General relationship between the band-gap energy and iodine-oxygen bond distance in metal iodates

Abstract

A wide band gap is a fundamental requirement for an ideal nonlinear optical material. A two-step investigation has been implemented to determine the relationship between the band-gap energy and the iodine-oxygen bond distance in optically nonlinear metal iodate materials. Firstly, we utilized high-pressure conditions on Mg and Zn iodates to correlate the pressure-induced changes in band-gap energy with changes in the iodine-oxygen bond distances. On both cases, the band-gap energy shows a nonlinear decrease under compression. The nonlinear behavior is a consequence of the interplay between the pressure-induced increase of the first-nearest neighbor iodine-oxygen bonds, which favors a narrowing of the band gap, and the decrease of the second-nearest neighbor iodine-oxygen bonds, which favors an opening of the band gap. The inverse correlation between the band-gap energy and the iodine-oxygen bond distance is confirmed in the second part of the investigation by collating and comparing the band-gap energies and corresponding average iodine-oxygen bond distances of the metal iodates reported in the literature. In the comparison, only nontransition and closed-shelled transition metals were included, without regard for their chemical formula, crystal structure, or stoichiometry.