**3. Equilibrium states in M-F systems**

Temperature dependencies of equilibrium compositions in the M-F systems (M = V, Nb, Ta, Mo, W, Re) are presented at the Fig.1. The data represent the thermodynamic stability of the refractory metal fluorides with different valencies both monomer and polymer states depending on the place of the metal in the Periodic table. The gas phase composition depends on both the heat of the fluoride formation and the vaporation heat of the fluorides. The thermodynamic analysis of M-F systems shows that the highest fluorides of the metals are stable at temperatures up to 2000 K. The exceptions contain the fluorides VF5, MoF6, ReF6 that decompose slightly at the high temperature range and their thermal stability increase according to the following order: VF5 > MoF6 > ReF6 .

The gas low-valent fluoride concentrations, which depend upon the metal place in the periodic system, rise with the increase of atomic number within each group and decrease with the increase of atomic number within each period. Thus tantalum fluorides are most strongly bonded halids and vanadium fluorides are most unstable among considered fluorides. It is nesessary to note that partial pressures of low valent fluorides in Re-F system are close to each other but low valent fluorides in Ta-F system have very different concentrations.

Nevertheless the vaporation temperature of fluorides varies depending upon the metal place in the periodic system in opposite direction than the gas low-valent fluorides concentration. The most refractory fluorides are VF2 and VF3 (above 1500 K), the low-valent fluorides of Nb and Mo possess the mean vaporation temperature (900-1100 K). Th lowvalent fluorides of tantalum, tungsten, rhenium have the lowest vaporation temperature (500-550 K).

The peculiarity of the fluorides is the possibility of their polymerization. It is known that dimers or threemers are observed in gas state but tetramer clasters of Nb, Ta, Mo, W fluorides and chains of V, Re fluoride polymers are forming in solid state [30]. For example, fluorides W2F8, W2F10 and Mo2F6, Mo2F8, Mo2F10 exist in W-F and Mo-F system, correspondingly. The main structural state of Nb, Ta, Mo, W, Re fluoride polymers are threemers but vanadium pentafluoride does form polymer state. M3F15 polymers are forming by the single M-F-M bonds but the fluoride dimers have double fluorine bridge bonds. The exception are dimer molecules V2F6, V2F8, Re2F8 with the M-M bonds. All polymer states are presented in tables 1-3.
