Experimental determination and modeling of structural, vibrational and optical properties of the ZnAl2−xCrxO4 (x = 0 and 0.05) spinels

Abstract

The structural, vibrational and optical properties of the ZnAl2−xCrxO4 (x = 0 and 0.05) spinels (Fd 3‾ m space group), prepared by a conventional solid-state reaction, have been addressed in this work. The room temperature Raman and infrared (ATR-IR) spectra have been obtained and interpreted. The optical band gap Eg has been determined from reflectance and absorption UV/vis. spectra. Tauc's law confirmed the direct transition behavior. The observed increase in the Urbach energy Eu for ZnAl1.95Cr0.05O4 suggests that disorder and defects concentration are larger than in the undoped ZnAl2O4 spinel. From the absorption and photoluminescence excitation (PLE) spectra, the electronic structure of Cr3+ was determined by a crystal field study in the Oh symmetry site. The theoretical study led to a precise attribution of the Cr3+ d-d transitions. Using Group Theory and the notion of potential energy surface (diabatic and adiabatic), the origin of the spin-forbidden transitions in the studied materials was also investigated. The PLE spectra show the presence of vibronic progressions of about 485 cm−1, which were attributed to an O–Al–O asymmetric stretching vibration detected in the ATR-IR study. The contribution of a non-totally symmetric mode to the PLE spectrum proves the occurrence of the Jahn-Teller effect leading to a distorted CrO6 octahedral site. A detailed crystal field study is also presented for the D3d distorted symmetry site.