Abstract
1 mol% Re2O3/MnO – 99 mol% MAl2O4 (Re = Eu, Sm, Dy, Er and M = Ca and Sr) powder samples were prepared by solid state sintering in the temperature range of 1386–1450 °C and their structural, thermal and photoluminescence properties were compared. X-ray diffraction and Rietveld refinement studies of calcium aluminate samples confirm the formation of monoclinic CaAl2O4 along with a small concentration (1.9–4.6 wt %) of monoclinic CaAl4O7 while the strontium aluminate samples contain monoclinic SrAl2O4 and a small amount (0.7–3.5 wt %) of cubic Sr3Al2O6. The short-range structure of CaAl2O4 consists of CaO6, CaO9 and AlO4 structural units while that of SrAl2O4 contains SrO9 and AlO4 units. The Sr–O bond-lengths (2.32–3.58 Å) are longer than Ca–O bond-lengths (2.04–3.18 Å) and Al–O bond-lengths are of the range 1.35–2.12 Å. Differential scanning calorimetry studies reveal high thermal stability of calcium aluminates, while strontium aluminate samples show reversible monoclinic to - hexagonal structural phase transition at 688 °C during heating cycle. Raman spectra exhibit Al–O vibrational modes. Photoluminescence studies show reddish-orange emission in Eu, Sm and Mn-doped samples, Er doped samples show green emission and Dy-doped samples emit yellow-white light. Calcium aluminate samples show significantly higher luminescence intensity and quantum yield than strontium aluminates.
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