Pressure stabilization effect on the donor–acceptor polyiodide chains in tetraethylammonium bis(diiodine) triiodide – insights from Raman spectroscopy

: Polyiodides present high bonding flexibility already at ambient conditions, and undergo significant pressure-induced structural deformations. Resonant Raman spectroscopy has been widely used to study I-I bonds in various polyiodides, but it carries a risk of photodecomposition due to the high visible-light absorption of iodine. In this study, tetraethylammonium (bis)diiodine triiodide (TEAI) has been investigated by resonant Raman spectroscopy up to 12.02(3) GPa. The effect of pressure on the intensities and positions of Raman bands has been evaluated and correlated with the interatomic I-I distances derived from high-pressure X-ray diffraction experiments. Pressure was shown to effectively stabilize TEAI against laser-induced photodecomposition, even after a long course of irradiation with the resonant laser light. Examination of a freshly exposed crystal surface revealed that TEAI superficially passivates with the layer of lower polyiodides, which prevents further iodine loss, and shows distinct pressure-induced behaviour.