Luminescence and energy transfer processes in Gd_0.99Er_0.01Al_0.995Cr_0.05O₃

Gd_0.99Er_0.01AlO₃ and Gd_0.99Er_0.01Al_0.995Cr_0.05O₃ samples were synthesized using a solid-state reaction method. Structural analysis revealed that the samples crystallized in an orthorhombic structure phase with a Pbnm space group. The average crystallite sizes were around 283 nm and 574 nm for Gd_0.99Er_0.01AlO₃ and Gd_0.99Er_0.01Al_0.995Cr_0.05O₃, respectively. Derivative absorption spectrum fitting (DASF) and first-derivative reflectance (dR /dλ) methods confirmed that the samples possess a direct wide band gap, with energies of 5.93 eV and 5.90 eV, respectively. The photoluminescence (PL) spectrum of Gd_0.99Er_0.01AlO₃ under λ _ex = 377 nm excitation exhibits a green emission and intense sharp red lines at 680 nm, 697 nm, 705 nm, 717 nm and 758 nm. The green emission corresponds to the transitions ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 of Er³⁺ ions, while the sharp red lines are attributed to transitions between intrinsic defect centers related to the GdAlO₃ host coupled to B_3g (4) and B_1g (7) vibrational modes. Efficient energy transfer via resonant phonon-assisted and cross-relaxation processes from Er³⁺ and intrinsic defect centers to Cr³⁺ is responsible for the decrease in green and red emission line intensities in Gd_0.99Er_0.01Al_0.995Cr_0.05O₃. The energy transfer from Er³⁺ and intrinsic defect centers indicates that red emission lines at 697 nm and 726 nm in Gd_0.99Er_0.01Al_0.995Cr_0.05O₃ mainly originate from the ²T₁ (²G) → ⁴A₂ (⁴F) and ²E_g (²G) → ⁴A_2g (⁴F) transitions of Cr³⁺ ions.