## 3 Results
The strong affinity between nitrogen and tungsten provides the driving force to form tungsten nitrides, as a result of a redox reaction between metal and molecular nitrogen. Nitrogen molecules are chemisorbed on tungsten and the triple bond is then progressively weakened. Further reduction of nitrogen proceeds with the formation of an \"azo-complex\", where each nitrogen atom is bonded to metal atoms and the double bond is hence reduced to a nitride derivative. The whole process requires high temperatures or, as in the biochemical system of nitrogen fixation (where nitrogen is reduced to ammonia derivatives), the aid of an enzyme nitrogenase that uses molybdenum or tungsten cofactors [25]. In a sputtering reactor, high vacuum conditions and ejection of reactive species from the target make nitride synthesis possible at relatively low temperatures. It is reasonable that processes at different powers should determine the formation of different-sized metallic clusters able to adsorb nitrogen and react with it. In fact the arrangement of metal atoms in a metallic cluster can exert a powerful influence on the surface chemical behavior with respect to the adsorption and dissociation of nitrogen. In other words, only a small amount of nitridation occurs by direct reaction with nitrogen-activated ions, while the greater part proceeds by a first stage of gas adsorption from different-sized metallic W clusters, followed by progressive reduction reactions. Hydrogen present in the sputtering chamber, or derived from water molecules, binds itself to the metal clusters and contributes to a minor extent to nitrogen reduction. Another aspect to be considered is that the amorphous W\({}_{\rm x}\)N, a solid solution of nitrogen in alpha tungsten, can itself be a target, especially at low target powers, and become sputtered itself. Once it has appeared on substrates, this amorphous phase can in turn adsorb other nitrogen and if the temperature is raised it can crystallize in W\({}_{2}\)N, as already described for the 300 W and 500 W films, or can evolve, expelling excess nitrogen and producing metallic tungsten, like in the 700 W.  
Annealing of the 700 W film at 470 \({}^{\circ}\)C resulted in a partial crystallization of the amorphous phase, forming a two-phase structure containing the thermodynamic one, W\({}_{2}\)N.  
Figure 9: (**a**) FTIR Reflectance comparison; (**b**) 300 W FTIR inset with evidenced signals. Further annealing at 580 \({}^{\circ}\)C introduced a second modification: together with the W\({}_{2}\)N it a second crystalline phase, bcc W, appeared. These modifications progressed with time and temperature because they are based on redox reactions at the interfaces. Another aspect of the evolution of tungsten nitrides after annealing can be observed when they are part of cermet materials (e.g., with AlN as the ceramic part in absorbing layers of the patented solar double nitride coatings described above [4]). For fabricating a AlN-WN cermet, AlN material (cermet matrix) was produced at a constant cathode power, while WN\({}_{\mathrm{x}}\) phases dispersed in the matrix were produced by varying the cathode power (with a power value decreasing in a stepwise manner for each layer. When created in this way, a graded cermet has a high metallic content at the start and a low metallic content at the end of process. As for single materials, the same applies as for cermet and the nitridation stage of metallic W influences its stability: barely nitridated materials (700 W) after annealing can dismutate into W and W\({}_{2}\)N, while an excess of nitrogen (300 W) undergoes crystalline readjustment with nitrogen release. For the purpose of verifying their stability inside the cermet, thin films of cermet nitrides deposited at the same three representative powers (700 W, 500 W and 300 W) were characterized by means of vibrational analysis before and after annealing.