Boride-based nano-laminates with MAX-phase-like behaviour

MAX-phases being usually composed of transition metals, group A elements and carbon/nitrogen are considered interesting materials for many applications because of their tremendous bulk modulus, “reversible” plasticity, and machinability. This is mainly due to their unique kind of bonding comprising covalent, ionic as well as metallic bonds providing “easy” planes of rupture and deformability due to the layered crystal structures.In transition metal boride systems, similar types of bonding are available. In particular the W₂B₅-structure type and its stacking variations allow the synthesis of strongly layered crystal structures exhibiting unique delamination phenomena.The paper presents ab initio calculations showing the similarities of bonding between the ternary carbides and the corresponding ternary or quaternary borides. Formation of boride-based nano-laminates from auxiliary liquid phases, from the melt as well as during sintering and precipitation from supersaturated solid solutions will be discussed by means of SEM and TEM studies. The role of impurities weakening the interlayer bonding will be addressed in particular. The pronounced cleavage parallel to the basal plane gives rise for crack deflection and pull-out mechanisms if the laminates are dispersed in brittle matrices such as boron carbide, silicon carbide or other transition metal borides.     

Thermal decomposition reactions of metal carboxylato complexes in the solid state. I.: Thermographic and differential thermal studies of metal oxalato,malonato and succinato complexes

Thermal investigation of metal carboxylato complexes of the first transition metals, Mn(II), Fe(II), Fe(III), Co(II), Ni(II) and Cu(II) and non-transition metals like Zn(II) and Cd(II) in the solid state has been carried out under non-isothermal conditions in nitrogen atmosphere by simultaneous TG and DTA. TG and DTA curves inferred that the thermal stability of the complex decreased approximately with the increase of the standard potential of the central metal ion. The thermal parameters like activation energy, E_a, enthalpy change, ΔH, and entropy change, ΔS, corresponding to the dehydration and decomposition of the complexes are determined from TG and DTA curves by standard methods. A linear correlation is found between ΔH and ΔS and E_a and ΔS in dehydration and decomposition processes. DTA curves show an irreversible phase transition for Na₂Mn(mal)₂], Na₂[Cu(mal)₂] and Na₂,[Co(suc)₂] complexes. The residual products in these decomposition processes being a mixture of two oxides, of oxide and carbonate or a mixture of two carbonates.     

Structural chemistry of complex carbides and related compounds

Complex carbides formed in ternary systems of a transition element (M), a B-group element (M′), and carbon and having a formula M₂M′C (H-phase) or M₃M′C (perovskite carbide) occur frequently. This reflects the simple geometry of the atomic arrangement of the metals and the filling mode by an interstitial stabilizer such as carbon or nitrogen. The phase relationship of the ternary combinations {Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, and Ni}-aluminum-carbon was investigated. New complex carbides were found with the corresponding zirconium, hafnium, and tantalum combinations. The crystal structures in the case of Zr- and Hf-containing complex carbides can be characterized by a twelve-metal-layer sequence and by a ten-metal-layer sequence with carbon atoms again filling octahedral voids. The transition of structure types from TiC, Ti₂AlC, Ti₃SiC₂, ZrAlC₂, Zr₂Al₃C₅, to Al₄C₃ is also discussed.     

Origin of Extraordinary Stability of Square‐Planar Carbon Atoms in Surface Carbides of Cobalt and Nickel

Surface carbides of cobalt and nickel are exceptionally stable, having stabilities competitive with those of graphitic C on these surfaces. The unusual structure of these carbides has attracted much attention: C assumes a tetracoordinate square‐planar arrangement, in‐plane with the metal surface, and its binding favors a spontaneous p4g clock surface reconstruction. A chemical bonding model for these systems is presented and explains the unusual structure, special stability, and the reconstruction. C promotes local two‐dimensional aromaticity on the surface and the aromatic arrangement is so powerful that the required number of electrons is taken from the void M₄ squares, thus leading to Peierls instability. Moreover, this model predicts a series of new transition‐metal and main‐group‐element surface alloys: carbides, borides, and nitrides, which feature high stability, square‐planar coordination, aromaticity, and a predictable degree of surface reconstruction.     

