Mass Spectrometric Study of Thermal Decomposition Process of Niobium Clusters Nb 3 Cl 8 and Nb 6 Cl 14

The thermal decomposition process of solid niobium clusters Nb₃Cl₈ (NbCl_2.67) and Nb₆Cl₁₄ (NbCl_2.33) was studied by effusion methods with mass spectral registration of the vapour composition and pressure and their dependence on temperature. It is shown that the NbCl_2.67 cluster decomposes to the NbCl_2.33 cluster with formation of niobium tetrachloride in the gas phase. The process is monovariant. The NbCl_2.33 cluster decomposes to niobium dichloride. The process is bivariant because the homogeneity region is formed on the basis of the NbCl_2.33 crystal lattice. The composition of the vapour phase changes with an Nb: Cl atomic ratio which decreases during the process. At the beginning of the process the vapour consists of niobium tetrachloride and at the end the vapour is only niobium trichloride. The enthalpy of thermal decomposition of NbCl_2.67 (1.67 Nb₃Cl_{8 (s)} = 0.67 Nb₆Cl_{14 (s)} + NbCl_{4 (g)}) which is Δ_rH°(700K) = 287.4 ± 3.8 kJ mol⁻¹ and the formation enthalpy of NbCl_2.33, which is Δ _f ^H °(Nb₆Cl₁₄, 298K) = – 479 ± 10 kJ mol⁻¹, were calculated from experimental data.