| 1. |
- Eklof, Daniel, et al.
(författare)
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Mysterious SiB3: Identifying the Relation between alpha- and beta-SiB3
- 2019
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Ingår i: ACS Omega. - : AMER CHEMICAL SOC. - 2470-1343. ; 4:20, s. 18741-18759
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Tidskriftsartikel (refereegranskat)abstract
- Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric alpha-SiB3-x, which relates to the alpha-rhombohedral phase of boron, and as strictly ordered and stoichiometric beta-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of alpha-SiB3-x and beta-SiB3 and analyze their relative thermodynamic stabilities. alpha-SiB3-x is metastable (with respect to beta-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200-1300 degrees C. At the same time, alpha-SiB3-x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable beta-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:11:4 at temperatures 1175-1200 degrees C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of beta-SiB3 was seen at 1100 degrees C for samples pressurized to 5.5-8 GPa. beta-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 degrees C. The highly ordered nature of beta-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of alpha-SiB3-x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered alpha-SiB3-x were ascertained by a combination of single-crystal X-ray diffraction and Si-29 magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T amp;lt;= 1200 degrees C) and single crystal samples (obtained from Si-rich molten Si-B mixtures at T amp;gt; 1400 degrees C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B-12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of alpha-SiB3-x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B-12 icosahedron. Accordingly, alpha-SiB3-x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of alpha-SiB3-x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition beta-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that alpha-SiB3-x only becomes more stable than beta-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that alpha-SiB3-x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise alpha-SiB2.5 and beta-SiB3, whereas alpha-SiB3 represents a p-type conductor.
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| 2. |
- Fischer, A., et al.
(författare)
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Thermal and vibrational properties of thermoelectric ZnSb : Exploring the origin of low thermal conductivity
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:22
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Tidskriftsartikel (refereegranskat)abstract
- The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9-400 K) and powder x-ray diffraction (300-725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below 60 cm(-1). Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures Theta(D) = 195 K, Theta(E1) = 78 K, and Theta(E2) = 277K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-delta.
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| 3. |
- Ghanta, Sivaprasad, et al.
(författare)
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Synthesis, structure, and physical properties of a Y-Au-Ge 1/1 Tsai-type quasicrystal approximant and Y-14(Au,Ge)(51) with the Gd14Ag51 structure type
- 2023
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Ingår i: Journal of Solid State Chemistry. - 0022-4596 .- 1095-726X. ; 327
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Tidskriftsartikel (refereegranskat)abstract
- Exploration of the Au-Ge rich part of the pseudo-binary section Y-x(Au0.70Ge0.30)(100-x) (3 < x < 18) in the Y-Au-Ge system afforded the intermetallic compounds Y-3(Au0.7054Ge0.2946)(19-x) (x 0.38) and Y-14(Au0.8313Ge0.1687)(51), which are new representatives of the cubic 1/1 Tsai-type approximant crystal (YbCd6 parent type, space group Im (3) over bar) and the hexagonal Gd14Ag51 structure type (space group P6/m), respectively. Y-3(Au0.7054Ge0.2946)(19-x) decomposes peritectically into Y-14(Au, Ge)(51) phase and melt at around 700 degrees C. In this respect the Y-Au-Ge system deviates from the Y-Au-Si system which firstly contains a second type of 1/1 approximant crystal phase with higher Y content, Y-3.25(Au,Si)(18-x) (x approximate to 0.11) and, secondly, adjacent at higher Y composition there is a stoichiometric and ordered tetragonal phase YAu3Si. Y-3(Au,Ge)(19-x) and Y-14(Au,Ge)(51) exhibit bulk superconductivity of conventional type-II BCS type albeit with very low transition temperatures (below 1 K), which is very similar to the corresponding compounds in the Y-Au-Si system.
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| 4. |
- Gordeeva, Alisa, et al.