Ternary rare earth and actinoid transition metal carbides viewed as carbometalates

Ternary carbides A_xT_yC_z (A=rare earth metals and actinoids; T=transition metals) with monoatomic species C⁴⁻ as structural entities are classified according to the criteria (i) metal to carbon ratio, (ii) coordination number of the transition metal by carbon atoms, and (iii) the dimensionality of the anionic network [T_yC_z]ⁿ⁻. Two groups are clearly distinguishable, depending on the metal to carbon ratio. Those where this ratio is equal to or smaller than 2 may be viewed as carbometalates, thus extending the sequence of complex anions from fluoro-, oxo-, and nitridometalates to carbometalates. The second group, metal-rich carbides with metal to carbon ratios equal to or larger than 4 is better viewed as typical intermetallics (“interstitial carbides”). The chemical bonding properties have been investigated by analyzing the Crystal Orbital Hamilton Population (COHP). The chemical bonding situation with respect to individual T-C bonds is similar in both classes. The main difference is the larger number of metal-metal bonds in the crystal structures of the metal-rich carbides.     

Magnetic Susceptibility as a Method of Investigation of Short-Range Ordering in Highly Nonstoichiometric Carbides

This paper reviews the results of magnetic susceptibility studies on ordered and disordered Group IV and V transition metal carbides. It is shown that the variations of susceptibility resulting from deviations from stoichiometry are associated with the electronic spectrum features of these compounds. Using magnetic susceptibility as a tool for structural order–disorder transition analysis is discussed. The lower susceptibility of the nonstoichiometric carbides is due to short-range ordering, changing the contribution of orbital paramagnetism. The long- and short-range order parameters have been estimated for NbC _y , TaC _y , TiC _y , and HfC _y based on the experimental susceptibility data.     

Epitaxial oxide thin films by pulsed laser deposition: Retrospect and prospect

Pulsed laser deposition (PLD) is a unique method to obtain epitaxial multi-component oxide films. Highly stoichiometric, nearly single crystal-like materials in the form of films can be made by PLD. Oxides which are synthesized at high oxygen pressure can be made into films at low oxygen partial pressure. Epitaxial thin films of highT _c cuprates, metallic, ferroelectric, ferromagnetic, dielectric oxides, superconductor-metal-superconductor Josephson junctions and oxide superlattices have been made by PLD. In this article, an overview of preparation, characterization and properties of epitaxial oxide films and their applications are presented. Future prospects of the method for fabricating epitaxial films of transition metal nitrides, chalcogenides, carbides and borides are discussed.     

Kinetics and Determination of Some Transition Metals with Peri-Dihydroxynaphthindenone

Abstract

A simple sensitive and selective spectrophotometric method has been developed for determination of some transition metal ions. It is based on the reaction of the metal ions with excess peri-dihydroxynaphthindenone in 80% v/v ethyl alcohol-water mixture at 30°C. The colored products obtained display maximum absorption band at 560–590 nm depending upon the type of transition metal ion used, and E_{1 cm} ^1% in the range 70–480. Under the optimum conditions results with an average recovery of 96%, (mean standard deviation ±3% are obtained for 6 different transition metal ions without any significant interference from Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺, Ba²⁺, Sr²⁺, Ni²⁺, Bi³⁺, Co²⁺, Mn⁺², Cd⁺² and Al⁺³. Kinetic data reveal that the reaction proceeds via a second order route (first order with respect to either the metal cation or ligand). The activation parameters and a suggested mechanism have been presented, and the nature of bonding in the solid chelate products has been verified.     

Organometallic motifs in the structures of solid state ternary carbides of the late transition metals

