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Bo B. Iversen - One of the best experts on this subject based on the ideXlab platform.
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Application of Atomic Hirshfeld surface analysis to intermetallic systems: is Mn in cubic CeMnNi4 a thermoelectric rattler Atom?
Inorganic chemistry, 2012Co-Authors: Mads R. V. Jørgensen, Mogens Christensen, Iben Skovsen, Henrik F. Clausen, Eiji Nishibori, Mark A. Spackman, Bo B. IversenAbstract:The Mn Atom in the cubic polymorph of CeMnNi(4) appears to be located in an oversized cage-like structure, and anomalously large Atomic displacement parameters (ADPs) for the Mn Atom indicate that it is a potential "rattler" Atom. Here, multitemperature synchrotron powder X-ray diffraction data measured between 110 and 900 K are used to estimate ADPs for the Mn "Guest" Atom and the "host" structure Atoms in cubic CeMnNi(4). The ADPs are subsequently fitted with Debye and Einstein models, giving Θ(D) = 301(2) K for the "host" structure and Θ(E) = 165(2) K for the Mn Atom. This is higher than typical Einstein temperatures for rattlers in thermoelectric skutterudites and clathrates (Θ(E) = 50-80 K), indicating that the Mn Atom in cubic CeMnNi(4) is more strongly bonded. In order to probe the chemical interactions of the potential Mn rattler Atom, Atomic Hirshfeld surface (AHS) analysis is carried out and compared with AHS analysis of well-established Guest Atom rattlers in archetypical skutterudites, MCoSb(3). Surprisingly, the skutterudite rattlers have more deformed AHSs than the Mn Atom in cubic CeMnNi(4). This is related to the highly ionic nature of the skutterudite rattlers, which is not taken into account in the neutral spherical Atom approach of the AHS. Additionally, visualization of void spaces in the two materials using the procrystal electron density shows that while the Mn Atom is tightly fitting in the CeMnNi(4) structure then the La Atom in the skutterudite is truly situated in an oversized cage of the host structure. Overall, we conclude that the Mn Atom in cubic CeMnNi(4) cannot be coined a rattler.
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Clathrate Guest Atoms under pressure
Journal of Applied Physics, 2009Co-Authors: Mogens Christensen, Simon Johnsen, Fanni Juranyi, Bo B. IversenAbstract:Powder inelastic neutron scattering (INS) has been used to determine the Guest Atom “rattling” energy in thermoelectric clathrates Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8,Ga16) under different applied conditions. Chemical pressure was exerted by the Atomic substitution, and a physical pressure of 9 kbars was applied using a clamp cell. The volume reduction induced by the physical pressure increases the energy of the Guest Atom rattling mode, but the local chemical environment in the cage also appears to have a similar effect. The Guest Atom energies were investigated as function of temperature, and softening of the Guest Atom modes was observed upon cooling the sample. Ba8Ga16Ge30 with holes (p-type) and electrons (n-type) as charge carriers reveal similar temperature behavior, suggesting anharmonic potentials of similar shape for the Ba Guest Atom independent of the charge carrier type. For Sr8Ga16Ge30 a much stronger anharmonic potential was observed compared with Ba8Ga16Ge30. The Guest Atom energies for Ba8YxGe46−...
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Crystal Structures of Thermoelectric n- and p-type Ba8Ga16Ge30 Studied by Single Crystal, Multitemperature, Neutron Diffraction, Conventional X-ray Diffraction and Resonant Synchrotron X-ray Diffraction
Journal of the American Chemical Society, 2006Co-Authors: Mogens Christensen, Nina Lock, Jacob Overgaard, Bo B. IversenAbstract:Comprehensive single-crystal structural investigations of n- and p-type Ba8Ga16Ge30 have been carried out using multitemperature neutron and conventional X-ray diffraction as well as resonant synchrotron X-ray diffraction. The data show that the Guest Atom positions and dynamics are very similar in the two structures, although the barium Atoms are slightly more displaced from the cage centers in the p-type structure than in the n-type structure (Deltad = 0.025 A). For both structures Fourier difference maps calculated from very high-resolution neutron diffraction data (sin theta/lambda > 2 A-1) show that the Ba nuclear density at lowest temperatures (15 K) is distributed in a torus around the crystallographic 6d site with maxima in the 24j positions. At room temperature the maxima have shifted to the 24k position. Analysis of Atomic displacement parameters give Einstein temperatures of approximately 60(1) K for both structures. Thus, the fundamental difference in the low temperature thermal conductivity observed for p- and n-type Ba8Ga16Ge30 appear not to be directly related to the Guest Atom behavior as is commonly assumed in thermoelectric research. The neutron data and the resonant synchrotron X-ray data facilitate refinement of Ga/Ge framework occupancies. The Ga Atoms have a clear preference for the 6c site with the preference being somewhat stronger for the n-type structure.
