Roger Marchand

Nitrides and oxynitrides: Preparation, crystal chemistry and properties

Abstract As compared to oxides, nitride-type compounds can have specific properties and applications. The outline of this review focuses on the crystal chemistry of nitrides and oxynitrides. Given the nitride-oxynitride association, this article mainly deals with compounds which are predominantly covalent in character and does not include compounds which are metallic in character and in which nitrogen can be considered as an interstitial element. The classification which is used thus highlights the two main coordination polyhedra: nitrided or oxynitrided tetrahedra or octahedra.

Ternary lanthanoid-transition metal pnictides with ThCr2Si2-type structure

Abstract Fifteen new compounds AB 2 X 2 were synthesized where A is a lanthanoid, B = Fe, Co, or Ni, and X = P, As, or Sb. They crystallize with the ThCr 2 Si 2 (BaAl 4 , CeAl 2 Ga 2 ) type of structure. Their lattice constants are reported. Single crystals of the ternary phosphides were obtained using tin as a flux. The crystal structure of EuCo 2 P 2 (ThCr 2 Si 2 -type) was refined from single-crystal counter data to a conventional R value of 0.047 for 210 unique structure factors. While in most silicides with ThCr 2 Si 2 -type structure, the Si atoms form pairs, the P atoms in EuCo 2 P 2 are isolated from each other. Thus in an extreme ionic formulation, the compound can be rationalized as Eu 2+ Co 2+ 2 P 3− 2 . Bonding in these and the Zintl compounds is briefly discussed.

Characterization of nitrogen containing phosphate glasses by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been applied to phosphorus oxynitride glasses NaPO3 − xN2x3(O < x ⩽ 1), prepared by melting sodium metaphosphate (NaPO3)n in flowing ammonia, in order to determine the mechanism of the nitrogen/oxygen substitution. The formation of phosphorus-nitrogen bonds in place of phosphorus-oxygen bonds involves two kinds of nitrogen atoms N (N1) and N (N2) in mixed tetrahedra P(O, N)4. They substitute bridging oxygen O and non-bridging oxygen =0 according to rules which have been defined from obvious chemical considerations, while another part of oxygen atoms carries the negative charges required for the electronegativity with respect to Na+ ions. This study also brings information about the kinetical aspect of nitridation. N1 and N2 are simultaneously involved; the N2 content, which is first predominant, reaches a limiting value which corresponds to one atom per tetrahedron.

Ternary and higher order rare-earth nitride materials: synthesis and characterization of ionic-covalent oxynitride powders

The possibility to introduce nitrogen as a substitute for oxygen within the anionic network of ternary and higher order rare-earth oxides has been examined. The higher anionic formal charge resulting from this N3−/O2− substitution can be compensated either through cross-substitutions or by the creation of anionic vacancies. Resulting oxynitrides belong to different structures types which are well known in the oxide crystal chemistry. A commonly used synthesis method consists of a thermal nitridation in flowing ammonia of corresponding rare-earth oxide compositions. In such a solid–gas process the reactivity of the oxide precursors is an important reaction parameter, and a soft chemistry route involving a complexing method may be advantageously utilized to prepare them. The N3−/O2− anionic substitution induces also a more covalent character illustrated, for example, by strongly colored oxynitride powders that gives the possibility to tune the absorption edge of the UV–visible spectra by suitable adjustment of the N/O ratio.

Nitrogen-containing phosphate glasses

Abstract A new family of nitrided glasses has been discovered. Nitrogen-containing phosphate glasses are prepared by reaction between ammonia and alkaline polyphosphates (MIPO3)n(MILi, Na, K) at 700°C and characterized as oxynitrides. High nitrogen contents are obtained. With MNa, for example, the vitreous field appears in the range NaPO3NaPO2N0.67 corresponding to a ratio N/O+N ⩽ 0.25. The existence of KPO3−3xN2x glasses exemplifies the role of nitrogen in the glass structure, which is built up on P(O,N)4 tetrahedra. At 700°C, the reaction between NH3 and P4O10 leads to glassy phases belonging to the HPON system.

