Effects of Cr content and morphology on the luminescence properties of the Cr-doped alpha-Al2O3 powders

Al₂O₃:Cr³⁺ samples were synthesized via hydrothermal and microwave solvothermal methods and thermal decomposition of Cr³⁺ doped precursors. The sample characterizations were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD results indicated that Cr³⁺ doped samples were pure α-Al₂O₃ phase after being calcined at 1573 K. SEM results showed that the length and diameter of these Cr³⁺ doped alumina microfibers by hydrothermal route were about 2–5 μm and 100–300 nm, respectively; the obtained α-Al₂O₃ based powders via the microwave solvothermal method were microspheres with an average diameter about 1–2 μm. PL spectra showed that the Al₂O₃:Cr³⁺ samples presented a broad R band at 696 nm. It is shown that the 0.3 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ is optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 24 Å. It is found that the curve followed the single-exponential decay. Furthermore, the luminescence properties of the samples are also dependent on the morphology.