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Maximum entropy method analysis of thermal motion and disorder in thermoelectric clathrate Ba8Ga16Si30
Journal of Applied Physics, 2002Co-Authors: Anders Bentien, Bo B. Iversen, J. D. Bryan, Galen D. Stucky, A. E. C. Palmquist, Arthur J. Schultz, R. W. HenningAbstract:Multitemperature (15, 100, 150, 200, 300, 450, 600, 900 K) single crystal neutron diffraction data on the type I clathrate Ba8Ga16Si30 are reported. For the framework Atoms reciprocal space structural refinements give total occupancies in the unit cell of Ga/Si=3.8/2.2, 1.8/14.2, 10.2/13.8 for the 6c, 16i, and 24k sites respectively, thus showing that Ga avoids the tetrahedral 16i positions. The Guest Atom displacement parameters obtained from structure factor fitting are analyzed with semianharmonic Einstein models giving Einstein temperatures (ΘE) of 69(1), 98(7), and 124(2) K for Ba(2)(100), Ba(2)[100], and Ba(1), respectively. The analysis furthermore suggests that all Guest Atoms are structurally disordered, and the disorder appears to be temperature dependent with increased host-Guest interaction at high temperatures. The structure factors are subsequently used in the maximum entropy method calculations to obtain direct space nuclear densities. These are modeled with anharmonic one-particle potentia...
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Maximum Entropy Method Charge Density Distributions of Novel Thermoelectric Clathrates
MRS Proceedings, 1999Co-Authors: Bo B. Iversen, George S Nolas, Anders Bentien, Galen D. Stucky, Anders Palmqvist, D. Bryan, Susan E. Latturner, Nick P. Blake, Horia Metiu, David E. CoxAbstract:Recently materials with promising thermoelectric properties were discovered among the clathrates. Transport data has indicated that these materials have some of the characteristics of a good thermoelectric, namely a low thermal conductivity and a high electrical conductivity. Based on synchrotron powder and conventional single crystal x-ray diffraction data we have determined the charge density distribution in Sr 8 Ga 16 Ge 3O using the Maximum Entropy Method. The MEM density shows clear evidence of Guest Atom rattling, and this contributes to the reduction of the thermal conductivity. Analysis of the charge distribution reveals that Sr 8 Ga 16 Ge 30 contains mixed valence alkaline earth Guest Atoms. The Sr Atoms in the small cavities are, as expected, doubly positively charged, whereas the Sr Atoms in the large cavities appear negatively charged. The MEM density furthermore suggests that the Ga and Ge Atoms may not be randomly disordered on the framework sites as found in the conventional leastsquares refinements.
Mogens Christensen - One of the best experts on this subject based on the ideXlab platform.
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Combined X-ray and neutron diffraction study of vacancies and disorder in the dimorphic clathrate Ba8Ga16Sn30 of type I and VIII.
Dalton transactions (Cambridge England : 2003), 2013Co-Authors: Sebastian Christensen, Marcos A. Avila, Toshiro Takabatake, Koichiro Suekuni, Ross O. Piltz, Mogens ChristensenAbstract:We report detailed structural investigations of the dimorphic clathrate Ba8Ga16Sn30 that crystallizes in both type I and VIII clathrate structures. Single crystals of type I and VIII have been examined using single crystal X-ray and Laue neutron diffraction in the temperature range T = 10 K–500 K. The utilization of both X-ray and neutron diffraction gives a unique ability to reveal the occurrence of minute vacancy occupancies in the host structure. The vacancies are shown to be located on the 6c (type I) and 24g (type VIII) framework sites. Largest vacancy densities are observed for type I p-Ba8Ga16Sn30, 1.3(4)%, and type VIII n-Ba8Ga16Sn30, 0.7(2)%. The relation between Guest Atom disorder and occurrence of glasslike thermal conductivity in intermetallic clathrates was also investigated. In type VIII Ba8Ga16Sn30 neither n-type (crystalline thermal conductivity) nor p-type (glasslike thermal conductivity) showed any significant disorder of the Guest Atoms; they do however show anharmonic motion. The glasslike thermal conductivity of p-type Ba8Ga16Sn30 is interpretable as a result of higher effective mass of p-type charge-carriers affecting phonon scattering. In type I Ba8Ga16Sn30 Guest Atoms are highly disordered for both carrier types and samples of both charge carrier types have glasslike thermal conductivity.
