Syntheses and structures of new phases AeMxIn(4-x) (Ae = Sr, Ba; M = Mg, Zn): size effects and site preferences in BaAl4-type structures

The title phases were synthesized via high-temperature solid-state methods and structurally characterized by single-crystal X-ray diffraction. The phase widths of both SrMg(x)In(4-x) (0.85 相似文献   

KBi(2-x)Pbx (0 < x <or= 1): a Zintl phase evolving from a distortion of the cubic Laves-phase structure

The quasibinary system KBi(2-x)Pbx has been investigated, both experimentally and theoretically. Phases with compositions 0 < or = x < or = 1.2 were synthesized and structurally characterized by X-ray diffraction experiments. For low values of x (0 < or = x < 0.6), KBi(2-x)Pbx adopts the cubic Laves-phase structure MgCu2 (space group Fdm), which contains a rigid framework of corner-condensed symmetry-equivalent tetrahedra formed by randomly distributed Bi and Pb atoms. For compositions x > or = 0.6, these tetrahedra become alternately elongated and contracted. The distortion of the framework lowers the space-group symmetry to F43m (KBi(1.2)Pb(0.8), F43m, Z = 8, a = 9.572(1) A). Magnetometer measurements show that KBi2 (x = 0) is metallic and goes through a superconducting transition below 3.5 K. First principles calculations reveal that the Fd3m --> F43m distortion is largest for KBiPb (x = 1.0), which at the same time turns into a semiconductor. Thus, F43m KBiPb corresponds to a proper charge-balanced Zintl phase, K+[BiPb]-, with separated polyanionic tetrahedra, (Bi2Pb2)2-. However, it was not possible to prepare F43m KBiPb. Syntheses attempting to increase the Pb content in KBi(2-x)Pbx above x = 0.8 yielded additional, not yet characterized, ternary phases.     

Evolution of cobalt spin states and magnetic coupling along the SrCo(1-x)Sb(x)O(3-δ) system: correlation with the crystal structure

The oxides of the SrCo(1-x)Sb(x)O(3-δ) perovskite family have been recently designed, characterized and described as cathode materials for solid-oxide fuel cells with competitive power performance in the temperature range 750-850 °C. They feature a number of interesting properties including a good electronic conductivity, low electrode polarization resistance and adequate thermal expansion; the crystal structure adopts a 3C corner-linked perovskite network with a considerable number of oxygen vacancies. This paper reports on the effects of Sb-doping on the crystal structure features, the Co oxidation state and magnetic properties related to the presence of spin-state transitions in the Co cations. A phase transition was observed from the tetragonal P4/mmm space group for x≤ 0.15 to the cubic Pm ?3m space group in the x = 0.2 composition from neutron powder diffraction data. For the tetragonal phases the oxygen vacancies were found to be ordered and localized in the axial O2 and equatorial O3 atoms surrounding the Co2 positions. A noticeable distortion of CoO(6) octahedra is observed for x = 0.05 and 0.1, exhibiting a charge-ordering with a mixed oxidation state of Co(3+/4+) at Co1 sites and Co(3+) at Co2 positions: the Jahn-Teller Co(3+) in an intermediate-spin configuration is responsible for the octahedral distortion. Increasing Sb contents promotes a higher average oxidation state of cobalt, from a valence of 3.2+ for x = 0.05 to 3.4+ for x = 0.2, inducing a decrease of the oxygen vacancies and favouring a random distribution over a Pm ?3m cubic symmetry. All the samples present an antiferromagnetic behaviour with a G-type (k = 0) magnetic structure. The increase of the Sb content induces the weakening of the crystal field (Δ(cf)) in the octahedral environment promoting the Co spin-transition from the intermediate-spin to the high-spin configuration, as evidenced by the decrease of the octahedral distortion, increment of the unit-cell volume and enhancement of the ordered magnetic moment.     

