Vibrational Structures of Iodine-Vacancy Bismuth Oxyiodides Using Temperature-Dependent Low-Wavenumber Raman Spectroscopy

The reaction of BiOI to form iodine-poor derivatives under high temperatures was investigated by using temperature-dependent low-wavenumber Raman spectroscopy. The Raman spectra of BiOI and of the iodine-poor derivatives Bi4O5I2, Bi7O9I3, and Bi5O7I were recorded. The phonon mode displacement vectors were calculated in the software CRYSTAL, and the PBE0/pob-TZVP method was used to analyze these Raman bands. The appearance of a Raman band at 95 cm–1 of a heated sample of BiOI in the atmosphere was assigned to the production of Bi4O5I2, and up to 623 K, no other iodine-poor derivatives were generated. After 632.8 nm laser irradiation under vacuum for a few minutes before exposure to the atmosphere, Bi7O9I3 and β-Bi2O3 were produced at various laser powers. Red shifts of low-wavenumber bands with temperature were obtained, and the slopes of the linear dependence χ for three phonon modes of BiOI were obtained. The obtained χ values were more negative in the atmosphere than in a vacuum. This can be explained by the reaction of BiOI to form Bi4O5I2, during which the lattice volume may have been further expanded. The near-IR emission intensity of photoexcited BiOI was found to decrease with temperature. This turnoff process with a low activation energy is explained by thermal quenching of charge carriers.