Solid state ternary transition metal carbides containing carbon, a middle to late transition metal (Re to Ni), and a highly electropositive multivalent metal such as lanthanide, yttrium, or thorium exhibit a number of structural motifs resembling those in metal carbonyls and other transition metal derivatives of strong -acceptor ligands. This paper presents models for the chemical bonding in the transition metal—carbon subnetworks of the ternary late transition metal carbides LnCoC (Ln=lanthanide), Ln₂ReC₂, Th₂NiC₂, Ln₂FeC₄, Ln₃MC₄ (M=Fe, Co, Ni, Ru, Rh, Os, Ir), Ln₄NiC₅, Ca₄Ni₃C₅, and Er₈Rh₅C₁₂. Carbide ligands present in such materials include terminal C^4– in Th₂NiC₂ and Y₂ReC₂, ₂-C^4– in YCoC or Y₂ReC₂ similar to the central allene carbon atom, 1,2-bridging C ₂ ^4– in Sc₃CoC₄ formally derived from ethylene, ₃-bridging C ₂ ^4– in LnMC₂ (M=Fe, Co, Ni, Ru) formally derived from ethylene, and 1,1-bridging C ₂ ^2– in Ln₂FeC₄ isoelectronic with ₂-CO group in metal carbonyls.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1358–1366, August, 1994.     

Die Herstellung von Boriden der Übergangsmetalle aus dem Hilfsmetallbad

Zusammenfassung Das Hilfsmetallbadverfahren zur Herstellung von Hartstoffen—Reaktion der Ausgangskomponenten in einer inerten Metallschmelze und nachträgliche Entfernung des Hilfsmetalles—, —das sich für die Herstellung von Carbiden, Siliciden, Nitriden und Carbonitriden schon bewährt hat, ist nach den vorliegenden Versuchsergebnissen auch für die Herstellung von Boriden geeignet. Die im Vergleich zu anderen Hartstoffen etwas geringere chemische Beständigkeit der Boride bedingt größere Boridverluste beim Weglösen der Hilfsmetalle. Es konnten in Versuchen quantitative Angaben über die Beständigkeit von ZrB₂, TaB₂ und MoB₂ gegenüber verschiedenen wäßr. Medien erarbeitet werden. Es gelang, reines ZrB₂ und HfB₂ aus Kupfer-, Blei- und Zinnschmelzen herzustellen; ZrB und HfB scheinen nach den Versuchsergebnissen, wie schon früher von anderen Autoren vermutet, nur stabilisiert zu existieren. Ternäre, hilfsmetallhältige Phasen wurden nicht beobachtet.

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Preparation of transition metal borides from the auxiliary metal bath

The auxiliary metal bath process for the preparation of hard materials—reaction of the starting materials in an inert metal melt and subsequent removal of the auxiliary metal—which has already proved suitable for the preparation of carbides, silicides, nitrides and carbonitrides, is shown by the experiments described to be also suitable for preparing borides. Owing to the rather lower chemical stability of the borides compared with other hard materials, greater losses occur during dissolution of the auxiliary metals. Quantitative data concerning the stability of ZrB₂, TaB₂ and MoB₂ towards various aqueous media were obtained experimentally. Pure ZrB₂ and HfB₂ could be prepared from copper, lead and tin metls; experimental results indicate that ZrB and HfB exist only when stabilized, as previously supposed by other authors. Ternary phases containing auxiliary metals were not observed.

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Mit 3 Abbildungen     

Author index for volume 31

The solid state reactions of MMoO₄ (M = Mg²⁺, Fe²⁺, and Ni²⁺) and orthorhombic TeO₂ were investigated. A new metal telluromolybdate MgTeMoO₆ was obtained in the case of Mg; its structure belongs to the orthorhombic system with unit-cell dimensions a = 5.262Å, b = 5.028 Å, c = 8.880 Å. Fe₂(MoO₄)₃ and a new compound were formed in the case of Fe. The new compound is made up with Fe³⁺ ions and its chemical formula cannot be recognized as FeTeMoO₆. In the case of Ni a complex reaction mixture is obtained. An explanation is given of the ability of M²⁺ ions to form the metal telluromolybdates. The catalytic properties of MgTeMoO₆ are discussed and compared to those of the other metal telluromolybdates.     

Synthesis and characterization of divalent manganese,cobalt, nickel,copper and zinc complexes with nicotinic acid

Coordination compounds of the transition metal(II) acetylacetonates of the formula [M(NA)₂(acac)₂ ]_n (M = Mn, Co, Ni, Cu and Zn; NA = nicotinic acid and acac = acetyl-acetonate anion) have been synthesized and characterized by chemical analysis, magnetic susceptibility, ligand-field spectra, IR and far-IR spectral measurements as well as photoacoustic spectroscopy in the solid state. Tentative stereochemistries for the complexes isolated in the solid state are suggested. The ligand-field parameters 10 Dq, B, β, λ and CFSE are calculated for cobalt and nickel complexes and are in good agreement with the proposed geometries. The metal ions attain six-coordination through the four oxygens of the anion and two donor atoms of the nicotinic acid ligands acting always as monodentate ligands. The formation of the compound results in a considerable shift of v(M-O) to lower frequencies in all the compounds related to parent acetylacetonates.     

