Electrons valence, transition metal nitrides

Ti) solid solution and simple transition metal nitrides are classified using the radius ratio of nonmetal to metal atoms and the number of valence electrons. The relationship of the generalized number of valence electrons instead of the average number of valence electrons per atom to the thermal stability of transition metal nitride has been discussed. 

In this paper the thermal stability of several metal nitrides is discussed firstly with the number of valence electrons based on the information acquired experimentally. In the second part, the thermal stability is investigated by the electron theory using results of the discrete variational (DV)-Xct molecular orbital calculations for some transition metal nitrides. 

It has been found for examining empirically the thermal properties of the metal nitrides that the number of valence electrons is more advantageous than the average number of valence electrons per atom. DV-Xa molecular orbital calculations for several metal nitrides reveal that the thermal stability of transition metal nitride is intensely dominated by the bond overlap population of the metal-metal bond. 

A mixture of metallic, covalent and ionic components prevails in the bonding of transition metal carbides, nitrides, and carbonitrides. The metallic character is shown by the high electrical conductivities of these compounds. The bonding mechanism has been described extensively by a variety of approaches for calculating the density of states (DOS) and hence the electron density in f.c.c. transition metal carbides, nitrides, and oxides [11]. In the DOS of these compounds there is a minimum at a valence electron concentration (VEC) of 8, which corresponds to the stoichiometric composition of the group ivb carbides TiC, ZrC, and HfC. Transition metal carbides have a lower DOS at the Fermi level than the corresponding transition metal nitrides, hence the electrical properties such as electrical and thermal conductivity and the superconducting transition temperature, T, are lower than those of the nitrides. 

We note that the valence orbitals of metal atoms order in energy as AE>Ln>M. The d-levels of transition elements (M) range the lowest, and are therefore most sensitive for reduction, or to form a stable binary metal nitride. This may also explain the virtual absence of d-element compounds with 16 (valence) electron species, such as [N=N=N] , [N=C=N] , [N=B=N] T [C=C=CfT or [C=B=C] T at least through high-temperature syntheses. 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

The similar catalytic properties of transition metal carbides and nitrides to those of noble metals can be attributed to their similar electronic properties and stracture. The valence electron count of WC is similar to that of Pt (1). 

Titanium mononitride S-TiN c (0.42 < x < 1.0) has wide solubility from 10.93 to 22.63% of interstitial nitrogen [ 11. The TiN is stable golden compound of face-centered cubic (Bl) structure. Strictly speaking, stoichiometric composition TiNi.o cannot exist under atmospheric pressure, but substoichiometric composition TiNo.97 is stable. For this reason, the atomic vacancies [2] in sublattices of Ti and N reduce height of Fermi level based on valence electron concentration (VEC) of transition metal compounds [3], resulting in stabilization of band structure. It was experimentally reported that the VEC value of the transition metal compounds is stable at 8.8 under atmospheric pressure [4]. In case of titanium nitride, TiNo.ge is the most stable composition. 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

---