Raman and optical absorption spectroscopic study of the thermochromic phase transition of Ag2HgI4.

The thermochromic transition of Ag₂HgI₄ has been studied by optical absorption and Raman spectroscopy. In the absorption spectra, the shift of the absorption edge towards lower energies at high temperatures is responsible for the yellow to red colour change of Ag₂HgI>₄; this absorption edge is associated with a charge transfer process between electronic levels of Hg and I. The resulting hysteresis loop is similar to those found by reflectance measurements in Cu₂HgI₄. The Raman spectra show hysteresis loops for the position and half width of the ν₁(HgI₄²⁻, Td). There is a different behaviour between the first and subsequent heating cycles. The observed changes in the phase transition can be explained by assuming an increase of the interaction between the I and Ag atoms above the phase transition, and a consequent decrease of the IHg interaction. These changes would be induced by a decrease of the AgI distance above the phase transition, as a consequence of a structural distortion; consequently the HgI distance would increase and lead to the observed changes in the absorption spectrum.