Julia Dshemuchadse

Gram-scale synthesis of two-dimensional polymer crystals and their structure analysis by X-ray diffraction

The rise of graphene, a natural two-dimensional polymer (2DP) with topologically planar repeat units, has challenged synthetic chemistry, and has highlighted that accessing equivalent covalently bonded sheet-like macromolecules has, until recently, not been achieved. Here we show that non-centrosymmetric, enantiomorphic single crystals of a simple-to-make monomer can be photochemically converted into chiral 2DP crystals and cleanly reversed back to the monomer. X-ray diffraction established unequivocal structural proof for this synthetic 2DP, which has an all-carbon scaffold and can be synthesized on the gram scale. The monomer crystals are highly robust, can be easily grown to sizes greater than 1 mm and the resulting 2DP crystals exfoliated into nanometre-thin sheets. This unique combination of features suggests that these 2DPs could find use in membranes and nonlinear optics.

Large, larger, largest ‒ a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

This is the first of two parts, where we report the structure determination of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A(3) and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. These cluster-based structures are closely related to the class of Frank-Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase.

Structural building principles of complex face-centered cubic intermetallics

Fundamental structural building principles are discussed for all 56 known intermetallic phases with approximately 400 or more atoms per unit cell and space-group symmetry F43m, Fd3m, Fd3, Fm3m or Fm3c. Despite fundamental differences in chemical composition, bonding and electronic band structure, their complex crystal structures show striking similarities indicating common building principles. We demonstrate that the structure-determining elements are flat and puckered atomic {110} layers stacked with periodicities 2p. The atoms on this set of layers, which intersect each other, form pentagon face-sharing endohedral fullerene-like clusters arranged in a face-centered cubic packing (f.c.c.). Due to their topological layer structure, all these crystal structures can be described as (p × p × p) = p(3)-fold superstructures of a common basic structure of the double-diamond type. The parameter p, with p = 3, 4, 7 or 11, is determined by the number of layers per repeat unit and the type of cluster packing, which in turn are controlled by chemical composition.

Intermetallics : structures, properties, and statistics

Intermetallics are solid-state compounds exhibiting metallic bonding, defined stoichiometry and ordered crystal structure. It is now a century since it was recognized in an early book (Giua & Lollini Giua, 1918), written by Michele Giua, Professor of Industrial Chemistry at Turin University together with his wife Clara Lollini Giua, that numerous compounds could form. Many more were discovered in the following decades. A few gained increasing industrial interest, e.g. aluminides, but it was only in 1993 that a dedicated journal, Intermetallics, was founded by the late Robert Cahn. Now, here comes the book Intermetallics with subtitle Structures, Properties and Statistics by Walter Steurer and Julia Dshemuchadse which sets a new paradigm in the topic by analyzing the structure and classifying the tens of thousands intermetallics known to date. The book is divided in two parts: Concepts and Statistics are the content of Part I and Structure and Properties of Part II. The text is full of information filling more than 500 pages. It has an extensive literature section as well an index of chemical formulae. A useful list of abbreviations and a glossary are provided. The linguistic approach is rigorous in terminology and in making reference to theories, methods and rules. Chapter 1 gives the basic terminology concerning symmetry, lattices, atomic environment types. It is clear and well organized. I would suggest it to students of materials science courses to learn definitions properly. The second chapter summarizes the factors governing structure and stability of intermetallics with emphasis mainly on quantum chemistry. The quantum chemistry methods employed in the literature are mentioned with a short description of up to one page. For the reader not experienced in this topic, this section is of limited usefulness considering also the absence of illustrations. It is understood a more lengthy treatment would have diverted the text from its main objectives. The authors, however, direct the reader to the relevant literature for all methods. Being perhaps biased by my thermodynamic background, I felt the stability issue could have been tackled also by mentioning the methods employed to evaluate the Gibbs free energy of intermetallics, especially because the calculation of this quantity is an expanding topic for those performing phase diagram calculations including the calculation of the enthalpy of formation from first principles. The description of tilings in Chapter 3 is detailed, though concise, with good examples and images. This represents the basis for building up the structure of complex intermetallics through an accurate description of polyhedra and packings. The next step is the treatment of n-dimensional spaces to represent the structure of both complex periodic and aperiodic compounds in Chapter 4. The following Chapter 5, is the most innovative one dealing with a statistical analysis of the occurrence of intermetallics in binary (the largest number), ternary and higher order systems. This is a striking amount of work carried out with absolute competence and corroborated with several examples of structure types. The structure of compounds is classified according to the Pettifor chemical scale, which is used extensively. Apparently, the Pettifor scale is successful in indicating the zones of the plot of the Mendeleev number of constituents where structures can be found. This can be considered as an indication for predicting new compounds, although not explicitly stated in the text. I have a problem with the readability of such plots which are necessarily ISSN 2052-5206

