Structure-dependent luminescence of Tm³⁺-doped LaAlO₃: from crystalline to amorphous states

Tm³⁺-doped LaAlO₃ powders (x = 0.1-5 mol%) were synthesized in crystalline and amorphous forms to investigate the effect of structure and concentration on luminescent properties. Both crystalline and amorphous samples showed strong ¹D₂ → ³F₄ emission at 453 nm, with the strongest emission recorded at 5% and 2% Tm³⁺ for crystalline and amorphous sample, respectively. In amorphous LaAlO₃, excitation was limited to intra-4f transitions, with reduced emission intensity and shorter lifetimes, suggesting higher non-radiative losses. The low symmetry of the Tm³⁺ ions allowed for transitions from ¹G₄ level, which are absent in its polycrystalline analog. Energy transfer followed dipole-dipole (polycrystalline) and dipole-quadrupole (amorphous) mechanisms for ¹D₂ transitions. The ¹G₄ transitions in amorphous LaAlO₃:Tm³⁺ followed a dipole-dipole mechanism.