Correlation of crystal structure, dielectric properties and lattice vibration spectra of (Ba(1-x)Sr(x))(Zn(1/3)Nb(2/3))O3 solid solutions

(Ba(1-x)Sr(x))(Zn(1/3)Nb(2/3))O(3) (BSZN) (x = 0.0, 0.50, 0.60, 0.65, 0.70, 1.0) solid solutions were synthesized by a conventional solid-state sintering technique. Vibration spectra (Raman spectroscopy and Fourier transform far-infrared reflection spectroscopy, FTIR) and X-ray diffraction (XRD) were employed to evaluate the crystal structures and phonon modes of these solid solutions. Dielectric constants (ε(r)) and temperature coefficient of capacitances (τ(c)) were examined to reveal the correlation of the dielectric properties and the crystal structures. The results show that with the increase in Sr(2+) content, the lattice structures of ceramics turn gradually from disordered cubic structure to ordered structure because antiphase tilting of the oxygen octahedra occurs where x≥ 0.65, which is the main reason for the phase transitions and variation of crystal structure. The appearance of the phase transitions is associated with variation of the symmetry structure, from cubic (Pm ?3m, where x = 0) to pseudocubic (I4/mcm, where 0.65 ≤x < 1.0) and then to hexagonal (P ?3ml, where x = 1.0). New phonon modes appear at around 250 cm(-1) in Raman spectra where x≥ 0.65, and there is also a different phonon mode appearing at 156 cm(-1) in the FTIR spectra at the same x range. The appearance of the new phonon modes is the characteristic of ceramics whose oxygen octahedra have tilted with Sr(2+) concentration where x≥ 0.65. The Raman shifts are related to the rigidity of the oxygen octahedra, while the widths of peaks are correlated with τ(c). The FTIR spectra were subjected to the Kramers-Kronig analysis, and the imaginary part of the dielectric constant was analyzed in detail.     

SrAu4In4 and Sr4Au9In13: polar intermetallic structures with cations in augmented hexagonal prismatic environments

The title compounds were synthesized via high-temperature reactions of the elements in welded Ta tubes and characterized by single-crystal X-ray diffraction analyses and band structure calculations. SrAu(3.76(2))In(4.24) crystallizes in the YCo5In3 structure type with two of eight network sites occupied by mixtures of Au and In: Pnma, Z = 4, a = 13.946(7), b = 4.458(2), c = 12.921(6) A. Its phase breadth appears to be small. Sr4Au9In 13 exhibits a new structure type, P_6 m2, Z = 1, a = 12.701(2), c = 4.4350(9) A. The Sr atoms in both compounds center hexagonal prisms of nominally alternating In and Au atoms and also have nine augmenting (outer) Au + In atoms around their waists so as to define 21-vertex Sr@Au9M4In8 (M = Au/In) and Sr@Au9In12 polyhedra, respectively. The relatively larger Sr content in the second phase also leads to condensation of some of the ideal building units into trefoil-like cages with edge-shared six-member rings. One overall driving force for the formation of these structures can be viewed as the need for each Sr cation to have as many close neighbors as possible in the more anionic Au-In network. The results also depend on the cation size as well as on the flexibility of the anionic network and an efficient intercluster condensation mode as all clusters are shared. Band structure calculations (LMTO-ASA) emphasize the greater strengths (overlap populations) of the Au-In bonds and confirm expectations that both compounds are metallic.     

Phase evolution, phase transition, raman spectra, infrared spectra, and microwave dielectric properties of low temperature firing (K(0.5x)Bi(1-0.5x))(Mo(x)V(1-x))O4 ceramics with scheelite related structure

In the present work, the (K(0.5x)Bi(1-0.5x))(Mo(x)V(1-x))O(4) ceramics (0≤x ≤ 1.00) were prepared via the solid state reaction method and sintered at temperatures below 830 °C. At room temperature, the BiVO(4) scheelite monoclinic solid solution was formed in ceramic samples with x < 0.10. When x lies between 0.1-0.19, a BiVO(4) scheelite tetragonal phase was formed. The phase transition from scheelite monoclinic to scheelite tetragonal phase is a continuous, second order ferroelastic transition. High temperature X-ray diffraction results showed that this phase transition can also be induced at high temperatures about 62 °C for x = 0.09 sample, and has a monoclinic phase at room temperature. Two scheelite tetragonal phases, one being a BiVO(4) type and the other phase is a (K,Bi)(1/2)MoO(4) type, coexist in the compositional range 0.19 < x < 0.82. A pure (K,Bi)(1/2)MoO(4) tetragonal type solid solution can be obtained in the range 0.82 ≤ x ≤ 0.85. Between 0.88 ≤ x ≤ 1.0, a (K,Bi)(1/2)MoO(4) monoclinic solid solution region was observed. Excellent microwave dielectric performance with a relative dielectric permittivity around 78 and Qf value above 7800 GHz were achieved in ceramic samples near the ferroelastic phase boundary (at x = 0.09 and 0.10).     

