Atoms In Sublattice Research Articles

Crystal Structure and Magnetic Properties of Cu‐Rich Ce—Mn—Cu Intermetallics.

AbstractFor Abstract see ChemInform Abstract in Full Text.

SYMMETRY PROPERTIES OF IMPURITIES IN METALLIC SINGLE-WALL CARBON NANOTUBES

We use the effective mass model to describe spinless electrons near the Fermi level in metallic, single-wall carbon nanotubes. Taking into account two nonequivalent valleys (K-points) produces a four component Dirac equation for massless fermions, with the role of spin assumed by pseudospin due to the relative amplitude of the wavefunction on the two nonequivalent sublattice atoms. We show that the position of a short-ranged impurity within the hexagonal graphite unit cell produces a particular 4×4 matrix structure of the corresponding effective Hamiltonian. The symmetry of this Hamiltonian with respect to pseudospin flip is related to degeneracy breaking and, for an armchair tube, symmetry with respect to mirror reflection in the nanotube axis is related to pseudospin mixing. In a nanotube of finite length, we predict a sinusoidal oscillation of energy level shift as a function of energy with a period determined by the position of the impurity along the tube axis.

Surface and bulk electronic structure ofScN(001)investigated by scanning tunneling microscopy/spectroscopy and optical absorption spectroscopy

$\mathrm{ScN}(001)$ $1\ifmmode\times\else\texttimes\fi{}1$ surfaces have been prepared by growing $\mathrm{ScN}$ on $\mathrm{MgO}(001)$ using radio frequency molecular beam epitaxy. In situ ultrahigh vacuum scanning tunneling spectroscopy indicates that the Fermi level at the surface lies slightly above the $\mathrm{Sc}\phantom{\rule{0.3em}{0ex}}3d$ conduction band edge, which is attributed to a downward band bending at the surface. In situ scanning tunneling microscopy is used to image the $\mathrm{Sc}$ and $\mathrm{N}$ atom sublattices. While only one atom $(\mathrm{Sc})$ appears at small negative bias, both atoms ($\mathrm{Sc}$ and $\mathrm{N}$) appear at certain positive sample biases due to the partially ionic nature of the bonding. Charge accumulation near ionized subsurface donors is evident from the long-range topographic distortions at the surface. The combination of tunneling spectroscopy and optical absorption results show that $\mathrm{ScN}$ has an indirect bandgap of $0.9\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and a direct transition at $2.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Crystal structure and magnetic properties of Cu-rich Ce–Mn–Cu intermetallics

A study on the ternary intermetallic phases in the Cu-rich region of Ce–Mn–Cu system has been performed. Three compounds were identified and their transport and magnetic properties determined. The crystal structures were investigated by X-ray diffraction both on powders and single crystals: CeMn0.85Cu5.15, related to the CeCu5Au-type (oP28, Pnma, a=8.246(1)Å, b=5.0870(4)Å, c=10.346(1)Å); CeMn1.3Cu4.7, related to the YbMo2Al4 type (tI14, I4/mmm, a=6.5763(6)Å, c=5.0488(4)Å) and CeMn2.25Cu8.75, related to the CeMn6Ni5 type (tP24, P4/mbm, a=8.394(1)Å, c=4.973(1)Å). In all the three compounds the Mn atoms preferentially occupy sites, available on the non-rare earth atom sublattice, with higher coordination and larger interatomic distances. The Mn ions order ferromagnetically near 50K in the first compound, while the resistivity exhibits Kondo like behaviour. There is no clear evidence of a bulk magnetic phase transition below 300K in the latter two compounds. The heat capacity data indicate a strongly correlated electronic ground state in all the three alloys due to the Ce-4f hybridization with the conduction band states.

Crystal structure and magnetic properties of Cu-rich Ce?Mn?Cu intermetallics

A study on the ternary intermetallic phases in the Cu-rich region of Ce–Mn–Cu system has been performed. Three compounds were identified and their transport and magnetic properties determined. The crystal structures were investigated by X-ray diffraction both on powders and single crystals: CeMn 0.85 Cu 5.15 , related to the CeCu 5 Au-type (oP28, Pnma, a =8.246(1) Å, b =5.0870(4) Å, c =10.346(1) Å); CeMn 1.3 Cu 4.7 , related to the YbMo 2 Al 4 type (tI14, I4/mmm, a =6.5763(6) Å, c =5.0488(4) Å) and CeMn 2.25 Cu 8.75 , related to the CeMn 6 Ni 5 type (tP24, P4/mbm, a =8.394(1) Å, c =4.973(1) Å). In all the three compounds the Mn atoms preferentially occupy sites, available on the non-rare earth atom sublattice, with higher coordination and larger interatomic distances. The Mn ions order ferromagnetically near 50 K in the first compound, while the resistivity exhibits Kondo like behaviour. There is no clear evidence of a bulk magnetic phase transition below 300 K in the latter two compounds. The heat capacity data indicate a strongly correlated electronic ground state in all the three alloys due to the Ce-4f hybridization with the conduction band states.

