Thermodynamic properties of crystalline and gaseous lower niobium chlorides

Thermodynamic functions of crystalline and gaseous niobium chlorides in lower state of oxidation are presented. The thermodynamic properties of solid niobium chlorides including their clusters Nb3Cl8 (NbCl2.67) and Nb6Cl14 (NbCl2.33) in the temperature range from 298 to 1300 K were calculated using experimental low temperature isobaric heat capacity measurements, thermogravimetric and tensimetric data on the process of their thermal decomposition and mass-spectrometric determination of vapor phase composition during this process. The thermodynamic properties of gaseous lower niobium chlorides in the temperature range 298-2000 K were calculated using vibration frequencies of these molecules. The latter were obtained using suggested molecular structures by a method involving the application of the force constants estimated on the basis of vibration fre-quencies of vapors of titanium, zirconium and vanadium tetrachlorides in terms of modified valence force field. The vibration frequencies were used to calculate the thermodynamic functions of gaseous niobium lower chlorides in terms of the rigid rotator-harmonic oscillator approximation. The obtained thermodynamic data allowed us to calculate the composition of the vapor phase in the system Nb=NbCl5 (vapor) in the temperature range from 700 to 2000 K for pressures 10-6 and 1 atm. The mechanism of thermal decomposition of niobium cluster Nb6Cl14 (NbCl2.33) was determined under pressure of 1 atm and in vacuum.