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Guest host interaction and low energy host structure dynamics in tin clathrates
Journal of Applied Physics, 2013Co-Authors: Sebastian Christensen, Fanni Juranyi, Lasse Bjerg, Andreas Kaltzoglou, Thomas F. Fässler, Tobias Unruh, Mogens ChristensenAbstract:The two binary clathrates with vacancies (□) Rb8Sn44□2 and Cs8Sn44□2 have been examined using powder inelastic neutron scattering (INS). Rattling energies of Rb and Cs are found to be similar by both experiment and calculations, ℏωCs/ℏωRb|Exp.=0.98(1) and ℏωCs/ℏωRb|Calc.=1.0, despite the significant mass difference: mCs/mRb=1.6, which shows that Guest-host interaction is non-negligible for the studied system. For Rb8Sn44□2, a low energy phonon mode is observed at ≈3.5 meV, below the phonon mode which in the literature is attributed to the Guest Atom. The 3.5 meV mode is interpreted to have significant spectral weight of Sn host Atoms based on temperature dependence and comparison with published theoretical phonon calculations. The record of low thermal conductivity of the tin clathrates can be attributed to the host structure dynamics rather than the Guest Atom rattling.
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Application of Atomic Hirshfeld surface analysis to intermetallic systems: is Mn in cubic CeMnNi4 a thermoelectric rattler Atom?
Inorganic chemistry, 2012Co-Authors: Mads R. V. Jørgensen, Mogens Christensen, Iben Skovsen, Henrik F. Clausen, Eiji Nishibori, Mark A. Spackman, Bo B. IversenAbstract:The Mn Atom in the cubic polymorph of CeMnNi(4) appears to be located in an oversized cage-like structure, and anomalously large Atomic displacement parameters (ADPs) for the Mn Atom indicate that it is a potential "rattler" Atom. Here, multitemperature synchrotron powder X-ray diffraction data measured between 110 and 900 K are used to estimate ADPs for the Mn "Guest" Atom and the "host" structure Atoms in cubic CeMnNi(4). The ADPs are subsequently fitted with Debye and Einstein models, giving Θ(D) = 301(2) K for the "host" structure and Θ(E) = 165(2) K for the Mn Atom. This is higher than typical Einstein temperatures for rattlers in thermoelectric skutterudites and clathrates (Θ(E) = 50-80 K), indicating that the Mn Atom in cubic CeMnNi(4) is more strongly bonded. In order to probe the chemical interactions of the potential Mn rattler Atom, Atomic Hirshfeld surface (AHS) analysis is carried out and compared with AHS analysis of well-established Guest Atom rattlers in archetypical skutterudites, MCoSb(3). Surprisingly, the skutterudite rattlers have more deformed AHSs than the Mn Atom in cubic CeMnNi(4). This is related to the highly ionic nature of the skutterudite rattlers, which is not taken into account in the neutral spherical Atom approach of the AHS. Additionally, visualization of void spaces in the two materials using the procrystal electron density shows that while the Mn Atom is tightly fitting in the CeMnNi(4) structure then the La Atom in the skutterudite is truly situated in an oversized cage of the host structure. Overall, we conclude that the Mn Atom in cubic CeMnNi(4) cannot be coined a rattler.
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Clathrate Guest Atoms under pressure
Journal of Applied Physics, 2009Co-Authors: Mogens Christensen, Simon Johnsen, Fanni Juranyi, Bo B. IversenAbstract:Powder inelastic neutron scattering (INS) has been used to determine the Guest Atom “rattling” energy in thermoelectric clathrates Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8,Ga16) under different applied conditions. Chemical pressure was exerted by the Atomic substitution, and a physical pressure of 9 kbars was applied using a clamp cell. The volume reduction induced by the physical pressure increases the energy of the Guest Atom rattling mode, but the local chemical environment in the cage also appears to have a similar effect. The Guest Atom energies were investigated as function of temperature, and softening of the Guest Atom modes was observed upon cooling the sample. Ba8Ga16Ge30 with holes (p-type) and electrons (n-type) as charge carriers reveal similar temperature behavior, suggesting anharmonic potentials of similar shape for the Ba Guest Atom independent of the charge carrier type. For Sr8Ga16Ge30 a much stronger anharmonic potential was observed compared with Ba8Ga16Ge30. The Guest Atom energies for Ba8YxGe46−...
