Infrared study of high-pressure molecular phases of carbon dioxide

The infrared absorption spectra of the high-pressure crystalline phases II, III, and IV of solid CO2 were studied by using a resistively heated diamond anvil cell up to 30 GPa. The employment of crystal slabs having thickness of similar to 2 mu m allowed the study of the strongly absorbing fundamental bending and antisymmetric stretching modes without saturation. These are the first data for phases II and IV in the fundamental modes spectral region; furthermore the high sample quality allowed an improved characterization of the infrared spectra of phases I and III as compared with previous studies. A comparison of the spectral structure and of the frequency evolution with pressure of the crystal modes between phase I and the higher-pressure phases clearly indicates the close resemblance among all these phases. In particular, the dramatic change of the intermolecular interaction claimed for phases II (dimeric association) and IV (large molecular bending) can be ruled out and, as a consequence, the hypothesis of a transition from the molecular phase I to the silica-like phase V through intermediate nonmolecular phases can be discarded.