Raman spectra of InI, InI2 and InI3. Correlation between structure and thermodynamic properties of fused InxI1–x mixtures

Abstract

We have measured the Raman spectra of InI, InI₂ and InI₃ in both the high-temperature solid and the melt. Each of these materials, except for InI₃, undergoes a polymorphic transition at high temperatures that can be detected by recording the Raman spectra as a function of temperature, as well as by scanning calorimetric analyses. The Raman results for InI₂ demonstrate that the In⁺ ion is surrounded by units of the complex anion InI^–₄; since both In^I and In^III exist as separate entities in InI₂ it can be called an inhomogeneously mixed-valent compound. These results at room temperature are consistent with the previous i.r. study. The Raman spectra of molten InI₂ and InI₃ are given by parallel and perpendicular components. The decomposition of the parallel spectrum led to the conclusion that the complex ion InI^–₄ is stable as a host for anions in the molten InI₂. Previous work on the thermodynamic properties of fused In_xI_1–x mixtures showed that the partial molar entropy of In has a drastic excursion near x= 0.333 (or InI₂), whereas in the corresponding region there is a prominent change in the liquid structure from a Coulombic interaction between In⁺ and I^– in molten InI to that between In⁺ and InI^–₄ as an anion host. The complex anions are stabilized by covalent bonding centred on In^III because of the magnitude of the stretching force constant K of the In—I bond in InI^–₄.