Visible photoluminescence from MgAl2O4 spinel with cation disorder and oxygen vacancy

Abstract

Magnesium aluminate spinel, MgAl2O4, is a structurally and compositionally interesting crystal since it can exhibit significant deviations from the stoichiometric composition because of the ability of the lattice to tolerate substantial cation disorder. We show that MgAl2O4 spinels with excess Al2O3 non-stoichiometry can accommodate a large amount of positively charged oxygen vacancies (F+ centers) when they are heated under vacuum at ∼1900 °C using a high-frequency induction heating unit with a graphite crucible. Thus the obtained spinels show a bluish white photoluminescence (PL) with a PL quantum yield of ∼20%, which is attributed to excitation and recombination processes of the F+ center. The PL signal is characterized by a broad spectral band peaking at ∼2.7 eV (∼460 nm) and decays in a nonexponential manner on the time scale of several tens of microseconds. The F+ center is supposed to interact with nearby positively and negatively charged defects derived from cation disorder in the spinel structure, forming clusters of various defect centers.