The Mössbauer effect of SnI₄ at high pressures

Tetravalent tin iodide is a molecular crystal composed of SnI₄ tetrahedra loosely packed into a cubic configuration. Under pressure SnI₄ becomes metallic at about 15 GPa. We report a Mossbauer effect study of¹¹⁹Sn and¹²⁹I in SnI₄ to pressures of 26 GPa. The spectra exhibit dramatic changes with pressure starting at about 10 GPa and show large pressure hysteresis effects upon reducing the pressure from 26 GPa. In the intermediate region tin exists in both Sn⁴⁺ and Sn²⁺ states, and iodine exists in two nonequivalent sites characterized by a different symmetry and different sign and magnitude of the electric field gradient.