Radicals whose semisimple classes satisfy a generalised ADS condition

A class K of rings has the GADS property (i.e., generalized ADS property) if wheneverXl Il R with X∈ K, then there exists B l R with B ∈ K such that X ⊆ B ⊆ I. Radicals whose semisimple classes have the GADS property are called g-radicals. In this paper, we fully characterize the class of g -radicals. We show that ? is a g-radical if and only if either ? ⊆ I or S? ⊆ I , whereI denotes the class of idempotent rings and S? denotes the semisimple class of ?. It is also shown that the (hereditary) g-radicals form an (atomic) sublattice of the lattice of all radicals.

Ferroelectric and ferroelastic phase states of crystals caused by atomic ordering

The effect of atomic ordering on displacive-type structural phase transitions is considered for crystals with several atomic sublattices. On the basis of group-theoretical analysis and general crystallographic and thermodynamic principles, it is shown that ordering gives rise to qualitative restructuring of phase diagrams and changes in atomic states up to the formation of new ferroelectric and ferroelastic phases proper. Specific physical properties of crystals provided by atomic ordering are considered in the vicinity of structural transitions within the framework of the phenomenological theory of phase transitions.

Chemical Bonding in Thermodynamically Labile Oxyanion Crystals

Crystal valence density and difference density distributions were calculated for NaNO2, NaNO3, KClO3, and NaClO4 using local density functional theory. Hybridization of the oxygen states is enhanced in compounds with lower symmetry, leading to the formation of oxygen chains in the lattice. Interaction between the oxygen and anion's central atom sublattices gives rise to electron charge maxima in the middle of the A—O bonds and to two terminal oxygen maxima. The complex character of the intraanion electron transfer and the charge transfer between the nonequivalent oxygen sublattices make these compounds susceptible to external energy effects that lead to their solid‐state decomposition.

Search for structure-forming components in molecular crystals of binary compounds: a topological approach

Molecular crystal structures of 81 polymorphs of binary compounds are investigated with the method of coordination sequences and with the uniformity criterion for atomic sublattices. It is shown that using these tools one can estimate semi-quantitatively the relative force of intermolecular interactions and to find so-called structure-forming lattice, i.e. the atomic sublattice, where the most stable long-wavelength phonons are likely to spread. When a crystal consists of molecules with distinguished central atoms, the polarizability of surrounding atoms is found to dictate mainly the composition of the structure-forming lattice. If the polarizability is small, crystal structure is formed by molecules as a whole, otherwise surrounding atoms form the structure basis themselves, and it is their lattice that often is one of the close packings. It is shown that if the central atom of a molecule has stereoactive lone pair the central atom can form the structure-forming lattice together with surrounding atoms.

Linear approximations for determining atomic sublattice occupancies by ALCHEMI

Linear approximations for determining atomic sublattice occupancies by ALCHEMI

Search for structure-forming components in molecular crystals of binary compounds: a topological approach

Abstract Molecular crystal structures of 81 polymorphs of binary compounds are investigated with the method of coordination sequences and with the uniformity criterion for atomic sublattices. It is shown that using these tools one can estimate semi-quantitatively the relative force of inter-molecular interactions and to find so-called structure-forming lattice, i.e. the atomic sublattice, where the most stable long-wavelength phonons are likely to spread. When a crystal consists of molecules with distinguished central atoms, the polarizability of surrounding atoms is found to dictate mainly the composition of the structure-forming lattice. If the polarizability is small, crystal structure is formed by molecules as a whole, otherwise surrounding atoms form the structure basis themselves, and it is their lattice that often is one of the close packings. It is shown that if the central atom of a molecule has stereoactive lone pair the central atom can form the structure-forming lattice together with surrounding atoms.

Diffusion in nickel-based intermetallic compounds taking the L1 2 structure

In this paper, we review the present status of understanding of diffusion in the three compounds Ni 3Al, Ni 3Ge and Ni 3Ga. We discuss the six-jump-cycle mechanism, vacancy pair mechanism, intersublattice mechanism, majority atom sublattice mechanism, and the mixed inter/intrasublattice mechanisms, in the light of the available tracer diffusion data. We also present Monte Carlo results for the computer diffusion coefficients using available migration and defect formation energies in Ni 3Al. We show that overall the data are consistent with the majority atom species diffusing largely on its own sublattice with the minority atom species probably diffusing rapidly on the majority atom sublattice as antistructural atoms.

