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γ1-AuZn2.1 in the Au-Zn binary system has been synthesized and its structure analyzed by single-crystal X-ray diffraction. It crystallizes in the trigonal space group P31m (No. 157) with â¼227 atoms per unit cell and represents a \surd3a × \surd3a × c superstructure of rhombohedrally distorted γ-Au5-xZn8+y. The structure is largely tetrahedrally closed packed. The formation of γ1-AuZn2.1 can be understood within the framework of a Hume-Rothery stabilization mechanism with a valence electron concentration of 1.68â e/a (valence electrons per atom).
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4-[4'-Cyanophenoxy-carbonyl-phenyl-4-(trimethylsilyl)ethynyl]benzoate a rod shaped liquid crystal (SmA) is synthesized and characterized. The single-crystals were grown in triclinic crystal system in the space group of Pi - with unit cell parameters a = 5.9577(2) Å, b = 8.0398(3) Å, c = 25.8842(9) Å, α = 86.096(2)o, ß = 89.912(2)o, γ = 2.919(2)o, Z = 2, and V = 1182.16(7). The crystal structure is stabilized by C-H···O intra-molecular interactions. Further, the structure also involves C-H···π interactions and weak π-π stacking interactions [centroid-centroid separation = 3.806 (3) Å].
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The results of crystallographic analysis, magnetic characterization, and theoretical assessment of ß-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These ß-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of â¼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the ß-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed ß-Mn structure type, a result that offers greater insight into the ß-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).
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The region of the Au-Zn phase diagram encompassing γ-brass-type phases has been studied experimentally from 45 to 85 atom % Zn. The γ phases were obtained directly from the pure elements by heating to 680 °C in evacuated silica tubes, followed by annealing at 300 °C. Powder X-ray and single-crystal diffraction studies show that γ-"Au(5)Zn(8)" phases adopt a rhombohedrally distorted Cr(5)Al(8) structure type rather than the cubic Cu(5)Zn(8) type. The refined compositions from two single crystals extracted from the Zn- and Au-rich loadings are Au(4.27(3))Zn(8.26(3))â¡(0.47) (I) and Au(4.58(3))Zn(8.12(3))â¡(0.3) (II), respectively (â¡ = vacancy). These (I and II) refinements indicated both nonstatistical mixing of Au and Zn atoms as well as partially ordered vacancy distributions. The structures of these γ phases were solved in the acentric space group R3m (No. 160, Z = 6), and the observed lattice parameters from powder patterns were found to be a = 13.1029(6) and 13.1345(8) Å and c = 8.0410(4) and 8.1103(6) Å for crystals I and II, respectively. According to single-crystal refinements, the vacancies were found on the outer tetrahedron (OT) and octahedron (OH) of the 26-atom cluster. Single-crystal structural refinement clearly showed that the vacancy content per unit cell increases with increasing Zn, or valence-electron concentration. Electronic structure calculations, using the tight-binding linear muffin-tin orbital method with the atomic-sphere approximation (TB-LMTO-ASA) method, indicated the presence of a well-pronounced pseudogap at the Fermi level for "Au(5)Zn(8)" as the representative composition, an outcome that is consistent with the Hume-Rothery interpretation of γ brass.
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A series of rhombohedrally distorted γ-brass structures containing a mixture of magnetically active 3d elements, Cr and Fe, Cr(1-x)Fe(x)Ga, is investigated crystallographically. These structures consist of chains of trans-face-sharing Ga-centered transition metal icosahedra. Neutron powder diffraction specifically on Cr(0.5)Fe(0.5)Ga, which could be prepared as a single phase material, gave lattice constants (11 K) a = 12.5172(2) Šand c = 7.8325(2) Šand a refined composition of Cr(0.502(6))Fe(0.498)Ga = Cr(6.523)Fe(6.477)Ga(13) and revealed partial ordering of Cr and Fe atoms among three crystallographic sites. Magnetic susceptibility and magnetization studies of Cr(0.5)Fe(0.5)Ga showed the onset of magnetic ordering at ca. 25 K. Theoretical calculations suggested both site-energy and bond-energy factors influencing the Cr/Fe distribution. Heteroatomic interactions significantly affect exchange interactions and create low local magnetic moments. Models created to mimic Cr(0.5)Fe(0.5)Ga showed ferromagnetic Fe-Fe and antiferromagnetic Cr-Fe interactions, with an overall ferrimagnetic ordering.
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Novel ternary phases, (Pd(1-x)Zn(x))(18)(Zn(1-y)Al(y))(86-delta) (0
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Palladium zinc aluminium (2.28/10.37/0.35), Pd(2.28(1))Zn(10.37(1))Al(0.35(1)), represents the upper limit of Al substitution into the parent cubic γ-brass Pd(2+x)Zn(11-x). The structure can be described in terms of a 26-atom cluster consisting of an inner tetra-hedron (IT), an outer tetra-hedron (OT), an octa-hedron (OH) and a cubocta-hedron (CO), with the substituted Al atoms partially occupying the IT (.3m) and CO (..m) sites.
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The crystal structure, its variation within the homogeneity range and some physical properties of the new zinc-rich, partly disordered phase Ir7+7deltaZn97-11delta (0.31< or =delta< or =0.58) are reported. The structures of three phases with distinct composition were determined by means of single crystal X-ray diffraction. Ir7+7deltaZn97-11delta exhibits a significant homogeneity range, adopts a complex gamma-brass related cubic structure (cF403-406), is stable up to 1201(2) K, and transforms sluggishly below 1048(4) K into a phase with 394 atoms in the monoclinic primitive unit cell. It is a diamagnetic, moderate metallic conductor. Six distinguishable clusters consisting of 22-29 atoms comprise the structure. The clusters are situated about the 16 high symmetry points of the cubic F lattice. The structure can be subdivided into two partial structures, one with constant composition IrZn5 and 192 atoms per unit cell and a second being significantly richer in zinc with variable composition and 211-214 atoms per unit cell. The meandering triply periodic minimal surface of two interpenetrating diamond-like nets separates the compositionally variable from its complementary invariant part. The phase width is coupled with substitutional and positional disorder. A comprehensive analysis of composition-dependent site occupancy factors reveals a linear correlation between the various types of disorder which can be conclusively interpreted in terms of an incoherent intergrowth of distinctive partial structures in variable proportions on a length scale comparable to the size of the approximately 2 nm large unit cell. On the basis of the structural findings we derive the structure chemically meaningful formula Ir7+7delta)Zn97-11delta which quantitatively accounts for the interrelation between substitutional and positional disorder and provides a measure for the homogeneity range in structural terms.
