Intrinsic and Dopant-Related Luminescence of Undoped and Tb Plus Tm Double-Doped Lithium Magnesium Phosphate...

In this work, the luminescence properties of undoped, Tm³⁺ doped, and Tb³⁺ plus Tm³⁺ double-doped crystals of the lithium magnesium phosphate (LiMgPO₄, LMP) compound were investigated. The crystals under study were grown from melt using the micro-pulling-down method. The intrinsic and dopant-related luminescence of these crystals were studied using cathodo-, radio-, photo-, and thermoluminescence methods. Double doping with Tb³⁺ and Tm³⁺ ions was analyzed as these dopants are expected to exhibit an opposite trapping nature, namely to create the hole and electron-trapping sites, respectively. The spectra measured for the undoped samples revealed three prominent broad emission bands with maxima at around 3.50, 2.48, and 1.95 eV, which were associated with intrinsic structural defects within the studied compound. These were expected due to the anion vacancies forming F⁺-like centers while trapping the electrons. The spectra measured for Tb and Tm double-doped crystals showed characteristic peaks corresponding to the 4f-4f transitions of these dopants. A simplified model of a recombination mechanism was proposed to explain the temperature dependence of the measured thermally stimulated luminescence spectra. It seems that at low temperatures (below 300 °C), the charge carriers were released from ⁵D₃-related Tb³⁺ trapping sites and recombination took place at Tm-related sites, giving rise to the characteristic emission of Tm³⁺ ions. At higher temperatures, above 300 °C, the electrons occupying the Tm³⁺-related trapping sites started to be released, and recombination took place at ⁵D₄-related Tb³⁺ recombination centers, giving rise to the characteristic emission of Tb³⁺ ions. The model explains the temperature dependence observed for the luminescence emission from double-doped LiMgPO₄ crystals and may be fully applicable for the consideration of emissions of other double-doped compounds.