New routes to transition metal nitrides: and characterization of new phases

Transition metal nitrides form a class of materials with unique physical properties which give them varied applications, as high temperature ceramics, magnetic materials, superconductors or catalysts. They are commonly prepared by high temperature conventional processes, but alternative synthetic approaches have also been explored, more recently, which utilize moderate-temperature conditions. For example, high surface area γ-Mo 2 N nitride powders (fcc phase) are prepared from commercial oxide MoO 3 through a topotactic transformation process. Of prime importance is the nature of the precursor, because it may yield new nitride phases unattainable by other synthetic routes. A novel promising method to nitride synthesis has been developed using sulfides as starting materials. The ammonolysis reaction has been applied first to the preparation of two binary molybdenum nitrides: Mo 5 N 6 (filled 2H-MoS 2 structure) and δ-MoN (NiAs-type structure) from MoS 2 , and then extended to other metals such as W, Cr or Ti, as well as molybdenum- and tantalum-based ternary systems. Fine reactive molybdenum sulfide precursor powders (S g ≥200 m 2 g –1 ) have been synthesized in thiocyanate melt. On the other hand, alkali metal ternary oxides offer potential as nitridation precursors. For example, a binary nitride Nb 4 N 5 (defect NaCl-type structure) results from ammonolysis of sodium or potassium niobates whereas LiNb 3 O 8 is transformed into a mixed valent ternary nitride LiNb 3 N 4 (filled 2H-MoS 2 structure). Another illustration of the Li + inductive effect is given in the direct synthesis of LiMN 2 from Li 2 MO 4 (M=Mo, W). The nitrides Mo 5 N 6 , δ-MoN and Nb 4 N 5 show superconducting behavior at T<12 K.

Etude structurale des perovskites oxyazotees BaTaO2N et BaNbO2N: Structural study of BaTaO2N and BaNbO2N oxynitrided perovskites

Resume The structure of the BaTaO 2 N and BaNbO 2 N compounds has been refined by profile analysis of time-of-flight neutron diffraction data. These oxynitrided perovskites, which are characterized by high and stable dielectric constants in a large scale of temperature or frequency, have a cubic cell and no phase transformation was observed until 4 K : oxygen and nitrogen atoms are statistically distributed on one position of the Pm3m space group.

Influence of Nitrogen-content of a New Aluminophosphate Oxynitride Catalyst - Alpon in Knoevenagel Condensation

A new high surface area solid basic catalyst with tunable acid-base properties has recently been proposed [R. Marchand, R. Conanec, Y. Laurent, Ph. Bastians, P. Grange, L.M. Gandia, M. Montes, J. Fernandes and I. Odriozola, patent application FR 940108 1; R. Conanec, R. Marchand, Y. Laurent, Ph. Bastians and P. Grange, in Soft Chemistry Routes to New Materials, Nantes, France, in press; P. Grange, Ph. Bastians, R. Conanec, R. Marchand and Y. Laurent, Chem. Commun., submitted for publication). Starting from aluminophosphate (AlPO4) oxide precursor and with careful adjustment of the nitridation procedure, it is possible to modify the global composition, changing the O/N content of the nitrided aluminophosphate (AlPON). This letter evidences that conversion in the malononitrile-benzaldehyde condensation largely depends on the O/N ratio of these new catalysts. Such basic solids are compared with conventional MgO catalyst.

Synthesis and energetics of yellow TaON

From the standard enthalpy of formation of tantalum oxynitride TaON, determined by oxidative drop solution calorimetry, it is possible to explain its particular synthesis conditions from Ta 2O5 and NH3, i.e. the use of wet ammonia which prevents the formation of tantalum nitride Ta3N5. Thermogravimetric analyses were performed to study both the ammonolysis of Ta2O5 and the oxidation in air of Ta3N5 and TaON. TaON powder was prepared, which bright yellow color agrees well with its electronic structure.  2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

A new class of inorganic compounds containing dinitrogen-metal bonds

Abstract TGA of particular oxynitrides shows an unexpected intermediate phase between starting oxynitride and final oxide. Transition between this intermediate phase and the oxide step, studied by high resolution mass spectrometry, reveals that this phenomenon can be ascribed to a nitrogen retention. We deduce the general formulation M n+ O n 2 N x . Particular properties of those phases are: low density, structure type analogous to the one of their oxynitride precursor and a nitrogen binding energy, measured by XPS, near 403 eV. By comparison to the binding energy range of organometallic dinitrogen complexes, we assume the presence of M-NN-M entities included in an oxide lattice with cationic defect structure.