Tetracyanoborate salts M[b(CN)4] with M = singly charged cations: properties and structures

A series of tetracyanoborate salts M[B(CN)4] with the singly charged cations of Li+, Na+, Rb+, Cs+, [NH4]+, Tl+, and Cu+ as well as the THF solvate tetracyanoborates Na[B(CN)4] x THF and [NH4][B(CN)4] x THF were synthesized and their X-ray structures, vibrational spectra, solubilities in water, and thermal stabilities determined and compared with already known M[B(CN)4] salts. Crystallographic data for these compounds are as follows: Na[B(CN)4], cubic, Fd3m, a = 11.680(1) A, Z= 8; Li[B(CN)4], cubic, P43m, a = 5.4815(1) A, Z= 1; Cu[B(CN)4], cubic, P43m, a = 5.4314(7) A, Z= 1; Rb[B(CN)4], tetragonal, /4(1)/a, a = 7.1354(2) A, c= 14.8197(6) A, Z= 4; Cs[B(CN)4], tetragonal, /4(1)/a, a = 7.300(2) A, c = 15.340(5) A, Z= 4; [NH4][B(CN)4], tetragonal, /4(1)/a, a = 7.132(1) A, c = 14.745(4) A, Z= 4; Tl[B(CN)4], tetragonal, /4(1)/a, a = 7.0655(2) A, c = 14.6791(4) A, Z= 4; Na[B(CN)4] x THF, orthorhombic, Pnma, a = 13.908(3) A, b = 9.288(1) A, c = 8.738(1) A, Z= 4; [NH4][B(CN)4] x THF, orthorhombic, Pnma, a = 8.831(1) A, b = 9.366(2) A, c = 15.061(3) A, Z= 4. The cubic Li+, Na+, and Cu+ salts crystallize in a structure consisting of two interpenetrating independent tetrahedral networks of M cations and [B(CN)4]- ions. The compounds with the larger countercations (Rb+, Cs+, Tl+, and [NH4]+) crystallize as tetragonal, also with a network arrangement. The sodium and ammonium salts with the cocrystallized THF molecules are both orthorhombic but are not isostructural. In the vibrational spectra the two CN stretching modes A1 and T2 coincide in general and the band positions are a measure for the strength of the interionic interaction. An interesting feature in the Raman spectrum of the copper salt is the first appearance of two CN stretching modes.     

On the influence of the vacancy distribution on the structure and ionic conductivity of A-site-deficient Li(x)Sr(x)La(2/3-x)TiO3 perovskites

The crystal structure and dielectric properties of slowly cooled A-site-deficient perovskites Li(x)Sr(x)La(2/3-x)□(1/3-x)TiO(3) (0.04 ≤ x ≤ 0.33) have been investigated by powder X-ray diffraction (XRD), impedance spectroscopy, and (7)Li NMR techniques. In this series, nominal vacancies decrease with Li content, but the total amount of A-site vacancies, n(t) = Li + □, participating in conduction processes remains basically constant. Rietveld analysis of the XRD patterns showed a change of symmetry from orthorhombic to tetragonal when the lithium and strontium contents increased above x = 0.08 and from tetragonal to cubic above x = 0.16. Structural modifications are mainly due to the cation vacancy ordering along the c axis, which disappear gradually when the lithium content increases. In agreement with the structural information, two lithium signals with different quadrupole constants are detected in (7)Li NMR spectra of orthorhombic/tetragonal phases, which have been associated with lithium in two crystallographic z/c = 0 and 1/2 planes of perovskites. In cubic samples, only a single narrow component, indicative of mobile species, was detected. Lithium motion was thermally activated, with activation energies going from 0.35 to 0.38 eV. Evolution of the bulk dc-conductivity preexponential factors along the series showed a maximum that has been first related to the dependence of lithium hopping on the lithium and vacancy concentrations. Finally, changes in the vacancy ordering, produced along the series, affect the dimensionality of the conductivity, indicating that not only the amount of vacancies but also its distribution are relevant.     

