The Ir3Ge7 (D8f) Structure: an Electron Phase Related to γ-Brass

1996 ◽  
Vol 453 ◽  
Author(s):  
D. Swenson
Keyword(s):  
Electron Concentration ◽  
Structure Type ◽  
Large Concentration ◽  
Crystal Chemical ◽  
Formula Unit

AbstractThe crystallographic similarity between the Ir3Ge7 (D8f) and γ-brass (Cu5Zn8, D82) structures is discussed. By considering all phases known to possess the Ir3Ge7 structure, it is shown that phases of this structure type obey essentially the same electron concentration rules as does γ-brass, possessing on average 22 atoms/formula unit. Based on crystal-chemical considerations, it is further suggested that the Ir3Ge7 structure may be considered to be a structural derivative of γ-brass, in which 12 atoms have dropped out of the γ-brass lattice and the remaining atoms have undergone a certain amount of rearrangement in order to accommodate such a large concentration of vacancies. Finally, as further support of this idea, the phase NiGa4 is presented as an example of another γ-brass derived structure which contains crystallographic features of both γ-brass and the Ir3Ge7 structure.

scholarly journals Geometrical parameterization of the crystal chemistry of P63/m apatites: comparison with experimental data and ab initio results

2005 ◽  
Vol 61 (6) ◽  
pp. 635-655 ◽  
Author(s):  
Patrick H. J. Mercier ◽  
Yvon Le Page ◽  
Pamela S. Whitfield ◽  
Lyndon D. Mitchell ◽  
Isobel J. Davidson ◽  
...  
Keyword(s):  
Single Crystal ◽  
Ab Initio ◽  
Minimum Energy ◽  
Structure Type ◽  
Chemical Model ◽  
Crystal Chemical ◽  
Chemical Parameters ◽  
Rietveld Refinements ◽  
Cell Parameters ◽  
Cell Data

Experimental structure refinements and ab initio simulation results for 18 published, fully ordered P63/m (A^{\rm I}_4)(A^{\rm II}_6)(BO4)6 X 2 apatite end-member compositions have been analyzed in terms of a geometric crystal-chemical model that allows the prediction of unit-cell parameters (a and c) and all atom coordinates. To an accuracy of ± 0.025 Å, the magnitude of c was reproduced from crystal-chemical parameters characterizing chains of …–A II–O3–B–O3–A II–... atoms, whereas that of a was determined from those describing (A IO6)–(BO4) polyhedral arrangements. The c/a ratio could be predicted to ±0.2% using multi-variable functions based on geometric crystal-chemical model predictions, but could not be ascribed to the adjustment of a single crystal-chemical parameter. The correlations observed between algebraically independent crystal-chemical parameters representing the main observed polyhedral distortions reveal them as the minimum-energy solution to accommodate misfit components within this flexible structure type. For materials with given composition, good agreement (within ± 0.5–2.0%) of ab initio crystal-chemical parameters was observed with only those from single-crystal refinements with R ≤ 4.0%. Agreement with single-crystal work with R > 4.0% was not as good, while the scatter with those from Rietveld refinements was considerable. Accordingly, ab initio cell data, atomic coordinates and crystal-chemical parameters were reported here for the following compositions awaiting experimental work: (Zn,Hg)10(PO4)6(Cl,F)2, (Ca,Cd)10(VO4)6Cl2 and (Ca,Pb,Cd)10(CrO4)6Cl2.

Structure determination of KScS2, RbScS2and KLnS2(Ln = Nd, Sm, Tb, Dy, Ho, Er, Tm and Yb) and crystal–chemical discussion

2015 ◽  
Vol 71 (7) ◽  
pp. 623-630 ◽  
Author(s):  
Lubomír Havlák ◽  
Jan Fábry ◽  
Margarida Henriques ◽  
Michal Dušek
Keyword(s):  
Rare Earth ◽  
Structure Determination ◽  
Structure Type ◽  
Trivalent Cation ◽  
Crystal Chemical ◽  
Ionic Radii ◽  
Structural Family ◽  
Trivalent Cations ◽  
Ytterbium Sulfide

