Transition metal nitrides melting

Some transition metal nitrides,10 MN, of Ti, Zr, and Hf have cubic (Nad type) structures. Others which are often not exactly stoichiometric (being N deficient), are chemically very inert and extremely hard with high melting points. The electronic band structure of the metal persists, the appearance is metallic and the compounds are electrically conducting. As an example, VN has mp 2570°C and hardness 9-10. 

A patent has been issued for the preparation of pure ThN or its mixture with either U, Pu, or any other of a number of the high-melting transition metal nitrides [31]. To prepare pure ThN, an intimate fine powder mixture of C with Th oxalate in the mole ratio of 1 to 2 is heated in streaming H2 or Ar + H2 to above 600X (the oxalate decomposes at 750 to 800°C) after which the product is evacuated and the carbothermal reduction completed at 1100 to 1300°C. The product is converted to the nitride in streaming N2 by heating to 1750 to 1800°C until evolution of CO is complete. 

Table 1.2 compares the melting points of transition metals with those of the carbides and nitrides. The trends in these values were already discussed in connection with the electronic properties of the compounds. Here, note is made of the elevated temperatures of the carbides and nitrides compared to the pure metals. They are among the highest for any type of material, and are more akin to those of the ceramic materials. The melting points are indicative of the high cohesive strengths in the materials. 

The metal nitrides of the transition metals of the subgroups of the IVth, Vth and Vlth groups are very similar to the corresponding carbides in their. structures (interstitial compounds of the MN type) and their properties such as hardness, melting point and electrical conductivity (see Table 5.6-3). Tungsten and molybdenum nitrides are exceptions with different structures and decompose with nitrogen loss above 800°C. 

Many carbides and silicides of composition AX are formed by transition metals. These carbides and silicides are characterized by very high melting points, extreme hardness, optical opacity and relatively high electrical conductivity. Many of them have the sodium chloride structure but they are not ionic compounds rather do they resemble the corresponding nitrides and phosphides in simulating alloy systems in many of their properties. For this reason they will be discussed later. 

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