodal decomposition. The microstructure evolution with temperature depends on the initial composition. In the case where the decompositon starts with only spinodal decomposition the microstructure at 1100 °C consists of domains that are larger than in the case where the decomposition occurs by nucleation and growth. ScMN2 (M=V, Nb, Ta) phases have been experimentally demonstrated for M=Nb and Ta in a few studies, but have not been much investigated. In this theseis, their crystal structure, stability, elastic properties, electronic structure and thermoelectric properties have been studied. At 0 K and 0 GPa it has been shown that these three phases are thermodynamically and elastically stable. Additionally, these are narrow-bandgap semiconductors and their thermoelectric properties can be tuned by doping. Pressure has a stabilizing effect on these structures. When pressure increases from 0-150 GPa the elastic constants and moduli increases in the range 53-317 %.
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