Temperature-dependent phase evolution of the Gd³⁺ doped strontium compounds: Structural and optical properties

Mixed-phases of the SrCO₃/Sr₃Al₂O₆/SrAl₂O₄/SrAl₄O₇/SrAl₁₂O₁₉ (hereafter called SSSSS) doped with Gd³⁺ (SSSSS: 0.05 % Gd³⁺) nanopowders were prepared using the citrate sol–gel method. The effect of annealing temperature (T_a) on the structure, morphology, and photoluminescence (PL) properties was investigated. The T_a was varied in a range of 600–1300 °C, while the dopant concentration was kept constant at 0.05 % Gd³⁺. X-ray diffraction (XRD) results revealed that the prepared nanopowders consist of a mixture of orthorhombic (SrCO₃), cubic (Sr₃Al₂O₆), monoclinic (SrAl₂O₄ and SrAl₄O₇), and hexagonal SrAl₁₂O₁₉ structures. It was noted that increasing the phase composition depends on the T_a. The Fourier-transform infrared spectroscopy (FTIR) shows several peaks at around 529, 874, 1181, 1442, 2947, and 3421 cm⁻¹. The Energy dispersive spectroscopy (EDS) confirmed the elementary composition of the Sr, C, O, and Al. The scanning electron microscope (SEM) images showed that the phase composition highly depends on the T_a. Transmission electron microscope (TEM) showed that the particle sizes are in the nanoscale. The ultraviolet–visible (UV–vis) diffuse reflection spectroscopy results showed that the absorption peaks below 300 nm are influenced by the T_a. The PL results showed three emission peaks located around 431, 541, and 657 nm when excited at 272 nm. These emission peaks were attributed to SrCO₃, SrAl₂O₄, and Sr₃Al₂O₆ crystal lattice, respectively. The results showed that the luminescence intensity and lifetime of the prepared samples depends on the T_a. The chromaticiy coordinate showed that all samples emit in a blue region and the color purity was influenced by the T_a.