Some Statistics on Intermetallic Compounds

It is still largely unknown why intermetallic phases show such a large variety of crystal structures, with unit cell sizes varying between 1 and more than 20 000 atoms. The goal of our study was, therefore, to get a general overview of the symmetries, unit cell sizes, stoichiometries, most frequent structure types, and their stability fields based on the Mendeleev numbers as ordering parameters. A total of 20829 structures crystallizing in 2166 structure types have been studied for this purpose. Thereby, the focus was on a subset of 6441 binary intermetallic compounds, which crystallize in 943 structure types.

More statistics on intermetallic compounds - ternary phases

How many different intermetallic compounds are known so far, and in how many different structure types do they crystallize? What are their chemical compositions, the most abundant ones and the rarest ones? These are some of the questions we are trying to find answers for in our statistical analysis of the structures of the 20,829 intermetallic phases included in the database Pearsons Crystal Data, with the goal of gaining insight into some of their ordering principles. In the present paper, we focus on the subset of 13,026 ternary intermetallics, which crystallize in 1391 different structure types; remarkably, 667 of them have just one representative. What makes these 667 structures so unique that they are not adopted by any other of the known intermetallic compounds? Notably, ternary compounds are known in only 5109 of the 85,320 theoretically possible ternary intermetallic systems so far. In order to get an overview of their chemical compositions we use structure maps with Mendeleev numbers as ordering parameters.

A new complex intermetallic phase in the system Al–Cu–Ta with familiar clusters and packing principles

The structure of hP386-Al(57.4)Cu(3.6)Ta(39.0) was determined by single-crystal X-ray diffraction analysis. It can be described as a hexagonal close-packing of two types of endohedral fullerene-like clusters with different Frank-Kasper polyhedra filling the gaps. The description of the structure as a superstructure and as a layered structure illustrates other characteristic structural building principles. The diffuse scattering, which can be observed in some of the crystals, is qualitatively well reproduced by a disorder model. A comparison with the structures of the other complex intermetallics in the system Al-Cu-Ta indicates the decisive role that Cu plays in the constitution and packing of the clusters.

Real space crystallography of a complex metallic alloy: high-angle annular dark-field scanning transmission electron microscopy of o-Al4(Cr,Fe)

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) has been performed along the low-index zone axes of the o-Al4(Cr,Fe) complex metallic alloy to obtain a real-space representation of the crystal structure and to elucidate the materials inherent structural disorder. By comparing experiments with multislice STEM simulations, the model previously suggested by X-ray diffraction is further refined to provide a new set of positions and occupancies for the transition metal atoms. Pmnb is suggested as the new space group for the o-Al4(Cr,Fe) phase. A nonperiodic layer-type modulation, averaged out in bulk diffraction methods, is detected, corroborating the need for complementing bulk diffraction analysis with real-space imaging to derive the true crystal structure of Al4(Cr,Fe).

Simple particles, complex structures

Quasicrystalline structures, and complex compounds in general, have been found in a wide range of intermetallic systems over the last half-century. Their occurrence and stability is most commonly attributed to chemical and electronic factors and the systems that compose them are viewed with a focus on composition and stoichiometry. Recent studies, however, have shown that the same complex structures are found in soft-matter systems, either on vastly different length scales in experimental investigations of colloids or nanoparticles or in computational work that can agnostically explore phase space with almost unlimited flexibility.

Real space crystallography of a complex metallic alloy

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) has been performed along the low-index zone axes of the o-Al4(Cr,Fe) complex metallic alloy to obtain a real-space representation of the crystal structure and to elucidate the materials inherent structural disorder. By comparing experiments with multislice STEM simulations, the model previously suggested by X-ray diffraction is further refined to provide a new set of positions and occupancies for the transition metal atoms. Pmnb is suggested as the new space group for the o-Al4(Cr,Fe) phase. A nonperiodic layer-type modulation, averaged out in bulk diffraction methods, is detected, corroborating the need for complementing bulk diffraction analysis with real-space imaging to derive the true crystal structure of Al4(Cr,Fe).