A.L.N. Stevels

Effect of non-stoichiometry on the luminescence of Eu2+-doped aluminates with the β-alumina-type crystal structure

Abstract In Al 2 O 3 rich Ba-aluminate: Eu phosphors a green luminescence band involving Eu is found next to the blue Eu 2+ -luminescence. The green band is ascribed to associates of Eu and oxygen ions at Ba sites. These O Ba ions also influence the concentration quenching behaviour of the Ba aluminate with high Eu content. The drop in the quantum efficiency of Ba-aluminate: Eu 2+ on decreasing the Eu content of phosphors is not related to the presence of O Ba . It is explained by the presence of Ba-vacancies. No green luminescence was observed in Mg-rich β-alumina type Eu 2+ phosphors. Also, the quenching of the luminescence at higher Eu concentrations is less. Most probably, Mg 2+ ions, positioned in between O Ba and Eu 2+ , block the interaction between last-named ions.

Red Mn2+-luminescence in hexagonal aluminates

Abstract On UV excitation, a red luminescence band peaking at 690 nm is observed in Eu, Mn doped aluminate lattices containing large trivalent cations and high Mn2+-concentrations. The green Mn2+ luminescence band occuring at lower Mn2+ contents is quenched by the appearance of the red band. The excitation spectra of the red luminescence show that energy transfer takes place from the green Mn2+ centres which are located at tetrahedrally surrounded sites to the red emitting Mn2+ centres. The last-named are most probably Mn2+ ions surrounded octahedrally by oxygen ions. One of these oxygen ions is located at the crystallographic position normally occupied by the large trivalent cation. The dependence of the intensity of the red luminescence on the Mn2+ concentration in La-aluminate samples can be explained if it is assumed that the Mn2+ ions in tetrahedral sites (the green emitting ones) are all located near the oxygen ions at the large cation sites.

Effects of defects on the quantum efficiency of Eu2+ -doped aluminates with the magnetoplumbite-type crystal structure

The luminescence of SrAl12O19 : Eu2+ phosphors can be quenched by substitution of a small amount of Mg for Al, or by preparing samples with excess Al2O3. It is most probable that oxygen vacancies near the Eu2+ activator ions act as the radiationless sinks of the excitation energy. In CaAl12O19 : Eu2+ the quenching of the luminescence by the introduction of defects is much less effective, so that less oxygen vacancies are supposed to be formed. In view of the fact that the stability of CaAl12O19, with respect to less oxygen containing phases, is higher than that of SrAl12O19, this is a reasonable assumption. The homogeneity range of La1−xAl1123 + xO19 : Eu2+ is at such values of x that a large number of Al vacancies exist. Quenching of its luminescence by the introduction of oxygen vacancies was not observed. In LaMgAl11O19 : Eu2+ phosphors Al3+ interstitials rather than oxygen defects seem to play a dominate role in achieving charge neutrality.

Luminescence spectra of non-stoichiometric aluminates doped with Eu2+ ions

Abstract Luminescence measurements indicate that in Al 2 O 3 -deficient aluminate phosphors with the magnetoplumbit crystal structure, aprt of the Eu 2+ activator ions occur in β-alumina-type oxygen surroundings rather than in magnetoplumbite-type oxygen surroundings. In Al 2 O 3 -rich β-alumina type aluminate phosphors part of the Eu 2+ ions has a magnetoplumbite-type oxygen surrounding.

Recent developments in the application of phosphors

Abstract A review is given of developments in the application of phosphors in the years 1972–1975.

Models for energy transfer and their application to aluminate phosphors

Abstract Energy transfer efficiencies and decays of the luminescence in aluminate phosphors are discussed as a function of the activator (A) and sensitizer (S) concentration. A model with continuously variable SA distance and a model with discrete SA distances and site occupancies have been compared. Examples are given in the Ce 1-υy Tb υy ] 0.92 O 0.08 MgAl 11.13 O 19 and Ba 1-z Eu z Mg 1-υy Mn υy Al 10 O 17 phosphor systems. The consideration of the sensitizer decay curves leads to discrimination between discrete and continuum models for energy transfer, even in cases where efficiency data only do not allow such a distinction.

A noncontinuum model for energy transfer in europium-activated oxytungstates

Abstract Transfer from sensitizer ( S = W - ion ) to activator ( A = Eu ) in Y 2 WO 6 : Eu occurs by single steps. The S decays are best described by a discrete model of strong transfer to the 10 nearest A sites around S and weak transfer to the 20 next-nearest sites. New crystal structure data on Y 2 WO 6 are presented as well.