The title structures of KScS2(potassium scandium sulfide), RbScS2(rubidium scandium sulfide) and KLnS2[Ln = Nd (potassium neodymium sufide), Sm (potassium samarium sulfide), Tb (potassium terbium sulfide), Dy (potassium dysprosium sulfide), Ho (potassium holmium sulfide), Er (potassium erbium sulfide), Tm (potassium thulium sulfide) and Yb (potassium ytterbium sulfide)] are either newly determined (KScS2, RbScS2and KTbS2) or redetermined. All of them belong to the α-NaFeO2structure type in agreement with the ratio of the ionic radiir3+/r+. KScS2, the member of this structural family with the smallest trivalent cation, is an extreme representative of these structures with rare earth trivalent cations. The title structures are compared with isostructural alkali rare earth sulfides in plots showing the dependence of several relevant parameters on the trivalent cation crystal radius; the parameters thus compared arec,aandc/a, the thicknesses of the S—S layers which contain the respective constituent cations, the sulfur fractional coordinatesz(S2−) and the bond-valence sums.

Intermediate-valent Cerium in CeRu2Mg5

2010 ◽  
Vol 65 (10) ◽  
pp. 1185-1190 ◽  
Author(s):  
Stefan Linsinger ◽  
Matthias Eul ◽  
Ute Ch. Rodewald ◽  
Rainer Pöttgen
Keyword(s):  
High Frequency ◽  
Three Dimensional ◽  
Magnetic Ordering ◽  
Structure Type ◽  
Formula Unit ◽  
Temperature Dependent ◽  
Magnetic Susceptibility Data ◽  
Susceptibility Data ◽  
Intermediate Valent ◽  
Cerium Atom

The magnesium-rich compound CeRu2Mg5 was synthesized by high-frequency melting of the elements in a sealed tantalum ampoule. CeRu2Mg5 crystallizes with a new tetragonal structure type: P42/ncm, a = 961.1(1), c = 723.2(1) pm, wR2 = 0.0284, 481 F2 values and 25 variables. The striking structural motifs in CeRu2Mg5 are short Ce-Ru distances of 232 pm. Each cerium atom is connected to two ruthenium atoms within a three-dimensional [Ru2Mg5] network. CeRu2Mg5 has a pronounced magnesium substructure with short Mg-Mg distances in the range 302 - 341 pm. The short Ce-Ru distances are a consequence of the almost tetravalent character of the cerium atoms. Temperature-dependent magnetic susceptibility data show intermediate-valent behavior of the cerium atoms (0.9(1) μB per formula unit) and no magnetic ordering down to 3 K.

scholarly journals Structure variations within RSi2 and R 2 TSi3 silicides. Part I. Structure overview

2020 ◽  
Vol 76 (2) ◽  
pp. 177-200 ◽  
Author(s):  
M. Nentwich ◽  
M. Zschornak ◽  
M. Sonntag ◽  
R. Gumeniuk ◽  
S. Gemming ◽  
...  
Keyword(s):  
Density Functional ◽  
Alkaline Earth ◽  
Structural Parameters ◽  
Earth Metal ◽  
Structure Type ◽  
Underlying Structure ◽  
Alkaline Earth Metal ◽  
Formula Unit ◽  
Functional Theory ◽  
Subgroup Scheme

Here, structural parameters of various structure reports on RSi2 and R 2 TSi3 compounds [where R is an alkaline earth metal, a rare earth metal (i.e. an element of the Sc group or a lathanide), or an actinide and T is a transition metal] are summarized. The parameters comprising composition, lattice parameters a and c, ratio c/a, formula unit per unit cell and structure type are tabulated. The relationships between the underlying structure types are presented within a group–subgroup scheme (Bärnighausen diagram). Additionally, unexpectedly missing compounds within the R 2 TSi3 compounds were examined with density functional theory and compounds that are promising candidates for synthesis are listed. Furthermore, a correlation was detected between the orthorhombic AlB2-like lattices of, for example, Ca2AgSi3 and the divalence of R and the monovalence of T. Finally, a potential tetragonal structure with ordered Si/T sites is proposed.

