https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A25804/datastream/PDF2/Lothenbach-2019-Cemdata18-%28accepted_version%29.pdf

Cemdata18: A chemical thermodynamic database for hydrated Portland cements and alkali-activated materials

Persistent luminescence phenomena in materials doped with rare earth ions

Al coordination and speciation in calcium aluminosilicate glasses: Effects of composition determined by 27Al MQ-MAS NMR and Raman spectroscopy

Thermoluminescence properties of the Eu2+-, R3+-doped calcium aluminate materials, CaAl2O4:Eu2+,R3+, were studied above room temperature. The trap depths were estimated with the aid of the preheating and initial rise methods. The seemingly simple glow curve of CaAl2O4:Eu2+ peaking at ca. 80 degrees C was found to correspond to several traps. The Nd3+ and Tm3+ ions, which enhance most the intensity of the high-temperature TL peaks, form the most suitable traps for intense and long-lasting persistent luminescence, too. The location of the 4f and 5d ground levels of the R3+ and R2+ ions were deduced in relation to the band structure of CaAl2O4. No clear correlation was found between the trap depths and the R3+ or R2+ level locations. The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R3+ ions. A new persistent luminescence mechanism presented is based on the photoionization of the electrons from Eu2+ to the conduction band followed by the electron trapping to an oxygen vacancy, which is aggregated with a calcium vacancy and a R3+ ion. The migration of the electron from one trap to another and also to the aggregated R3+ ion forming R2+ (or R3+-e-) is then occurring. The reverse process of a release of the electron from traps to Eu2+ will produce the persistent luminescence. The ability of the R3+ ions to trap electrons is probably based on the different reduction potentials and size of the R3+ ions. Hole trapping to a calcium vacancy and/or the R3+ ion may also occur. The mechanism presented can also explain why Na+, Sm3+, and Yb3+ suppress the persistent luminescence.

Thermoluminescence study of persistent luminescence materials: Eu2+- and R3+-doped calcium aluminates, CaAl2O4:Eu2+,R3+.

Persistent luminescence or afterglow is caused by a gradual release of charge carriers from trapping centers. The energy needed to release these charge carriers is determined by the trap depths. Knowledge of these trap depths is therefore crucial in the understanding of the persistent luminescence mechanism. Unfortunately, the trap depths in persistent phosphors are often difficult to evaluate in an accurate and reliable way. The existing analysis methods are mostly based on single experiments, or they ignore the possibility of a continuous distribution of trap depths. We present a procedure to accurately probe the activation energies, even in the presence of a continuous distribution of energy levels. By performing a series of thermoluminescence experiments with varying excitation duration and at varying excitation temperature, and employing the initial rise analysis method, the depth and shape of such a distribution can be estimated. As an example, we investigated the trap system in the violet persistent phosphor CaAl2O4:Eu,Nd, and show that it consists of a Gaussian-shaped distribution of trap depths. The maximal density of traps lies in the region around 0.9 eV, but the distribution extends to 0.7 eV on the shallow side and 1.2 eV on the deep side. The described procedure can be used to obtain a clear view of the trap system in other persistent phosphors as well. This can lead to a better understanding of the nature of these trapping centers, and the role they play in the persistent luminescent mechanism.

/pdf/revealing-trap-depth-distributions-in-persistent-phosphors-2iv3mvlbuf.pdf

Revealing trap depth distributions in persistent phosphors

Zur kristallstruktur von CaAl2O4

Einkristalle von BaAl2O4 wurden dargestellt und mit einem Vierkreisdiffraktometer rontgenographisch untersucht (Raumgruppe: C – P63; Gitterkonstanten: a = 1047,0 und c = 881,9 pm). Parameterverfeinerungen in der grosen Elementarzelle mit den Uberstrukturreflexen zeigen, das BaAl2O4 nicht mit BaGa2O4 isotyp ist. Es gibt im BaAl2O4 nur eine Verknupfung der Tetraedersechsringe vom Typ ABABAB. Die Uberstruktur wird als eingefrorener Zustand einer verzerrten Idealstruktur aufgefast.



About the Crystal Structure of BaAl2O4



Single crystals of BaAl2O4 were prepared and investigated by X-ray diffractometer data (space group: C – P63; lattice constants: a = 1047.0 and c = 881.9 pm). Parameter refinement within the large unit cell and the superstructure intensities indicate that BaAl2O4 and BaGa2O4 are not isotypic. In BaAl2O4 there is only one type of linkage in 6-rings of tetrahedra, i. e. ABABAB. The superstructure is interpreted as metastable form of a distorted substructure.

Zur Kristallstruktur von BaAl2O4

Single crystals of β-CaCr2O4 were prepared by solid state reaction (1300°C, 7d) of CaO-Cr2O3 mixture. The so far unknown parameters of the atomic positions show no distortion of the octahedral surrounding [Cr2O4]2- in respect to other compounds of this type.

W. Hörkner

Papers

Zur kristallstruktur von CaAl2O4

Zur Kristallstruktur von BaAl2O4

Notizen: Einkristalluntersuchungen von β-CaCr2O4.