Rapid Solid-State Synthesis of Tantalum, Chromium, and Molybdenum Nitrides

• Published in: Inorganic chemistry 2001-05, Vol.40 (10), p.2240-2245
• Main Authors: O'Loughlin, Jennifer L, Wallace, Charles H, Knox, Meredith S, Kaner, Richard B
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic001265h

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 °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 γ-Mo2N cannot easily be synthesized under ambient conditions. Here metathesis reactions are demonstrated to produce the cubic nitrides TaN, CrN, and γ-Mo2N when sufficient pressure is applied before the reaction is initiated. By pressing a pellet of TaCl5 and Li3N 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 CrCl3 and Li3N initiated under 49 kbar of pressure. Crystalline γ-Mo2N is produced from MoCl5 and Ca3N2 (since MoCl5 and Li3N 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 NH4Cl is added to CrCl3 and Li3N, crystalline CrN forms when the reaction is initiated with a resistively heated wire under ambient conditions. Cubic γ-Mo2N also forms at ambient pressure when 3 mol of NH4Cl is added to the reactants MoCl5 and Ca3N2 and ignited with a resistively heated wire. A potential advantage of synthesizing γ-Mo2N 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 m2/g using a Langmuir model for γ-Mo2N produced by a metathesis reaction at ambient pressure. The enhanced surface area is confirmed using scanning electron microscopy.