Fluorine-doped lithium magnesium phosphate has been studied for the first time. It has been shown that fluorine significantly enhances the intensity of thermally stimulated luminescence. To find the preferred positions of fluorine and structural distortions caused by aliovalent substitution, ab initio calculations have been performed, which demonstrate that fluorine is not included into the (PO4)3– anion; rather, it promotes the formation of clusters simultaneously containing lithium and fluorine ions.