Structures of New Nitrides Prepared using Solid Oxide Precursors
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Several novel transition metal nitrides and oxynitrides were synthesized via ammonolysis of solid state oxide precursors at temperatures ranging from 700°C-900°C and reaction times ranging from 12 hours to 4 days. The products were characterized by powder X-ray diffraction and their structures were determined by powder X-ray Rietveld refinement. The relationships between the structures of these nitrides and oxynitrides, and their similarity to the structures of the transition metal dichalcogenides, is discussed. INTRODUCTION
The synthesis of ternary and quaternary nitrides provides many interesting challenges to the synthetic solid state chemist. Because of its high bond energy (941 KJ/mol), the use of nitrogen gas requires high temperatures
for a reaction to occur. This often results in the formation of intermetallic nitrides or the simple binaries. Recently, several groups have explored alternative synthetic routes to facilitate the formation of ionic/covalent ternary nitrides[1-14]. This effort has resulted in the synthesis of a number of new ternary and quaternary nitrides. The use of oxide precursors has been well documented as an experimentally simple and inexpensive method for the synthesis of a wide variety of nitride and oxynitride products[15-19]. Of particular note are the recent syntheses of ternary nitrides containing two transition metal starting from the appropriate ternary oxide, demonstrating that complete conversion to a nitride product is possible. Also, the ammonolysis of lithium molybdate
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Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
and tungstate precursors yields ternary nitride materials with structures remarkably similar to those found in lithium intercalated molybdenum and tungsten dichalcogenides. In addition, ammonolysis reactions of ternary oxides containing rare earths[20-221 or vanadium[23] yield distinct oxynitride products. Interestingly, while the precursors used in the ammonolysis reactions vary both in structure and composition, the nitrides and oxynitrides which result demonstrate remarkable structural similarity. We have prepared several new ionic/covalent nitrides and oxynitrides, FeWN 2118], (Feo.sWo. 2 )WN 2 , (Feo.sMoo. 2 )MoN 2[151, MnMoN2114], MnWN 2 114], M(MO. 67O0.33)N 2-SOy (M = Ta or Nb; 01 = vacancy)[24], using transition metal molybdate, tungstate, tantalate and niobate solid state precursors. The structures of all the compounds are closely related, containing as the common structural motif layers of trigonal prismatically and octahedrally coordinated transition metals. In this proceeding, the syntheses of these nitrides and their structural relationships are presented. EXPERIMENTAL The transition metal tungstates (FeWO4, MnWO4 , Fe2W 3012) molybdates (Fe 2Mo3012, MnMoO 4 ), niobates (FeNbO 4, FeNb 20 6) and tantalates (FeTaO 4, FeTa 20 6) were used respectively as precursors for the transition metal nitrides FeWN 2, MnWN2, (FeO.8WO.2)WN2, (FeO.sMoO.2)MoN 2, MnMoN2 and the oxynitrides , M(MO. 67 0o. 33)N2-5Oy (M = Ta or Nb; 0 = vaca
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