(författare)
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Layered Zinc Hydroxide Dihydrate, Zn-5(OH)(10)center dot 2H(2)O, from Hydrothermal Conversion of epsilon-Zn(OH)(2) at Gigapascal Pressures and its Transformation to Nanocrystalline ZnO
- 2020
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Ingår i: ACS Omega. - : AMER CHEMICAL SOC. - 2470-1343. ; 5:28, s. 17617-17627
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Tidskriftsartikel (refereegranskat)abstract
- Layered zinc hydroxides (LZHs) with the general formula (Zn2+)(x)(OH-)(2x-my ),(A(m-))(y)center dot nH(2)O (A(m-) = Cl- , NO3- , ac(-) , SO42-, etc) are considered as useful precursors for the fabrication of functional ZnO nanostructures. Here, we report the synthesis and structure characterization of the hitherto unknown "binary" representative of the LZH compound family, Zn-5(OH)(10)center dot 2H(2)O, with A(m-) = OH- , x = 5, y = 2, and n = 2. Zn-5(OH)(10)center dot 2H(2)O was afforded quantitatively by pressurizing mixtures of epsilon-Zn(OH)(2) (wulfingite) and water to 1-2 GPa and applying slightly elevated temperatures, 100-200 degrees C. The monoclinic crystal structure was characterized from powder X-ray diffraction data (space group C2/c, a = 15.342(7) angstrom, b = 6.244(6) angstrom, c = 10.989(7) angstrom, beta = 100.86(1)degrees). It features neutral zinc hydroxide layers, composed of octahedrally and tetrahedrally coordinated Zn ions with a 3:2 ratio, in which H2O is intercalated. The interlayer d(200) distance is 7.53 angstrom. The H-bond structure of Zn-5(OH)(10)center dot 2H(2)O was analyzed by a combination of infrared/Raman spectroscopy, computational modeling, and neutron powder diffraction. Interlayer H2O molecules are strongly H-bonded to five surrounding OH groups and appear orientationally disordered. The decomposition of Zn-5(OH)(10)center dot 2H(2)O, which occurs thermally between 70 and 100 degrees C, was followed in an in situ transmission electron microscopy study and ex situ annealing experiments. It yields initially 5-15 nm sized hexagonal w-ZnO crystals, which, depending on the conditions, may intergrow to several hundred nm-large two-dimensional, flakelike crystals within the boundary of original Zn-5(OH)(10)center dot 2H(2)O particles.
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| 5. |
- Kal, Subhadeep, et al.
(författare)
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High-pressure modifications of CaZn2, SrZn2, SrAl2, and BaAl2 : Implications for Laves phase structural trends
- 2008
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 181:11, s. 3016-3023
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Tidskriftsartikel (refereegranskat)abstract
- High-pressure forms of intermetallic compounds with the composition CaZn2, SrZn2, SrAl2, and BaAl2 were synthesized from CeCu2-type precursors (CaZn2, SrZn2, SrAl2) and Ba21Al40 by multi-anvil techniques and investigated by X-ray powder diffraction (SrAl2 and BaAl2), X-ray single-crystal diffraction (CaZn2), and electron microscopy (SrZn2). Their structures correspond to that of Laves phases. Whereas the dialuminides crystallize in the cubic MgCu2 (C15) structure, the dizincides adopt the hexagonal MgZn2 (C14) structure. This trend is in agreement with the structural relationship displayed by sp bonded Laves phase systems at ambient conditions.
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| 6. |
- Mikhaylushkin, Arkady S, et al.
(författare)
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High-pressure structural behavior of large-void CoSn-type intermetallics: Experiments and first-principles calculations
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : Institutionen för fysik, kemi och biologi. - 1098-0121 .- 1550-235X. ; 77:1, s. 014102-
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Tidskriftsartikel (refereegranskat)abstract
- The high-pressure structural behavior of the binary intermetallic compounds CoSn, FeSn, and NiIn with the peculiar void containing CoSn (B35)-type structure has been studied by means of room-temperature diamond anvil cell and high-temperature multianvil experiments, as well as by first-principles calculations. All three compounds remain structurally stable at pressures up to at least 25 GPa, whereas first-principles calculations predict high-pressure structural changes below 20 GPa. A plausible explanation for the discrepancy is that at room temperature, a sizable activation barrier inhibits kinetically the transformation into more close-packed polymorphs. It is supported by our experiments at temperatures around 1000 °C and a pressure of 10 GPa. At these conditions, NiIn transforms into the temperature-quenchable stoichiometric CsCl-type high-pressure phase, which has been predicted in our first-principles calculations. However, CoSn and FeSn decompose into a mixture of compounds richer and poorer in tin, respectively. Nevertheless, it might be possible that lower temperatures and higher pressures may afford theoretically predicted polymorphs. In particular, a phase transformation to the FeSi-type structure predicted for CoSn is of interest as materials with the FeSi-type structure are known for unusual thermal and transport properties.
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| 7. |
- Nylén, Johanna, et al.
