Abstract
The phosphor powders of Ca(m/2)−xEuxSi12−(m+n)Alm+nOnN16−n (m = 1.6, n = 0.8, x in the range of 0–0.08) were synthesized by means of a solid state reaction in flowing nitrogen in a carbon resistant furnace and the influence of Eu concentration on the crystal structure and photoluminescent properties was thoroughly studied. The optical properties of selected α-sialon:Eu2+ samples at temperatures in the range of 10 to 500 K and pressures up to 240 kbar are presented. The crystal lattice parameters were affected by doping with europium and some increase of the unit cell volume was observed up to 6 mol % of Eu. The higher concentration of europium led to subtle changes in the overall structure of the produced sialon phosphors. It was shown that the chemical composition of Ca, Eu-α-sialon phosphor was slightly different from the designed one and the phosphor powders were contaminated by AlN. The phosphor particle surface showed significant europium and oxygen enrichment with Eu3+ but below the thin surface layer Eu2+ was dominant and higher nitrogen content was observed. After examination of absorption, excitation, and emission spectra it was found that the emission peak position shifted toward longer wavelengths with rising Eu2+ concentration from 565 nm (0.1 mol % Eu2+) to 585 nm (10 mol % Eu2+). The quantum yield of the phosphors reached the maximum at a rather low concentration of 4 mol % of Eu. Excitation spectra depend on the monitored wavelength which is typical for multisite Eu2+. The existence of many Eu2+ sites in the sample was supported by the dependence of the decay time on the monitored wavelength.
Highlights
The application of white light emitting diodes (LEDs) in general lighting and in display devices is an active issue in todays energy saving world
The obtained Ca(0.8−x) Eux Si9.6 Al2.4 O0.8 N15.2 (0 < x < 0.08) series samples are all indexed by the Ca-α-sialon phase (ICDD-98-020-1683) but all were contaminated by an amount of AlN, increasing in samples with Eu concentration over 4 mol % (Figure 1, Table 1)
From our experience in manufacturing various sialons we know that a slight change in the initial composition usually leads to various chemical compositions of the resultant sialon powder. α- and β-sialons form as solid solutions with the concurrent and equivalent substitution of oxygen for nitrogen the actual chemical composition of the given sialon is sensitive to the local concentration of elements and partial pressure of bothof gases
Summary
The application of white light emitting diodes (LEDs) in general lighting and in display devices is an active issue in todays energy saving world. A relatively long wavelength of the Eu2+ emission in nitride based phosphors is observed and attributed to the high covalence of the Si-N lattice bonds and the large crystal field splitting of the 4f6 5d1 electronic configuration of the Eu2+ coordinated by nitrogen anions. These properties together with strong absorption of light in the range from ultraviolet to blue together with high thermal resistance and chemical stability enables them to be used as excellent phosphors in white LED systems [1,7]. One of the purposes of this research was to investigate the changes of luminescence properties with respect to bond lengths changed by hydrostatic pressure instead of by varying the chemical composition
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