Spectroscopic and crystallographic anomalies of (Co1−xZnx)Al2O4 spinel oxide

Abstract

This work investigates the spectroscopic properties of (Co_1−xZn_x)Al₂O₄ with a range of x of 0 ≤ x ≤ 1. Spectroscopic and crystallographic evaluations using XRD, Raman, FT-IR and UV-Vis spectroscopy reveal that Zn²⁺ substitution systematically changes the lattice constant, which mainly depends on the Co–O bonds, and the related optical characteristics of this material. The x dependence of these properties shows two trends, and the mutation point seems to be at x ≈ 0.5. This implies that the electronic structure of (Co_1−xZn_x)Al₂O₄ is not changed monotonically by Zn²⁺ substitution. Interestingly, some of the optical phenomena observed in this study become prominent for samples with x ≥ 0.5. That is, we observed sideband peaks near the main peaks in the Raman spectra, and their relative intensities systematically and significantly increased with increasing Zn²⁺ substitution. The rates of increase are not constant, and are fast for samples with x ≥ 0.5. The sideband peaks are considered to reflect the unique changes in the local electronic structure of (Co_1−xZn_x)Al₂O₄, and they are useful for evaluating the substitution level without the influence of the site change phenomenon. Thus, clarifying them is expected to be important for understanding and controlling the electronic structure of the spinel oxide. On the other hand, investigation of the visible light absorption due to the d–d transition of Co²⁺ reveals that the efficiency is also high for samples with high Zn²⁺ substitution (x ≥ 0.5). This is also considered to be valuable information for investigation of the optical properties and/or the catalytic function of the spinel oxide. Moreover, the fluorescence of the (Co_1−xZn_x)Al₂O₄ samples is also identified as a novel functional property of this material. The intensity of the fluorescence peak also dramatically increases for samples with x ≥ 0.7. The effect of Zn²⁺ substitution on the local electronic structure of (Co_1−xZn_x)Al₂O₄ has not been clarified yet. However, some of the interesting characteristics reviewed in this study are worth investigating from the viewpoint of materials science and applications.