Single crystal X-ray structure study of the Li(2-x)Na(x)Ni[PO4]F system

The new compounds Li(2-x)Na(x)Ni[PO(4)]F (x = 0.7, 1, and 2) have been synthesized by a solid state reaction route. Their crystal structures were determined from single-crystal X-ray diffraction data. Li(1.3)Na(0.7)Ni[PO(4)]F crystallizes with the orthorhombic Li(2)Ni[PO(4)]F structure, space group Pnma, a = 10.7874(3), b = 6.2196(5), c = 11.1780(4) ? and Z = 8, LiNaNi[PO(4)]F crystallizes with a monoclinic pseudomerohedrally twinned structure, space group P2(1)/c, a = 6.772(4), b = 11.154(6), c = 5.021(3) ?, β = 90° and Z = 4, and Na(2)Ni[PO(4)]F crystallizes with a monoclinic twinned structure, space group P2(1)/c, a = 13.4581(8), b = 5.1991(3), c = 13.6978(16) ?, β = 120.58(1)° and Z = 8. For x = 0.7 and 1, the structures contain NiFO(3) chains made up of edge-sharing NiO(4)F(2) octahedra, whereas for x = 2 the chains are formed of dimer units (face-sharing octahedra) sharing corners. These chains are interlinked by PO(4) tetrahedra forming a 3D framework for x = 0.7 and different Ni[PO(4)]F layers for x = 1 and 2. A sodium/lithium disorder over three atomic positions is observed in Li(1.3)Na(0.7)Ni[PO(4)]F structure, whereas the alkali metal atoms are well ordered in between the layers in the LiNaNi[PO(4)]F and Na(2)Ni[PO(4)]F structures, which makes both compounds of great interest as potential positive electrodes for sodium cells.     

Vibrational modes and structural characteristics of (Ba(0.3)Sr(0.7))[(Zn(x)Mg(1-x))(1/3)Nb(2/3)]O3 solid solutions

(Ba(0.3)Sr(0.7))[(Zn(x)Mg(1-x))(1/3)Nb(2/3)]O(3) (BSZMN) (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) solid solution ceramics were synthesized by the conventional solid-state sintering technique. Vibration spectra (Raman spectroscopy and Fourier transform far-infrared reflection spectroscopy, short for FTIR) and X-ray diffraction (XRD) were employed to evaluate the correlation between crystal structures and vibration modes of these solid solutions as a function of Mg(2+) ions replaced by Zn(2+) ions. It is verified that these ceramics present a phase transition, i.e., the crystal structure changes from hexagonal phase (P ?3m1, where x≤ 0.4) to the pseudocubic phase (I4/mcm, where x≥ 0.8) with increasing Zn(2+) content. The phase transition is a gradual process, the sample where x = 0.6 is of the transition phase, i.e., at x = 0.6, phase transition begins to appear from hexagonal phase to pseudocubic phase but is not complete. The phase transition is also verified by the FTIR spectra. Tilting of oxygen octahedra is the main reason for the phase transition. The phonon modes of the vibration spectra were assigned, the position and width were determined, and the correlation of phonon vibrations with the microstructure for the different atoms substituted in B'-site was found.     

Scanning tunneling microscopy of template-stripped Au surfaces and highly ordered self-assembled monolayers

Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparation method for SAMs deposited from both vapor phase and solution phase.     

Ammoniated alkali fullerides (ND(3))(x)NaA(2)C(60): ammonia specific effects and superconductivity

The crystal structure of the superconducting (ND(3))(x)()NaA(2)C(60) (0.7 < or = x < or = 1, A= K, Rb) fullerides (T(c)= 6-15 K) has been studied by synchrotron X-ray and neutron powder diffraction. It is face-centered cubic (fcc) to low temperatures with Na(+)-ND(3) pairs residing in the octahedral interstices. These are disordered over the corners of two "interpenetrating" cubes with the Na(+) ions and the N atoms displaced by approximately 2.0 A and approximately 0.5 A from the center of the site and statically disordered over the corners of the inner and outer cube, respectively. Close contacts between the D atoms of the ND(3) molecules and electron rich 6:6 C-C bonds of neighboring C(60) units provide the signature of weak N-D.pi hydrogen-bonding interactions, which control the intermolecular packing in the crystal and may determine the unusual superconducting properties.     

