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

Abstract

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.