(författare)
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Characterization of a high pressure, high temperature modification of ammonia borane (BH3NH3)
- 2013
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 139:5, s. 054507-
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Tidskriftsartikel (refereegranskat)abstract
- At elevated pressures (above 1.5 GPa) dihydrogen bonded ammonia borane, BH3NH3, undergoes a solid-solid phase transition with increasing temperature. The high pressure, high temperature (HPHT) phase precedes decomposition and evolves from the known high pressure, low temperature form with space group symmetry Cmc2(1) (Z = 4). Structural changes of BH3NH3 with temperature were studied at around 6 GPa in a diamond anvil cell by synchrotron powder diffraction. At this pressure the Cmc2(1) phase transforms into the HPHT phase at around 140 degrees C. The crystal system, unit cell, and B and N atom position parameters of the HPHT phase were extracted from diffraction data, and a hydrogen ordered model with space group symmetry Pnma (Z = 4) subsequently established from density functional calculations. However, there is strong experimental evidence that HPHT-BH3NH3 is a hydrogen disordered rotator phase. A reverse transition to the Cmc2(1) phase is not observed. When releasing pressure at room temperature to below 1.5 GPa the ambient pressure (hydrogen disordered) I4mm phase of BH3NH3 is obtained.
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| 8. |
- Nylen, Johanna, et al.
(författare)
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Effect of metal doping on the low-temperature structural behavior of thermo electric beta-Zn4Sb3
- 2007
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 180:9, s. 2603-2615
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Tidskriftsartikel (refereegranskat)abstract
- The low-temperature structural phase transitions of Bi, Pb, In and Sn-doped samples of thermoelectric Zn4Sb3 have been characterized on crystals grown from molten metal fluxes, using electrical resistance and single crystal X-ray diffraction measurements. Room temperature stable, disordered, beta-Zn4Sb3 undergoes two phase transitions at 254 and 235 K to the consecutively higher ordered phases a and alpha', respectively. The ideal crystallographic composition of alpha-Zn4Sb3 is Zn13Sb10. The alpha-alpha' transformation is triggered by a slight and homogenous Zn deficiency with respect to this composition and introduces a compositional modulation in the alpha-Zn4Sb3 structure. When preparing beta-Zn4Sb3 in the presence of metals with low melting points (Bi, Sn, In, Pb) the additional metal atoms are unavoidably incorporated in small concentrations (0.04-1.3 at%.) and act as dopants. This incorporation alters the subtle balance between Zn disorder and Zn deficiency in Zn4Sb3 and has dramatic consequences for its low-temperature structural behavior. From molten metal flux synthesis it is possible to obtain (doped) Zn4Sb3 samples which (1) only display a P-a transition, (2) only display a beta-alpha' transition, or (3) do not display any low-temperature phase transition at all. Case (2) provided diffraction data with a sufficient quality to obtain a structural model for highly complex, compositionally modulated, alpha'-Zn4Sb3. The crystallographic composition of this phase is Zn84Sb65.
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| 9. |
- Nylen, Johanna, et al.
(författare)
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Low-temperature structural transitions in the phonon-glass thermoelectric material beta-Zn4Sb3 : Ordering of Zn interstitials and defects
- 2007
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 19:4, s. 834-838
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Tidskriftsartikel (refereegranskat)abstract
- The low-temperature phase transitions of thermoelectric Zn4Sb3 have been characterized using single-crystal X-ray diffraction, electrical resistance, and thermal conductivity measurements. Room-temperature stable, disordered beta-Zn4Sb3 undergoes a phase transition at 254 K to ordered alpha-Zn4Sb3, which has an ideal composition Zn13Sb10. Below 235 K, a second low-temperature phase (alpha'-Zn4Sb3) can be detected. The sequence of phase transitions beta-alpha-alpha' is reversible. The alpha-alpha' transformation originates from a slight Zn deficiency with respect to Zn13Sb10. The actual composition of Zn4Sb3 is Zn13-delta Sb10.
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| 10. |
- Wu, Yang, et al.
(författare)
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Zn5Sb4In2-delta - a Ternary Derivative of Thermoelectric Zinc Antimonides
- 2009
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 48:13, s. 5996-6003
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Tidskriftsartikel (refereegranskat)abstract
- The now intermetallic compound Zn5Sb4In2-delta has been synthesized and structurally characterized. In the temperature region 10-350K it is superior to Zn4Sb3 as a thermoelectric material.
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