Structure and magnetic properties of rare-earth chromium germanides RECrxGe2 (RE=Sm, Gd–Er)
TL;DR: In this article, the average structure of rare-earth chromium germanides has been determined, which corresponds to the CeNiSiSi{sub 2}-type, with Cr atoms entering more and more square pyramidal sites.
About: This article is published in Journal of Solid State Chemistry.The article was published on 2009-01-01. It has received 12 citations till now.
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TL;DR: The family of materials RE(4)TGe(8) exhibits directional zero thermal expansion (ZTE) via a process that is associated with the linking of planar square nets in the third dimension via a Poisson-like mechanism that allows the structure to contract along one direction as it expands only slightly in the perpendicular direction.
Abstract: The family of materials RE4TGe8 (RE = Yb, Gd; T = transition metal) exhibits directional zero thermal expansion (ZTE) via a process that is associated with the linking of planar square nets in the third dimension. The Ge square nets in these compounds exhibit commensurate long-range modulations similar to those observed in charge-density-wave compounds. The ZTE is manifested in the plane of the square nets from 10 to 300 K with negligible volume expansion below ∼160 K. The specific atomic arrangement in RE4TGe8 enables a Poisson-like mechanism that allows the structure to contract along one direction as it expands only slightly in the perpendicular direction.
46 citations
TL;DR: The measured ordering temperatures and calculated ones based on empirical rules and Ruderman-Kittel-Kasuya-Yosida interactions are shown to be in close agreement.
Abstract: Rare-earth metal aluminum germanides with the general formula REAl1–xGe2 (RE = Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y) have been synthesized by direct fusion of the corresponding elements. The structures have been studied by single-crystal X-ray diffraction and selected-area electron diffraction (SAED). The average structure represents a randomly “stuffed” variant of the orthorhombic ZrSi2 structure type, also known as the CeNi1–xSi2 type (Pearson symbol oC16; space group Cmcm). The SAED patterns for selected members of the family suggest the coexistence of commensurate and incommensurate structural modulations. The most prominent model for long-range vacancy ordering is the Tb4FeGe8 type (Pearson symbol mP26; space group P21/n), which is the commensurate 4-fold superstructure of CeNi1–xSi2 (x = 3/4). Short-range correlations cause additional deviations in the 4-fold superlattice. These results shed more light on the structural complexity as a function of the aluminum vacancies and size of the rare-earth metal...
16 citations
TL;DR: Physical property measurements indicate metallic conductivity and the absence of localized magnetism in the studied compounds, and first-principle calculations reveal hypervalent bonding in the homoatomic Bi subunits.
Abstract: Employment of liquid bismuth (Bi) allows the facile single-crystal growth of compounds containing elements with high melting points, provided that these elements have reasonably high solubility in ...
10 citations
10 citations
TL;DR: The magnetic and transport properties of ternary rare-earth chromium germanides (RCr{sub 0.3}Ge{sub 2} (R=Y and Tb-Er) have been determined.
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TL;DR: In this paper, a computer program has been written for the purpose of standardizing crystal structure data according to rules formulated by Parthe & Gelato, which can be used to recognize structures which are isopointal.
Abstract: A computer program has been written for the purpose of standardizing crystal structure data according to rules formulated by Parthe & Gelato [Acta Cryst. (1984). A40, 169–183]. From input consisting of space-group symbol, unit-cell parameters and positional coordinates of the atoms, a reordered and renumbered list of standardized atom coordinates is obtained. The space group is now in the standard setting and the cell is reduced if applicable. The origin and the orientation of the coordinate system have been chosen in such a way as to minimize the standardization parameter Γ. A second standardization parameter, based on the position of the centre of gravity of the atoms in the asymmetric unit, is introduced. The Wyckoff sequence, obtained from the standardized structure data, can be used to recognize structures which are isopointal. An example of the application of STRUCTURE TIDY is given.
1,202 citations
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08 Mar 1994
TL;DR: In this article, the magnetic properties of 3D-rich ternary linear compounds are described. But they do not specify the properties of the corresponding 3D lattice structures.
Abstract: Introduction. Crystal Structures of Ternary Lanthanide Intermetallics. Magnetic Properties of Intermetallics. Magnetic Properties of the Intermetallic RxTyXz Compounds with y/xGBP2. Magnetic Materials Based on 3D-Rich Ternary Compounds. Concluding Remarks.
283 citations
TL;DR: The hypervalent electron counting scheme developed in this paper, along with the classical Zintl-Klemm electron counting rules, gives an easy qualitative understanding of bonding in a wide variety of intermetallic compounds of heavy main group elements.
Abstract: We construct a theory for electron-rich polyanionic networks in the intermetallic compounds of heavy late main group elements, building a bonding framework that makes a connection to well-understood hypervalent bonding in small molecules such as XeF(4), XeF(2), and I(3)(-). What we do is similar in spirit to the analogy between the Zintl-Klemm treatment of classical polyanionic networks and the octet rule for molecules. We show that the optimal electron count for a linear chain of a heavy main group element is seven electrons per atom, six electrons per atom for a square lattice, and five electrons per atom for a simple cubic lattice. Suggestions that these electron counts are appropriate already exist in the literature. We also derive electron counts for more complicated topologies, including one-dimensional ladders and one dimensional strips cut from a square lattice. We also study pairing (Peierls) distortions from these ideal geometries as well as other deformations. The presence of s-p mixing (or its absence) plays a critical role in the propensity for pairing and, in general, in determining the geometrical and electronic structure of these phases. Hypervalent bonding goes along with the relative absence of significant s-p interaction; there is a continuum of such mixing, but also a significant difference between the second-row and heavier elements. We attribute the existence of undistorted metallic networks of the latter elements to diminished s-p mixing, which in turn is due to the contraction of less-screened s orbitals relative to p orbitals down the groups in the Periodic Table. The number of electrons in the polyanionic network may be varied experimentally. An important general principle emerges from our theoretical analysis: upon oxidation a hypervalent structure transforms into a classical one with the same lattice dimensionality, while upon Peierls distortion the hypervalent structures transform into classical ones with the lattice dimensionality reduced. Dozens of crystal structure types, seemingly unrelated to each other, may be understood using the unifying concept of electron-rich multicenter bonding. Antimonides, which are explored in great detail in the current work, conform particularly well to the set of electron counting rules for electron-rich nonclassical networks. Some deviation up and down from the ideal electron count is exhibited by known stannides and tellurides. We can also make sense of the bonding in substantially more complicated alloys, including La(12)Mn(2)Sb(30) and Tl(4)SnTe(3). The hypervalent electron counting scheme developed in this paper, along with the classical Zintl-Klemm electron counting rules, gives an easy qualitative understanding of bonding in a wide variety of intermetallic compounds of heavy main group elements.
242 citations
TL;DR: In this article, two new germaniures binaires are introduced: LuGe 2 and TmGe 2, isotypes de ZrSi 2, which are non-stoechiometriques, avec de larges domaines d'homogeneite.
Abstract: Resume De nombreux germaniures et stannures isotypes de CeNiSi 2 ont ete caracterises par diffraction X de poudres et analyse a la microsonde electronique. La plupart de ces composes sont non-stoechiometriques, avec de larges domaines d'homogeneite; leurs formules sont donc RM x X 2 avec 0 x min x x max 2 de type ZrSi 2 . De simples considerations geometriques ne permettent pas d'expliquer les fortes variations des indices x min et x max , observees en fonction de la nature du constituant M. Deux nouveaux germaniures binaires sont aussi decrits: LuGe 2 et TmGe 2 , isotypes de ZrSi 2 .
132 citations