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Crystal Structures of Thermoelectric n- and p-type Ba8Ga16Ge30 Studied by Single Crystal, Multitemperature, Neutron Diffraction, Conventional X-ray Diffraction and Resonant Synchrotron X-ray Diffraction
Journal of the American Chemical Society, 2006Co-Authors: Mogens Christensen, Nina Lock, Jacob Overgaard, Bo B. IversenAbstract:Comprehensive single-crystal structural investigations of n- and p-type Ba8Ga16Ge30 have been carried out using multitemperature neutron and conventional X-ray diffraction as well as resonant synchrotron X-ray diffraction. The data show that the Guest Atom positions and dynamics are very similar in the two structures, although the barium Atoms are slightly more displaced from the cage centers in the p-type structure than in the n-type structure (Deltad = 0.025 A). For both structures Fourier difference maps calculated from very high-resolution neutron diffraction data (sin theta/lambda > 2 A-1) show that the Ba nuclear density at lowest temperatures (15 K) is distributed in a torus around the crystallographic 6d site with maxima in the 24j positions. At room temperature the maxima have shifted to the 24k position. Analysis of Atomic displacement parameters give Einstein temperatures of approximately 60(1) K for both structures. Thus, the fundamental difference in the low temperature thermal conductivity observed for p- and n-type Ba8Ga16Ge30 appear not to be directly related to the Guest Atom behavior as is commonly assumed in thermoelectric research. The neutron data and the resonant synchrotron X-ray data facilitate refinement of Ga/Ge framework occupancies. The Ga Atoms have a clear preference for the 6c site with the preference being somewhat stronger for the n-type structure.
Serkan Caliskan - One of the best experts on this subject based on the ideXlab platform.
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a first principles study on spin resolved electronic properties of x c70 x n b endohedral fullerene based molecular devices
Physica E-low-dimensional Systems & Nanostructures, 2019Co-Authors: Serkan CaliskanAbstract:Abstract A first principles study, through Density Functional Theory combined with Non-Equilibrium Green's Function Formalism, is addressed on the spin dependent electronic properties of X@C70 (X = N, B) fullerene molecules attached to Au electrodes (Au-X@C70-Au). These endohedral fullerenes across a molecular junction are examined by spin resolved transmission, density of states, molecular orbitals, differential conductance and current-voltage (I-V) characteristics. Au-X@C70-Au molecular devices exhibit spin-asymmetric behavior, emerging from the Guest Atom (N, B) encapsulated in the C70 fullerene cage. While the Au-N@C70-Au molecular junction reveals an almost linear I-V variation (implying a metallic property), the Au-B@C70-Au device exhibits a nonlinear change within the bias range from −1 V to 1 V for both spin orientations. The essential role of each Guest Atom, induced magnetic moment, spin polarization and other relevant quantities associated with the spin resolved electronic transport are elucidated.
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A first principles study on spin resolved electronic properties of X@C70 (X = N, B) endohedral fullerene based molecular devices
Physica E: Low-dimensional Systems and Nanostructures, 2019Co-Authors: Serkan CaliskanAbstract:Abstract A first principles study, through Density Functional Theory combined with Non-Equilibrium Green's Function Formalism, is addressed on the spin dependent electronic properties of X@C70 (X = N, B) fullerene molecules attached to Au electrodes (Au-X@C70-Au). These endohedral fullerenes across a molecular junction are examined by spin resolved transmission, density of states, molecular orbitals, differential conductance and current-voltage (I-V) characteristics. Au-X@C70-Au molecular devices exhibit spin-asymmetric behavior, emerging from the Guest Atom (N, B) encapsulated in the C70 fullerene cage. While the Au-N@C70-Au molecular junction reveals an almost linear I-V variation (implying a metallic property), the Au-B@C70-Au device exhibits a nonlinear change within the bias range from −1 V to 1 V for both spin orientations. The essential role of each Guest Atom, induced magnetic moment, spin polarization and other relevant quantities associated with the spin resolved electronic transport are elucidated.
Susan M Kauzlarich - One of the best experts on this subject based on the ideXlab platform.
