New routes to transition metal nitrides: preparation 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 gamma-Mo2N nilride powders (fcc phase) are prepared from commercial oxide MoO3 through a topotactic transformation process. Of prime importance is the nature of the precursor, because ii 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: Mo5N6 (filled 2H-MoS2 structure) and delta-MoN (NiAs-type structure) from MoS2, 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 greater than or equal to 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 Nb4N5 (defect NaCl-type structure) results from ammonolysis of sod-m or potassium niobates whereas LiNb3O8 is transformed into a mixed valent ternary nitride LiNb3N4 (filled 2H-MoS2 structure). Another illustration of the Li+ inductive effect is given in the direct synthesis of LiMN2 from Li2MO4 (M = Mo, W). The nitrides Mo5N6, delta-MoN and Nb4N5 show superconducting behavior at T < 12 K.