The molecular vibration frequencies of the gallium–chlorine gaseous species were selected. For the known species Ga2Cl6, GaCl3 and GaCl selection was performed by comparing previous experimental values with our own Raman spectrometric determinations. For the unidentified species Ga2Cl4, Ga2Cl2 and GaCl2 frequency values were evaluated from theoretical literature considerations. The Raman spectra of the vapours obtained by heating condensed gallium and chlorine mixtures are discussed using the selected frequencies and taking into account the Ga2Cl6 and GaCl3 detection limits. With our Raman recording conditions corresponding to well defined spectra, the species are detected up to 5 × 10−3 at. Under these conditions, for Cl/Ga = 3 and between 70 and 500 °C, the main gaseous species are GaCl3 and Ga2Cl6 and the system should be described by the single equilibrium: Ga2Cl6(g)⇌2GaCl3(g). For Cl/Ga = 0.122–3, and between 300 and 800 °C, the representation of the system should be simplified to a single heterogeneous equilibrium between the major species: 2Ga(I)+GaCl3(g)⇌3GaCl(g). In this case, the evolution of the Raman spectra with temperature is explained by taking into account contribution of Ga2Cl4 in very small amounts.
Procedures for the quantitative treatment of Raman spectra are proposed for the investigation of gallium+hlorine gaseous mixtures. They are based on correlations between pressures and Raman band intensities and require a knowledge of reliable entropy and heat capacity data for the main gaseous species. For the ratio CI/Ga = 3 and between 590 and 690 K, the pressure determinations from the band intensity evolution corresponding to the symmetric vibrational modes 'ul' of Ga,CI, and GaC13 confirm the starting consideration of the single equilibrium Ga,C16(g) 2GaCl,(g). They lead to an enthalpy of dissociation of Ga,C16 very close to the literature data. For CI/Ga< 3 and between 800 and 950 K, the treatment of the Raman spectra is performed by computation of the GaCl spectrum in order to discriminate between the GaCI, 'vl' band intensity and the GaCl 'us' branch intensity. This intensity difference and the GaCl 'uQ' branch intensity are used for pressure determinations. The results are consistent with the proposed equilibrium 2Ga(l) + GaC13(g) S 3GaCl(g). They lead to a calculated reaction enthalpy that allow one to select a reliable value for the enthalpy of formation of GaCI.
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