Syntheses，Structures and Properties of Some New Composition Perovskite Compounds：Sr_（0．6）Bi_（0．4）FeO_（2．7），Sr_（1－x）Bi_xFeO_（3－y） and Ba_（1．5）Pt_（0．5）Mn_2O_6

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Oligomeric Rare-Earth-Metal Halide Clusters. Three Structures Built of (Y(16)Z(4))Br(36) Units (Z = Ru, Ir)

Suitable reactions in sealed Nb tubing at 850-950 degrees C gave good yields of a family of oligomeric cluster phases that were characterized by single-crystal X-ray diffraction means. The basic Y(16)Z(4) units ( approximately &fourmacr; symmetry) can be derived from 2+2 condensation of centered Y(6)Br(12)Z-type clusters or as tetracapped truncated tetrahedra Y(16) that are centered by a large tetrahedral Z(4). These are surrounded by 36 bromine atoms which bridge edges or cap faces of the Y(16)Z(4) nuclei and, in part, bridge to metal atoms in other clusters. The principal bonding appears to be Y-Z and Y-Br, with weaker Y-Y (&dmacr; approximately 3.70 ?) and negligible Z-Z interactions. The phase Y(16)Br(20)Ru(4) (P4(2)/nnm, Z = 2; a = 11.662(1) ?, c = 16.992 (2) ?) is isostructural with Y(16)I(20)Ru(4) and with the new Sc(16)Br(20)Z(4) (Z = Fe, Os). Syntheses only in the presence of Ir and ABr-YBr(3) fluxes (A = K-Cs) produce Y(16)Br(24)Ir(4) (Fddd, Z = 8; a = 11.718(3) ?, b = 22.361(7) ?, c = 44.702(2) ?), in which the electron-richer Ir interstitials are compensated by four additional bromine atoms and altered bridging between macroclusters. Larger amounts of YBr(3) yield a third example, Y(20)Br(36)Ir(4) (Y(16)Br(24)Ir(4).4YBr(3), I4(1)a, Z = 4; a = 12.699(1) ?, c = 45.11(1) ?). Here infinite zigzag chains of YBr(6/2) octahedra that share cis edges lie between and bridge to the Y(16)Ir(4) clusters. All of these phases contain 60-electron, closed-shell macroclusters. Y(16)Br(20)Ru(4) and Y(20)Br(36)Ir(4) were found to exhibit temperature-independent (Van Vleck) paramagnetism with values typical of those found for other rare-earth-metal, zirconium, niobium, and tantalum cluster halides.     

Two Homologous Intermetallic Phases in the Na-Au-Zn System with Sodium Bound in Unusual Paired Sites within 1D Tunnels

The Na-Au-Zn system contains the two intermetallic phases Na(0.97(4))Au(2)Zn(4) (I) and Na(0.72(4))Au(2)Zn(2) (II) that are commensurately and incommensurately modulated derivatives of K(0.37)Cd(2), respectively. Compound I crystallizes in tetragonal space group P4/mbm (No. 127), a = 7.986(1) ?, c = 7.971(1) ?, Z = 4, as a 1 × 1 × 3 superstructure derivative of K(0.37)Cd(2) (I4/mcm). Compound II is a weakly incommensurate derivative of K(0.37)Cd(2) with a modulation vector q = 0.189(1) along c. Its structure was solved in superspace group P4/mbm(00g)00ss, a = 7.8799(6) ?, c = 2.7326(4) ?, Z = 2, as well as its average structure in P4/mbm with the same lattice parameters.. The Au-Zn networks in both consist of layers of gold or zinc squares that are condensed antiprismatically along c ([Au(4/2)Zn(4)Zn(4)Au(4/2)] for I and [Au(4/2)Zn(4)Au(4/2)] for II) to define fairly uniform tunnels. The long-range cation dispositions in the tunnels are all clearly and rationally defined by electron density (Fourier) mapping. These show only close, somewhat diffuse, pairs of opposed, ≤50% occupied Na sites that are centered on (I) (shown) or between (II) the gold squares. Tight-binding electronic structure calculations via linear muffin-tin-orbital (LMTO) methods, assuming random occupancy of ≤ ～100% of nonpaired Na sites, again show that the major Hamilton bonding populations in both compounds arise from the polar heteroatomic Au-Zn interactions. Clear Na-Au (and lesser Na-Zn) bonding is also evident in the COHP functions. These two compounds are the only stable ternary phases in the (Cs,Rb,K,Na)-Au-Zn systems, emphasizing the special bonding and packing requirements in these sodium structures.     

