Kravets O. Luminescence of magnesium and zinc gallate doped with manganese and europium

This thesis dedicated to investigate the effect of co-doping with Mn2+ and different concentration of Eu3+ ions on the optical-luminescent properties of magnesium and zinc gallate compounds (MgGa2O4, ZnGa2O4) with spinel structure and solid solutions of these compounds (Mg1-xZnxGa2O4), and to establish the role of crystal structure defects in light emission and excitation energy transition processes for application of these compounds as phosphor materials with multiple emission colors. The single phase spinel structure of all obtained ceramic materials was testified by X-ray diffraction measurements with application of Rietveld refinement. Positron annihilation time spectroscopy revealed cation vacancy and multi-vacancy defects in the structure of investigated spinels and verified successful incorporation of Eu3+ ions into the spinel host. The luminescence of matrix and Mn2+ ions in the MgGa2O4 and ZnGa2O4 compounds were found to be excited in the region of fundamental absorption edge. The ceramics of the Mg0.5Zn0.5Ga2O4: Mn2+, Eu3+ solid solution exhibits an order higher excitation intensity of the matrix luminescence band due to large number of structural defects. In all of the co-doped ceramic samples the Eu3+ ions were excited in the broad charge transfer band (O2--Eu3+) and narrow lines of intracenter transitions in Eu3+ ions. The photoluminescence emission spectra of all gallate ceramic samples co-doped with Mn2+ and Eu3+ ions show emission bands of matrix (350-475 nm), Mn2+ ions (475-575 nm), and Eu3+ ions (575-650 nm). It has been shown that incorporation of Eu3+ ions and rising of their concentration affects all of the observed luminescence bands. However, it changes neither character and spectral position. An optimal concentration of europium ions was found for MgGa2O4: Mn2+, Eu3+ and ZnGa2O4: Mn2+, Eu3+ ceramics, taking values approximately 4 and 3 mol.%, respectively. The increase of the Eu3+ ions concentration leads to suppression of the matrix luminescence intensity in co-doped magnesium and zinc gallate compounds. However, the emission intensity of Mn2+ ions increases with the introduction of Eu3+ ions and reaches a maximum around 5 mol.% in the magnesium gallate compound. The obtained concentration dependences indicate energy transfer between the matrix, the Mn2+, and Eu3+ ions. The reduction of the luminescence decay constant of Mn2+ ions after co-doping with Eu3+ ions confirms the assumption about the energy interaction between activator ions. The coordinates of luminosity CIE (Commission Internationale de l'Éclairage) diagram were calculated and showed that all ceramics exhibit variation of the emission color from blue to red. The color modification can be employed either by changing the composition of gallate ceramics or by tuning concentration of activator ions. This reveals that investigated ceramics of magnesium and zinc gallates together with their solid solutions co-doped with Mn2+ and Eu3+ ions can be used for application as phosphors with possibility of controlling the emission color.