Spectroscopic studies of Ti³⁺ ions speciation inside MgAl₂O₄ spinels

An electron paramagnetic resonance (EPR) investigation on Ti-doped MgAl₂O₄ spinels has been made in order to study the Ti³⁺ sites. The study we present here concerns the angular dependencies of the Ti³⁺ EPR lines and the variations of the EPR spectra due to a modification of the TiO₂ content or of the chemical composition from MgAl₂O₄ to MgAl₄O₇ and MgAl₆O₁₀. In all the studied samples except one, we observe the presence of both Mn²⁺ and Ti³⁺ ions. No correlation was observed between Ti³⁺ amount and TiO₂ content; the titanium ions are located in three different sites: the octahedral B site of the spinel structure; the tetrahedral A site and a last site which remains unclear. On the contrary, the major part of the Mn²⁺ ions is assumed to be in the tetrahedral A site of the spinel structure and a minor part in the octahedral B site. Our work demonstrates the overall interest of EPR spectroscopy in the study of the paramagnetic optically active ions inside optical materials.