Ternary rare-earth arsenides REZn3As3 (RE = La-Nd, Sm) and RECd3As3 (RE = La-Pr)

Ternary rare-earth zinc arsenides REZn(3)As(3) (RE = La-Nd, Sm) with polymorphic modifications different from the previously known defect CaAl(2)Si(2)-type forms, and the corresponding rare-earth cadmium arsenides RECd(3)As(3) (RE = La-Pr), have been prepared by reaction of the elements at 800 °C. LaZn(3)As(3) adopts a new orthorhombic structure type (Pearson symbol oP28, space group Pnma, Z = 4, a = 12.5935(8) ?, b = 4.1054(3) ?, c = 11.5968(7) ?) in which ZnAs(4) tetrahedra share edges to form ribbons that are fragments of other layered arsenide structures; these ribbons are then interconnected in a three-dimensional framework with large channels aligned parallel to the b direction that are occupied by La(3+) cations. All remaining compounds adopt the hexagonal ScAl(3)C(3)-type structure (Pearson symbol hP14, space group P6(3)/mmc, Z = 2; a = 4.1772(7)-4.1501(2) ?, c = 20.477(3)-20.357(1) ? for REZn(3)As(3) (RE = Ce, Pr, Nd, Sm); a = 4.4190(3)-4.3923(2) ?, c = 21.4407(13)-21.3004(8) ? for RECd(3)As(3) (RE = La-Pr)) in which [M(3)As(3)](3-) layers (M = Zn, Cd), formed by a triple stacking of nets of close-packed As atoms with M atoms occupying tetrahedral and trigonal planar sites, are separated by La(3+) cations. Electrical resistivity measurements and band structure calculations revealed that orthorhombic LaZn(3)As(3) is a narrow band gap semiconductor.     

Chemical pressure and rare-earth orbital contributions in mixed rare-earth silicides La(5-x)Y(x)Si4 (0 ≤ x ≤ 5)

A crystallographic study and theoretical analysis of the structural and La/Y site preferences in the La(5-x)Y(x)Si(4) (0 ≤ x ≤ 5) series prepared by high-temperature methods is presented. At room temperature, La-rich La(5-x)Y(x)Si(4) phases with x ≤ 3.0 exhibit the tetragonal Zr(5)Si(4)-type structure (space group P4(1)2(1)2, Z = 4, Pearson symbol tP36), which contains only Si-Si dimers. On the other hand, Y-rich phases with x = 4.0 and 4.5 adopt the orthorhombic Gd(5)Si(4)-type structure (space group Pnma, Z = 4, Pearson symbol oP36), also with Si-Si dimers, whereas Y(5)Si(4) forms the monoclinic Gd(5)Si(2)Ge(2) structure (space group P2(1)/c, Z = 4, Pearson symbol mP36), which exhibits 50% "broken" Si-Si dimers. Local and long-range structural relationships among the tetragonal, orthorhombic, and monoclinic structures are discussed. Refinements from single crystal X-ray diffraction studies of the three independent sites for La or Y atoms in the asymmetric unit reveal partial mixing of these elements, with clearly different preferences for these two elements. First-principles electronic structure calculations, used to investigate the La/Y site preferences and structural trends in the La(5-x)Y(x)Si(4) series, indicate that long- and short-range structural features are controlled largely by atomic sizes. La 5d and Y 4d orbitals, however, generate distinct, yet subtle effects on the electronic density of states curves, and influence characteristics of Si-Si bonding in these phases.     

