Cationic distribution and its influence on the luminescent properties of $Fe^{3+}$ -doped $LiAl_5O_8$ prepared by wet chemical methods

The cationic distribution during the phase transition of the synthetic mineral $LiAl_5O_8$ from the ordered primitive cube to high temperature spinel form is investigated using samples prepared by three different wet chemical routes, viz. gel to crystallite conversion, hydrothermal method, precipitation from sodium aluminate, and the ceramic method. The solid-state magic angle spinning nuclear magnetic resonance spectra (MAS NMR) of $^7Li\hspace{2mm}and\hspace{2mm}^{27}Al$ indicate that all the $Li^+$ ions are in octahedral sites and remain unaffected by the transformation, whereas the disorder involves the redistribution of $Al^{3+}$ ions in octahedral (o) and tetrahedral (t) sites. Substitution of aliovalent ions such as $Zn^{2+}$ and $Mg^{2+}$ alters the t /o ratios. The deep red emission spectra of $LiAl_5O_8 :Fe^{3+}$ are affected by cation distribution. The electron paramagnetic resonance spectra (EPR) indicate the presence of $(Fe^{3+} –V_0)$ defect complex, i.e. $Fe^{3+}$ ions associated with the oxygen vacancies, in addition to the prevalence of $Fe^{3+}$ in both 4- and 6- coordinated sites having axial distortion. The luminescence spectra of $LiAl_5O_8 :Fe^{3+}$ are explainable in terms of defect-related multiple centres involving ferric ions