Dinuclear complex formation in TaCl 5–AlCl 3 molten mixtures: Vibrational modes and thermodynamics

Abstract

Raman spectroscopy has been used to obtain structural information on TaCl 5–AlCl 3 molten mixtures ( X TaC l 5 0 = 0 – 1 ) at temperatures up to 235 °C under static equilibrium conditions. The observed band wavenumbers and polarization characteristics along with the composition effects and the temperature-dependent features of the Raman spectra obtained for the molten mixtures indicate the occurrence of the dinuclear TaAlCl 8 molecular liquid complex (m.p. ∼120 °C) in equilibrium with molten Ta 2Cl 10 and Al 2Cl 6. The spectra were interpreted in terms of a C 2 υ configuration for the TaAlCl 8 molecule consisting of a TaCl 6 octahedron sharing an edge with a AlCl 4 tetrahedron. The spectral changes observed upon freezing indicate that the proposed configuration for the dinuclear complex is maintained in the polycrystalline state. The liquid TaAlCl 8 complex undergoes a gradual dissociation to its components with increasing temperature. The thermodynamics and the extent of the reaction 1/2Ta 2Cl 10(l) + 1/2Al 2Cl 6(l) ⇆ TaAlCl 8(l) were probed by applying a formalism for correlating the relative Raman band intensities with the stoichiometric coefficients, the equilibrium constant and the thermodynamics of the reaction. The enthalpy of the reaction was determined from the Raman data as Δ H° = − 12.1 ± 1.5 kJ mol −1 and its value is discussed in terms of the coordination number and local geometry of Ta and Al atoms within the TaAlCl 8 molecule.