Mass spectrometric studies of the Decomposition and the Heat of Formation of CuInS2(s)

The equilibrium vaporization of CuInS₂(s) was studied by Knudsen cell mass spectrometric techniques in the temperature range 902—1110 K. Based on these and independent congruency studies, CuInS₂(s) decomposes under steady-state conditions according to the reaction 2CuInS₂(s) = Cu₂S(s) + In₂S(g) + S₂(g). During the initial transient period of the vaporization of CuInS₂, the ion intensities of In₂S⁺ and S₂⁺ change and assume steady-state values after some time. A second-law evaluation of intensity versus temperature data, obtained under steady-state conditions, yielded a value for the heat of the above decomposition reaction (2 moles of CuInS₂) of δH 630.9 ± 26 kJ. With this quantity and auxiliary thermochemical data, the enthalpy of formation of CuInS₂(s) was computed to be δH = ?221.7 ± 13 kJ/mol.