Long-Range Effects in the FePd2Au Alloy Under Ion Bombardment

The effect is studied of ion bombardment (Ar+, E = 20 keV, j = 100 μA/cm2, F = 5 × 1016 to 1018 cm−2) and thermal annealing on the atomic and magnetic structure of the FePd2Au alloy after 80% cold plastic deformation and quenching from 1200°C. It is established by the Mössbauer effect and X-ray diffraction that ion irradiation at 350°C (for 1.5 to 30 min) causes formation in the disordered face-centered-cubic matrix of a long-range atomic order (of an Fe atom sublattice at an anomalously large depth up to 20 μm, at an ion projected range of ~13 nm) accompanied by ferromagnetic to asperomagnetic phase transition (Tmeas = 77 K). Annealing at T = 350°C up to 30 min in the absence of irradiation does not result in any noticeable changes in the atomic and magnetic structure. Atom mobility (the ordered structure formation rate) in the course of irradiation at 350°C is approximately the same as observed in the case of annealing at 700°C.

Single-ion anisotropy of localized-electron compounds

We present a computational analysis of the influence of taking the full multiplet structure into account in calculations of the single-ion contribution to the magnetocrystalline anisotropy (MCA) of $3d$ transition-metal (TM) ions in localized-electron compounds at $T=0$ K. For atoms with ${d}^{n}$ $(n=1--4,6--9)$ configurations, at sites with a local cubic, tetragonal, or trigonal symmetry the single ion MCA on the basis of the Hund's rule ground state term only [crystal field (CF) theory] is compared with the single-ion MCA on the basis of a fully relativistic first principles atomic theory including the intra-atomic $d\ensuremath{-}d$ interaction. Solid state effects are taken into account by effective crystal fields and the exchange field. Under certain, realistic, conditions the use of the full multiplet theory is shown to have a significant effect on the calculated single-ion MCA. We also discuss the effect on the overall cubic anisotropy constants for cubic crystals containing transition metal atom sublattices for which the $d\ensuremath{-}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}$ atoms are located on sites with a local tetragonal and trigonal symmetry. Possible refinements of the theory are discussed.

The Role of Sublattices in Chemical Bonding in Essentially Ionic Crystals

A technique for calculating the self-consistent electron density of sublattice and crystal as a whole has been developed based on the local electron density functional theory (DFT). Difference density, obtained by subtraction of the densities of the individual sublattices from the total electron density, is introduced as a quantity defining the relationship between the sublattices. Self-consistent calculations have been performed for series of compounds with a NaCl lattice: alkali and noble metal halides and alkali metal oxides and sulfides with an antifluorite lattice. As a result of these calculations, we formulated the key principle of chemical bonding based on sublattices. In the anion sublattice, alike atoms form a covalent type of bond with each other by transferring some portion of the charge to the interstice. The same mechanism of bonding is realized in sublattices of metal atoms containing the levels of the d-states in the valence region. In sublattices of simple metals, the valence density behaves as “metallic,” changing slightly within the unit cell, with electrons again moving from the metal sites into the interstice. Addition of the valence densities of sublattices leads to two essentially different situations: in one case, the metal sites coincide with the density maxima at the interstice of the anion sublattice, and in the other, they do not. In the first case, the superposition of the sublattice densities at the lattice sites leads to anion-cation hybridization, forming an essentially ionic chemical bond with a possible admixture of the covalent bond. In the second case, the anion sublattice is a covalently bonded framework incorporating the metal sublattice. This takes place in alkali metal oxides and sulfides.

Synthesis and Crystal Structure of trans-Rb2[Pd(NO2)2Cl2]. Structure Refinement of Na2[Pd(NO2)4

The reaction products of palladium(II) nitration with sodium nitrite in the presence of rubidium salts are described. The crystal structure of rubidium dinitrodichloropalladoate(II) — trans-Rb2[Pd(NO2)2Cl2] — has been studied (KUMA automatic diffractometer, MoKα radiation, graphite monochromator, θ/2θ scan mode for 2θ 2σ(I)). Crystal data: a = 15.123(3), b = 7.750(2), c = 7.776(2) A, V = 911.4(4) A3, space group Cmca, Z = 4, dcalc = 3.209 g/cm3. The structure consists of [Pd(NO2)2Cl2]2– complex anions and Rb+ cations. The planar square coordination of the Pd atom is completed to distorted octahedral by the chlorine atoms of the neighboring complexes at Pd...Cl distances of 3.18 A. Chains of complexes with alternating orientations are formed. The geometrical characteristics of the complex anions, the coordination polyhedra of the cations, and the heavy atom sublattice are analyzed. The crystal structure of Na2[Pd(NO2)4] has been refined, and refinement data are given.

