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- Akselrud, Lev, et al.
(author)
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Order and disorder in the modulated phase hp-Mg9Ge5.1
- 2022
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In: Zeitschrift fur Anorganische und Allgemeine Chemie. - : Wiley. - 0044-2313 .- 1521-3749. ; 648:10
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Journal article (peer-reviewed)abstract
- Single crystals of Mg9Ge5.1(2) were obtained at a pressure of 5 GPa in the temperature range 723 to 1273 K. The metastable high-pressure phase exhibits a crystal structure, which may be described as a host-guest variety of an UCl3-type framework. The atomic arrangement is characterized by a host crystal structure Mg9Ge3 with disordered germanium guests (≈2 Ge per formula unit) in the channels. The composite arrangement undergoes incommensurate modulation. Upon heating at ambient pressure, the phase transforms into the thermodynamically stable compounds Mg2Ge and Ge at 496(5) K. Measurements of the magnetic susceptibility indicate diamagnetic behavior between 75 and 350 K.
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- Feuerbacher, Michael, et al.
(author)
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The Samson phase, β-Mg2Al3, revisited
- 2007
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In: Zeitschrift fur Kristallographie. - : Walter de Gruyter GmbH. - 0044-2968. ; 222:6, s. 259-288
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Research review (peer-reviewed)abstract
- The Al-Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg38.5Al61.5, this cubic phase, space group Fd3̄m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214°C a first-order phase transition to rhombohedral β′-Mg2Al3, (hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd3̄m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400°C. It essentially agrees with Samson's model, even in most of the many partially occupied and split positions. The structure of β′-Mg 2Al3 is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.
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4. |
- Fredrickson, Daniel, et al.
(author)
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Re4Si7, First in a New MoSi2-Based Family of 14-Electron Phases
- 2009
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In: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 15:33, s. 8108-8112
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Journal article (peer-reviewed)abstract
- The compound Re4Si7 poses a long-standing structural mystery at the intersection of the quantum chemical and material properties areas of interest. Its atomic structure has gradually emerged over X-ray and electron diffraction studies since 1941 to be a complex ordering of Si vacancies on a MoSi2-type lattice, the details of which remain unresolved. In 1941, it was originally identified as ReSi2, adopting the MoSi2-type crystal structure (Figure 1).1a In this structure, Re atoms form a body-centered tetragonal lattice, while the Si atoms lie in the square-pyramidal interstitial spaces of this Re lattice. This was followed by two structure reports in progressively lower symmetry: an orthorhombic Immm solution (1983),1b and a triclinic P1 solution (1995),1c in which fractional occupancies on the Si sites adjust the composition to the Re4Si7 stoichiometry reported in a redetermination of the Re–Si phase diagram.1d Electron microscopy investigations then revealed complex superstructures and incommensurability in Re4Si7 samples.2 This work culminated in an attempt to solve its structure as an eight-fold supercell in the space group Cm from single-crystal synchrotron X-ray diffraction data (2004).1e Severe refinement instabilities were encountered, and the published structure shows chemically implausible Re–Si and Si–Si distances
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