Crystal Chemistry of Complex Carbides and Related Compounds

Numerous metal carbides can be discussed in a rather uniform way by means of structural features, mainly characterized by the mode of linking of octahedral and occasionally trigonal prismatic [M₆C] groups (M = transition element). From this point of view perovskite carbides (M₃M′C, M′ = another transition or A-group element) and derivatives, β-Mn carbides (M₃M′₂C), κ-carbides, carbides with V₃AsC- and Cr₂AlC type structures and derivatives, η-carbides (M₃M′₃C) and carbides having the filled Mn₅Si₃ type structure will be treated. The high stability of these complex carbides is due to the strong bonding M? C and additional bonding of M? M′ atoms forming an ordered parent lattice. Besides the interstitial principle of filling of lattice holes (by isolated carbon atoms), substitution with A-group elements may also take place. Thus in borocarbides extended structural elements occur.     

Packing of chiral molecules of metal TRIS-acetylacetonates

The Cambridge Crystallographic Data Centre (CCDC) is searched to organize packings of metal tris-acetylacetonates M(aa)₃ (about 70 structures over the last 50 years). In all structures, planes with a hexagonal molecular environment are found. Three types of the nearest hexagonal environment are revealed in these planes with allowance for the chirality of molecules. Different superposition of these planes produces five types of packing of metal tris-acetylacetonates: α, β, γ, jg, and pg. Spectrally pure Al(aa)₃, Cr(aa)₃, Mn(aa)₃, and Fe(aa)₃ complexes are synthesized and their solid solutions are studied. The effect of impurities on crystallization is established. A temperature range of 210 K to 100 K, in which we have previously detected thermochromism of metal β-diketonates is distinguished. In the same range, a symmetry change was previously found for Al(aa)₃ and Mn(aa)₃, however, Cr(aa)₃ symmetry did not change in this temperature range. The ratios of components in solid solutions, at which low temperature symmetry change is observed in the complexes, are found experimentally. The relation between thermochromism and the symmetry change is discussed.     

An investigation on the solid state pyrolytic decomposition of bimetallic oxalate precursors of Ca,Sr and Ba with cobalt: A mechanistic approach

The mixed metal oxalate precursors, calcium(II)bis(oxalato)cobaltate(II)hydrate (COC), strontium(II)bis(oxalato)cobaltate(II)pentahydrate (SOC) and barium(II)bis(oxalato)cobaltate(II)octahydrate (BOC) have been synthesized and their thermal stability was investigated. The complexes were characterized by elemental analysis, IR spectral and X-ray powder diffraction studies. Thermal decomposition studies (TG, DTG and DTA) in air showed that the compound COC decomposed mainly to CaC₂O₄ and Co₃O₄ at 340 °C, and a mixture of CaCO₃ and Co₃O₄ identified at 510 °C. A mixture of CaCO₃ and Ca₃Co₂O₆ along with the oxides and carbides of both the cobalt and calcium were attributed at 1000 °C as end products. DSC study in nitrogen ascertained the formation of a mixture of CaO and CoO along with a trace of carbon at 550 °C. The mixture species, SrC₂O₄, CoC₂O₄ and Co₃O₄ were generated at 255 °C in case of SOC in air, which ultimately changed to CoSrO₃, SrCO₃ and oxides of strontium and cobalt at 1000 °C. The several mixture species also generated as intermediate at 332 and 532 °C. The DSC study in nitrogen indicated the formation of CoSrO_x (0.5 < x < 1) as end product. In case of BOC in air, a mixture of BaCoO₂, BaO, CoO and carbides are identified as end product at 1000 °C through the generation of several intermediate species at 350 and 530 °C. A mixture of BaO and CoO is identified as end product in DSC study in nitrogen. The kinetic parameters have been evaluated for all the dehydration and decomposition steps of all the three compounds using four non-mechanistic equations. Using seven mechanistic equations, the kind of dominance of kinetic control mechanism of the dehydration and decomposition steps are also inferred. The kinetic parameters, ΔH and ΔS of all the steps are explored from the DSC studies. Some of the decomposition products are identified by IR and X-ray powder diffraction studies.     