Geometrical laws in inorganic chemistry

On the basis of an original interpretation of the crystal state, it is shown that most of the structural types are determined by a small set of planar nets densely filled with the heaviest atoms of the structure. These nets are combined into atomic sublattices (matrices of the structure types) by the operations of (pseudo) symmetry, thus reducing the number of the degrees of freedom in the structure, which, in turn, provides structure stability. It is shown on examples of heavy-cation oxides, fluorides, and sulfides that the number of the types of such matrices (F-, I-, etc.) is limited. Each type is characterized by a certain structure-determining complex of mutually intersecting crystallographic planes parallel to the most densely filled atomic nets. Most of the atomic nets considered in crystal chemistry are, in fact, various sections of the sublattices-matrices formed by a limited number of densely filled atomic nets.

Effect of Stoichiometric Deviations on the Structural Properties of CuInSe2, CuIn0.5Ga0.5Se2 and CuGaSe2.

Ingots of CuInSe2, CuIn0.5Ga0.5Se2 and CuGaSe2 were synthesized under similar fusion at 1150°C in silisa ampoule, cut into slices and determined their composition by energy dispersive analysis of X-Rays. The structural properties, including the occupation fraction of each atomic sublattice, were studied by X-ray powder diffraction and Rietveld refinement. The Cu occupation number, N(Cu), is lower than one, The complex of two Cu vacancies and a substitutional catio, 2V(Cu)+B3+Cu (B3+Cu= In, Ga+In and Ga in CIS, CIGS and CGS respectively in a Cu site) is assumed and would explain the presence of Cu vacancies and allow an estimate of the B3+ concentration in its sublattice. When this increases a decrease in concentration of both Cu in its sublattice and the complex 2V(Cu)+B3+Cu are found. The strucutrual parameters depend on Cu and B3+ sublattice occupancy fractions for all compounds..In CuGaSe2, the lattice parameters increase as does Ga3+(subl) while the opposite arrives in CuInSe2 for In3+(subl). The distance of Se atom to z axis depends on Cu(subl) for all the compounds increasing in the case of CuInSe2 with V(Cu) and InCu antisites, decreasing the InSe bond distance. The opposite effect is observed in CuIn0.5Ga0.5Se2, and CuGaSe2.

Microstructural changes observed in a rapidly solidified Ni 3Al alloy during isothermal treatments

A series of metastable conditions arises in Ni 3Al alloys out of their rapid solidification, the most important one being the formation of a bimodal distribution of ordered domain sizes. The fine ordered domains are surrounded by elongated ones. Another characteristic is that the number of vacancies traps is higher than the equilibrium one. In this study, several tapes obtained by melt-spinning a Ni 3Al alloy with 76.5 at% Ni were annealed at 500°C for several periods from 0.5 to 120 h. These tapes were studied by means of transmission electron microscopy and X-ray diffraction. The latter technique allowed us to detect that the degree of ordering decreases in the first stages of the treatment; this occurs due to the formation of prismatic dislocation loops as a result of vacancy precipitation, this behavior is also due to the initial high long range order. Thus, the beginning of atomic movement causes the “right” sub-lattice atoms to occupy “wrong” sub-lattice positions. The structure recovers at the end of the process, followed by an increase in the degree of ordering. The vacancy precipitation was found to be heterogeneous; the region with higher density of fine ordered domains provides more sites to prismatic defects than the one with coarse domains.

Raman Scattering Studies of the Carbon Atom Vibrations in Superconducting Rare-Earth Carbide Halides RE2C2(X,X′)2 (RE = Y, Gd; X, X′ = Br, I)

The Raman spectra of the layered superconductors RE2C2(X,X′)2 (RE = Y, Gd; X, X′ = Br, I) were measured on single-crystal and polycrystalline specimens using a micro-Raman technique. Low energy modes (ω < 300 cm—1) correspond to mixed vibrations of rare-earth and halogen atom sublattices. Two high energy modes at ≈︂ 400 and ≈︂ 1600 cmm1) can be assigned to the tilting and stretching vibrations of the carbon C2 units. These carbon-related phonons were identified by analyzing the isotope shifts of the Raman lines in samples in which the natural carbon was replaced by isotope-enriched 13C. The energy of the stretching vibration is independent of the particular host compound while the energy of the tilting vibration correlates with the size of the rare-earth atom cage that surrounds the carbon atoms.