An application of the "coloring problem": structure-composition-bonding relationships in the magnetocaloric materials LaFe13-xSix

The LaFe(13)-(x)Si(x) (1.0 < or = x < or = 5.0) series is studied experimentally and theoretically to gain possible understanding for the relationships among geometrical structure, chemical composition, magnetic behavior, and physical properties as related to the magnetocaloric effect in these compounds. As the Si concentration increases, LaFe(13)-(x)Si(x) exhibits a structural transformation from the cubic NaZn(13) structure type to a tetragonal derivative due primarily to preferential ordering of Fe and Si atoms. At room temperature, LaFe(13)-(x)Si(x) crystallize in the cubic structure for the range 1 < or = x < or = 2.6 and in the tetragonal for 3.2 < or = x < or = 5. In the range 2.6 < or = x < or = 3.2, it shows a two-phase mixture. Temperature-dependent single-crystal X-ray diffraction experiments near the corresponding Curie temperatures were performed on the room-temperature cubic phases to examine the origin of the large isothermal magnetic entropy changes. A thorough statistical and structural analysis of the data indicates that the noncentrosymmetric F43c space group provides a more adequate atomic arrangement than the centrosymmetric Fm3c space group. This change in space group leads to divergence for specific sets of Fe-Fe distances below the Curie temperature that arises from tilting of Fe-centered [Fe(12)-(x)Si(x)] icosahedra. The noncentrosymmetric space group also agrees with the predominance of icosahedral clusters lacking local inversion symmetry. From extended Hückel and tight-binding linear muffin-tin orbital (TB-LMTO) electronic structure calculations on various model structures, the F43c model is more energetically favorable than the Fm3c model. Extended Hückel calculations on various icosahedral [Fe(12)-(n)Si(n)] (n = 1-5) clusters and TB-LMTO calculations on "LaFe(13)," LaFe(11)Si(2), and LaFe(9)Si(4) have also been carried out to study the effects of a main group element (Si) on stabilizing the cubic NaZn(13)-type structure, influencing the transformation between cubic and tetragonal symmetries, and to study relationships among their chemical bonding and magnetic properties.     

Relativistic effects and gold site distributions: synthesis, structure, and bonding in a polar intermetallic Na6Cd16Au7

Na(6)Cd(16)Au(7) has been synthesized via typical high-temperature reactions, and its structure refined by single crystal X-ray diffraction as cubic, Fm ?3m, a = 13.589(1) ?, Z = 4. The structure consists of Cd(8) tetrahedral star (TS) building blocks that are face capped by six shared gold (Au2) vertexes and further diagonally bridged via Au1 to generate an orthogonal, three-dimensional framework [Cd(8)(Au2)(6/2)(Au1)(4/8)], an ordered ternary derivative of Mn(6)Th(23). Linear muffin-tin-orbital (LMTO)-atomic sphere approximation (ASA) electronic structure calculations indicate that Na(6)Cd(16)Au(7) is metallic and that ～76% of the total crystal orbital Hamilton populations (-ICOHP) originate from polar Cd-Au bonding with 18% more from fewer Cd-Cd contacts. Na(6)Cd(16)Au(7) (45 valence electron count (vec)) is isotypic with the older electron-richer Mg(6)Cu(16)Si(7) (56 vec) in which the atom types are switched and bonding characteristics among the network elements are altered considerably (Si for Au, Cu for Cd, Mg for Na). The earlier and more electronegative element Au now occupies the Si site, in accord with the larger relativistic bonding contributions from polar Cd-Au versus Cu-Si bonds with the neighboring Cd in the former Cu positions. Substantial electronic differences in partial densities-of-states (PDOS) and COHP data for all atoms emphasize these. Strong contributions of nearby Au 5d(10) to bonding states without altering the formal vec are the likely origin of these effects.     

Rare-earth metal-rich antimonides: syntheses, structures, and properties of Tm(3)Sb and Lu(7)Sb(3)

The two most metal-rich lanthanide antimony phases known were obtained from high-temperature solid state syntheses, that for Tm3Sb being of greater difficulty because of its apparent incongruent melting. The Tm3Sb phase crystallizes in the tetragonal space group P42/n (No. 86) with a Ti3P-type (Pearson: tP32) structure, a = 12.2294(5) Angstrom, c = 5.9852(5) Angstrom, and Z = 8. The phase Lu7Sb3 exhibits a Sc7As3-type tetragonal structure, I4/mcm (No. 140) (tI56), with a = 15.5974(7) Angstrom, c = 8.8130(7) Angstrom, and Z = 8. Both structures are described in terms of compact arrays of condensed chains of metal polyhedra (tetrahedral, tetrahedral star, trigonal prismatic, cubic) together with six- to nine-coordinate Sb in metal polyhedra. Magnetic susceptibility data on the paramagnetic Tm3Sb are also reported.