Calculation of exchange coupling constants in solid state transition metal compounds using localized atomic orbital basis sets

The application of theoretical methods based on density functional theory using hybrid functionals and localized, atomic orbital type basis sets is shown to provide good estimates for exchange coupling constants in non-metallic, solid state transition metal compounds with relatively complex crystal structures. The accuracy of the calculated exchange coupling constants is similar to that previously obtained for dinuclear and polynuclear molecular compounds. As an application of this procedure, the magnetic properties of the high-temperature phase of CuGeO₃, the recently synthesized silver copper oxide Ag₂Cu₂O₃, and the family of M[N(CN)₂]₂ (M=Cr(II), Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)) compounds are analyzed via the computation of their most relevant exchange coupling constants.     

A new colorimetric chemodosimeter for Hg2+ based on charge-transfer compound of N-methylpyrrole with TCNQ

Reaction of N-methylpyrrole and 7,7,8,8-tetracyanoquinodimethane (TCNQ) furnishes an intense blue unsymmetrical charge-transfer compound through regioselective attachment of tricyanoquinodimethane at the 2-position of N-methylpyrrole which was found to be selective chemodosimeter for Hg²⁺ ions in CH₃CN:H₂O mixture (1:1 v/v, pH = 7.0, 0.01 M HEPES, 0.15 M NaCl) as well as in the solid state when supported on silica, over a variety of metal ions. A plausible mechanism for the sensing process has been proposed and supported through the characterization of the resulting Hg²⁺ complex and the density functional theory (DFT) studies.     

Catalytic asymmetric hydroxylation of α-alkoxycarbonyl amides with a Pr(OPr)3/amide-based ligand catalyst

A catalytic asymmetric hydroxylation of N-nonsubstituted α-alkoxycarbonyl amides is described. A new effective catalyst comprising Pr(OⁱPr)₃ and a fluoro-substituted amide-based ligand was identified using oxaziridine as the oxidizing reagent. The catalyst components were in dynamic equilibrium in the reaction mixture, which assembled to form the associated transition state through metal coordination and hydrogen bonding.     

Siliciding of titanium carbides with gaseous SiO

Titanium carbides of different stoichiometries were silicided with gaseous SiO at 1350°C. A mechanical mixture of silicon and silicon dioxide was used as a reaction source of SiO. Ti₃SiC₂, TiSi₂, and Ti₅Si₃ were the main reaction products, the phase composition of which strongly depended on the titanium carbide stoichiometry. The siliciding of carbides with a nearly stoichiometric carbon content resulted in the formation of Ti₃SiC₂, on the surface of which the other silicide phases, such as Ti₅Si₃ and TiSi₂, began to form. For titanium carbides with a low carbon concentration, Ti₅Si₃ was the only siliciding product.     

Bonding and doping of simple icosahedral-boride semiconductors

A simple model of the bonding and doping of a series of icosahedral-boride insulators is presented. Icosahedral borides contain clusters of boron atoms that occupy the 12 vertices of icosahedra. This particular series of icosahedral borides share both the stoichiometry B₁₂X₂, where X denotes a group V element (P or As), and a common lattice structure. The inter-icosahedral bonding of these icosahedral borides is contrasted with that of B₁₂O₂ and with that of α-rhombohedral boron. Knowledge of the various types of inter-icosahedral bonding is used as a basis to address effects of inter-icosahedral atomic substitutions. The inter-icosahedral bonding is maintained when an atom of a group V element is replaced with an atom of a group IV element, thereby producing a p-type dopant. However, changes of inter-icosahedral bonding occur upon replacing an atom of a group V element with an atom of a group VI element or with a vacancy. As a result, these substitutions do not produce effective n-type dopants. Moreover, partial substitution of boron atoms for atoms of group V elements generally renders these materials p-type semiconductors.