Sol-Gel Combustion Synthesis, Crystal Structure and Luminescence of Cr3+ and Mn4+ Ions in Nanocrystalline SrAl4O7

Abstract

A series of strontium dialuminate SrAl4O7 nanopowders with the grossite-type structure doped with chromium and manganese ions were synthesized by the combined sol–gel solution combustion method with use of two different strontium salts. The Cr3+ and Mn4+ ions concentrations were varied from 0.05 to 5 at.%. Evolution of phase composition, crystal structure, and microstructural parameters of the nanocrystalline materials depending on the synthesis conditions, temperature of thermal treatment, and dopant content were investigated by the X-ray powder diffraction and the scanning electron microscopy techniques. Photoluminescent properties of SrAl4O7 nanophosphors activated with Cr3+ and Mn4+ ions were studied at room temperature. The samples exhibit typical photoluminescence in the deep-red spectral region, corresponding to d-d transitions in Cr3+ or Mn4+ ions. The intensity of this deep-red emission is dependent on the dopant concentration and annealing temperature. Features of the formation of octahedral surroundings around Cr3+ or Mn4+ ions are discussed.

Synthesis, Crystal Structure and Photoluminescent Properties of Red-Emitting CaAl4O7:Cr3+ Nanocrystalline Phosphor

“…[15,16]). The inset in Figure 6b shows the R lines of this center in comparison with a similar center previously observed by us in SrAl4O7:Cr 3+ [13]. The second one also has two narrow lines at 683.3 and 686.7 nm, which are superimposed on a broad emission band stretching up to 800 nm.…”

“…This center marked as Cr2 in SrAl4O7:Cr 3+ manifests a broad emission band centered at about 780 nm, with excitation bands at about 470 and 630 nm. It should be mentioned that the Cr 3+ center in a weak crystal field exhibiting a broadband emission was not discussed in our recent study of SrAl 4 O 7 :Cr 3+ [13]. Having observed this new center in the studied calcium aluminate, we revisited the earlier results and carried out additional measurements for SrAl 4 O 7 :Cr 3+ samples calcined at 1400 • C. The examinations confirmed that the Cr center in a weak crystal field does exist in this host (see Figure 7).…”

“…[15,16]). It should be mentioned that the Cr 3+ center in a weak crystal field exhibiting a broad-band emission was not discussed in our recent study of SrAl4O7:Cr 3+ [13]. Having observed this new center in the studied calcium aluminate, we revisited the earlier results and carried out additional measurements for SrAl4O7:Cr 3+ samples calcined at 1400 °C.…”

“…Two remote oxygen atoms located at distances of 2.89 Å could also be assigned to the nearest coordination sphere of Ca atoms. In this case, the coordination number (CN) of Ca becomes seven, and the corresponding polyhedron is shaped as a pentagonal bipyramid, which is similar to Sr atoms in the related SrAl 4 O 7 compound [12,13]. Thus, the crystal structure of CaAl 4 O 7 can be seen as a 3D grid of CaO 7 polyhedra connected with two kinds of apex-shared AlO 4 tetrahedra (Figure 4).…”

“…The room temperature PLE (a) and photoluminescence (b) spectra of CaAl4O7:Cr 3+ nanocrystalline powders calcined at 1200 °C. The inset in Figure6bdemonstrates the R lines of Cr1 center at T = 300 K in CaAl4O7:Cr 3+ in comparison with those of SrAl4O7:Cr 3+ studied in[13] (red and blue lines, respectively).…”