Rapid solid-state synthesis of tantalum, chromium, and molybdenum nitrides.

Abstract

Solid-state metathesis (exchange) reactions can be used to synthesize many different transition-metal nitrides under ambient conditions including TiN, ZrN, and NbN. Typical metathesis reactions reach temperatures of greater than 1300 degrees C in a fraction of a second to produce these refractory materials in highly crystalline form. Likely due to the large amount of heat produced in these solid-state reactions, some transition-metal nitrides such as TaN, CrN, and gamma-Mo(2)N cannot easily be synthesized under ambient conditions. Here metathesis reactions are demonstrated to produce the cubic nitrides TaN, CrN, and gamma-Mo(2)N when sufficient pressure is applied before the reaction is initiated. By pressing a pellet of TaCl(5) and Li(3)N with an embedded iron wire, crystalline cubic TaN forms under 45 kbar of pressure after a small current is used to initiate the chemical reaction. Crystalline cubic CrN is synthesized from CrCl(3) and Li(3)N initiated under 49 kbar of pressure. Crystalline gamma-Mo(2)N is produced from MoCl(5) and Ca(3)N(2) (since MoCl(5) and Li(3)N self-detonate) initiated under 57 kbar of pressure. The addition of ammonium chloride to these metathesis reactions drastically lowers the pressure requirements for the synthesis of these cubic nitrides. For example, when 3 mol of NH(4)Cl is added to CrCl(3) and Li(3)N, crystalline CrN forms when the reaction is initiated with a resistively heated wire under ambient conditions. Cubic gamma-Mo(2)N also forms at ambient pressure when 3 mol of NH(4)Cl is added to the reactants MoCl(5) and Ca(3)N(2) and ignited with a resistively heated wire. A potential advantage of synthesizing gamma-Mo(2)N under ambient conditions is the possibility of forming high-surface-area materials, which could prove useful for catalysis. Nitrogen adsorption (BET) indicates a surface area of up to 30 m(2)/g using a Langmuir model for gamma-Mo(2)N produced by a metathesis reaction at ambient pressure. The enhanced surface area is confirmed using scanning electron microscopy.