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Tuning Thermoelectric Properties of Type I Clathrate K8–xBaxAl8+xSi38–x through Barium Substitution
Chemistry of Materials, 2016Co-Authors: Fan Sui, Susan M KauzlarichAbstract:The thermal stability and thermoelectric properties of type I clathrate K8Al8Si38 up to 873 K are reported. K8Al8Si38 possesses a high absolute Seebeck coefficient value and high electrical resistivity in the temperature range of 323 to 873 K, which is consistent with previously reported low temperature thermoelectric properties. Samples with Ba partial substitution at the K Guest Atom sites were synthesized from metal hydride precursors. The samples with the nominal chemical formula of K8–xBaxAl8+xSi38–x (x = 1, 1.5, 2) possess type I clathrate structure (cubic, Pm3n), confirmed by X-ray diffraction. The Guest Atom site occupancies and thermal motions were investigated with Rietveld refinement of synchrotron powder X-ray diffraction. Transport properties of Ba-containing samples were characterized from 2 to 300 K. The K–Ba alloy phases showed low thermal conductivity and improved electrical conductivity compared to K8Al8Si38. Electrical resistivity and Seebeck coefficients were measured over the tempera...
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crystal structure and thermoelectric properties of clathrate ba8ni3 5si42 0 small cage volume and large disorder of the Guest Atom
Journal of Solid State Chemistry, 2012Co-Authors: John H Roudebush, Mike Orellana, Sabah K Bux, Susan M KauzlarichAbstract:Samples with the type-I clathrate composition Ba{sub 8}Ni{sub x}Si{sub 46-x} have been synthesized and their structure and thermoelectric properties characterized. Microprobe analysis indicates the Ni incorporation to be 2.62{<=}x{<=}3.53. The x=3.5 phase crystallizes in the type-I clathrate structure (space group: Pm-3n) with a lattice parameter of 10.2813(3) A. The refined composition was Ba{sub 8}Ni{sub 3.5}Si{sub 42.0}, with small vacancies, 0.4 and 0.5 Atoms per formula unit, at the 2a and 6c sites, respectively. The position of the Ba2 Atom in the large cage was modeled using a 4-fold split position (24j site), displaced 0.18 A from the cage center (6d site). The volume of the large cage is calculated to be 146 A{sup 3}, smaller than other clathrates with similar cation displacement. The sample shows n-type behavior with a maximum of -50 {mu}V/K at 823 K above which the Seebeck coefficient decreases, suggesting mixed carriers. Lattice thermal conductivity, {kappa}{sub l}, is 55 mW/K above 600 K. - Graphical abstract: Seebeck coefficient and resistivity of the type-I clathrate Ba{sub 8}Ni{sub 3.5}Si{sub 41.0}. Structure show's large displacement of the Ba cation in the large cage (6c site). Highlights: Black-Right-Pointing-Pointer Crystal structure of the Ba{sub 8}Ni{sub 3.5}Si{sub 41.0} reported. Black-Right-Pointing-Pointer Vacancies at themore » 2a and 6c sites. Black-Right-Pointing-Pointer Large disorder of Ba Guest Atom, 0.18 A from cage center. Black-Right-Pointing-Pointer Structure is compared to Ba{sub 8}Si{sub 46} and other type-I clathrates. Black-Right-Pointing-Pointer Max Seebeck of -50.7 {mu}V/C at 798.4 K, thermal conductivity {approx}55 mW/K.« less
Eiji Kaneshita - One of the best experts on this subject based on the ideXlab platform.
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Phonon-glass electron-crystal thermoelectric clathrates : Experiments and theory
Reviews of Modern Physics, 2014Co-Authors: Toshiro Takabatake, Tsuneyoshi Nakayama, Koichiro Suekuni, Eiji KaneshitaAbstract:Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nano-scale tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the group 1 or 2 elements in the periodic table are encaged as the so-called rattling Guest Atom. It is remarkable that, though these compounds have crystalline cubic-structure, they exhibit glass-like phonon thermal conductivity over the whole temperature range depending on the states of rattling Guest Atoms in the tetrakaidecahedron. In addition, these compounds show unusual glass-like specific heats and THz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in topologically disordered amorphous materials or structural glasses, the so-called Boson peak. An efficient thermoelectric effect is realized in compounds showing these glass-like characteristics. This decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction experiments, thermal and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this article based on advances this decade. The subject of this article involves interesting phenomena from the viewpoint of not only physics but also from the view point of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.