The Ce2Li0.39Ni1.61Si2structure as a new derivative of the AlB2family

2014 ◽  
Vol 70 (6) ◽  
pp. 622-626 ◽  
Author(s):  
Andrij Stetskiv ◽  
Beata Rozdzynska-Kielbik ◽  
Volodymyr Pavlyuk
Keyword(s):  
Intermetallic Compounds ◽  
Title Compound ◽  
Classification Scheme ◽  
Tight Binding ◽  
Structure Type ◽  
Vertex Polyhedron ◽  
Orbital Method ◽  
Crystal Chemical ◽  
Nickel Disilicide ◽  
Metallic Bonding

A new quaternary dicerium lithium/nickel disilicide, Ce2Li0.39Ni1.61Si2, crystallizes as a new structure type of intermetallic compounds closely related to the AlB2family. The crystal–chemical interrelationships between parent AlB2-type, BaLiSi, ZrBeSi and the title compound are discussed using the Bärnighausen formalism. Two Ce atoms occupy sites of 3m. symmetry. The remainder,i.e.Ni, mixed Ni/Li and Si atoms, occupy sites of \overline{6}m2 symmetry. The environment of the Ce atom is an 18-vertex polyhedron and the Ni, Ni/Li and Si atoms are enclosed in tricapped trigonal prisms. The title structure can be assigned to class No. 10 (trigonal prism and its derivatives) according to the Krypyakevich classification scheme [Krypyakevich (1977). InStructure Types of Intermetallic Compounds. Moscow: Nauka]. The electronic structure of the title compound was calculated using the tight-binding linear muffin-tin orbital method in the atomic spheres approximation (TB-LMTO-ASA). Metallic bonding is dominant in this compound. The strongest interactions are Ni—Si and Ce—Si.

Crystal Chemical Constraints on the Formation of Actinide Pyrochlores

1984 ◽  
Vol 44 ◽  
Author(s):  
Bryan C. Chakoumakos ◽  
Rodney C. Ewing
Keyword(s):  
Electron Diffraction ◽  
Pyrochlore Structure ◽  
Type A ◽  
Structure Type ◽  
Crystal Chemical ◽  
Waste Forms ◽  
Naturally Occurring

AbstractThe pyrochlore structure type, A1−2 B2 O6Y0−1 Fd3m Z=8, is a common constituent of polyphase, crystalline waste forms. Naturally occurring minerals with the same structure often occur in the radiation damaged, electron-diffraction amorphous, “metamict” state; and therefore, a principal concern with phases of this structure type is the effect of alpha-recoil damage associated with the presence of actinides. In this paper we outline crystal chemical constraints on the formation of novel actinide pyrochlores.

Higher hydrates of lithium chloride, lithium bromide and lithium iodide

2018 ◽  
Vol 74 (2) ◽  
pp. 194-202 ◽  
Author(s):  
Julia Sohr ◽  
Horst Schmidt ◽  
Wolfgang Voigt
Keyword(s):  
Hydrogen Bond ◽  
Crystal Structures ◽  
Lithium Chloride ◽  
Lithium Bromide ◽  
Structure Type ◽  
Lithium Cation ◽  
Formula Unit ◽  
Lithium Iodide ◽  
Hydrogen Bond Networks ◽  
Solid Liquid

For lithium halides, LiX (X = Cl, Br and I), hydrates with a water content of 1, 2, 3 and 5 moles of water per formula unit are known as phases in aqueous solid–liquid equilibria. The crystal structures of the monohydrates of LiCl and LiBr are known, but no crystal structures have been reported so far for the higher hydrates, apart from LiI·3H2O. In this study, the crystal structures of the di- and trihydrates of lithium chloride, lithium bromide and lithium iodide, and the pentahydrates of lithium chloride and lithium bromide have been determined. In each hydrate, the lithium cation is coordinated octahedrally. The dihydrates crystallize in the NaCl·2H2O or NaI·2H2O type structure. Surprisingly, in the tri- and pentahydrates of LiCl and LiBr, one water molecule per Li+ ion remains uncoordinated. For LiI·3H2O, the LiClO4·3H2O structure type was confirmed and the H-atom positions have been fixed. The hydrogen-bond networks in the various structures are discussed in detail. Contrary to the monohydrates, the structures of the higher hydrates show no disorder.

Sign in / Sign up

Export Citation Format

Share Document