An estimator for the Coulomb repulsion parameter U to generate vacuum referred binding energy schemes for lanthanides in compounds

The U-value defined as the energy difference between the Eu4+/3+ and Eu3+/2+ charge transition levels (CTLs) is the most important parameter in constructing vacuum referred binding energy diagrams (VRBEs) with all the lanthanide CTLs with respect to the vacuum level of energy. The parameter is difficult to determine from experiment and the aim of this work is to establish a method to estimate the U-value from the average electronegativity of the cations in the compound. Since the U-value is controlled by the same physical processes, i.e., covalence and anion polarizability, as the centroid shift ϵc of the Ce3+ 5d configuration, one may estimate the U-value from that centroid shift. That method provides already good values for U for about 175 different compounds. Those U-values are compared with the average cation electronegativity χav, and relations will be established from which the U-value can be estimated with about ±0.1 eV accuracy from just the composition of the compound. It can be applied to all types of stoichiometric inorganic compounds like the halides (F, Cl, Br, I), chalcogenides (O, S, Se), and nitrides (N). The U-value complemented with the bandgap and the energy needed for electron transfer from the valence band top to a trivalent lanthanide dopant is then sufficient to construct a VRBE diagram with all lanthanide levels with respect to the vacuum level and the host valence and conduction bands.RST/Luminescence Material

The nephelauxetic effect on the electron binding energy in the 4f^q ground state of lanthanides in compounds

In the construction of a vacuum referred binding energy (VRBE) diagram with the lanthanide 4fq ground states, always a compound independent variation with the number q= 1 to 14 is assumed. Experimental data from thermo-luminescence, intervalence charge transfer bands, and thermo-bleaching studies provide first indications that a minor compound dependence does exist. To explain its origin we will first apply Jørgensen spin pairing theory to reproduce the VRBE in the ground states of the free di- and trivalent lanthanide ions which is equivalent to the 3rd and 4th ionization potentials of the lanthanide atoms. By combining experimental data and calculated trends therein, the relevant Racah E1, Racah E3, and spin orbit coupling ζff parameters for all di-, tri-, and tetravalent free ion lanthanides are derived. Using that as input for the spin pairing theory, the characteristic zigzag shapes in VRBE as function of q, as derived from ionization potentials, are nicely reproduced. Because of the nephelauxetic effect the parameter values are lowered when lanthanides are in compounds. How that reduction affects the VRBE curves will be treated in this work.Accepted Author ManuscriptRST/Luminescence Material

The hole picture as alternative for the common electron picture to describe hole trapping and luminescence quenching

Electronic level schemes with the host valence and conduction band together with the level locations of ground and excited states of defects are used to explain and predict luminescence and carrier trapping phenomena. These schemes are always constructed and interpreted by using the electron picture. In this work the alternative hole picture is presented. Such picture is sometimes used in the field of semi-conductors but hardly ever in the field of wide band gap inorganic compounds. We will focus on the lanthanides, and first show where to draw the hole ground state and excited hole states in our scheme. It leads to up-side-down Dieke diagrams and up-side-down configuration coordinate diagrams but for the rest everything is equivalent to the electron picture. With the hole picture, luminescence quenching via hole ionization to the valence band and hole trapping in defects can be illustrated much more conveniently than with the electron picture. As examples the quenching of the Tb3+ D45 emissions by electron ionization and the quenching of the Eu3+ D05 emissions by hole ionization are compared.Accepted Author ManuscriptRST/Fundamental Aspects of Materials and Energ

The quest for high resolution γ-ray scintillators

There are many properties of scintillators that are of importance for application. One property is the energy resolution for the detection of γ-rays, and during past 20 years we witnessed enormous progress. The state of the art resolution for the detection of 662 keV γ photons was 5–6% at the end of the 20th century, and today scintillators with 2.2% resolution are commercially available. This work will provide a review on the development of high resolution chloride, bromide, and iodide based scintillators that occurred since the discovery of the LaCl3:Ce3+ scintillator in 2000. Bandgap engineering and co-doping to eliminate afterglow or to improve proportionality have become new tools in optimizing scintillator performance. At the end of the review the prospects for the development of scintillators with resolution <2% are addressed together with new research strategies that might be required to accomplish that.RST/Luminescence Material

A review on how Lanthanide impurity levels change with chemistry and structure of inorganic compounds

The energy of the 4f-5d transitions of divalent and trivalent lanthanide impurities in compounds depends strongly on the type of lanthanide, its valence, and the type of compound. Despite this large variability there is much systematic in 4f-5d transition energy. Once it is known for one lanthanide that for all others when in the same compound can be predicted. The same applies for the energy of electron transfer from the valence band to the 4f-shell of lanthanides which also behaves in a systematic fashion with type of lanthanide and type of compound. This work reviews my studies during the past fifteen years that are based on an analysis of data on all divalent and all trivalent lanthanides in more than 1000 different inorganic compounds collected from the archival literature. The established redshift and charge transfer models that form the basis to construct binding energy schemes showing all lanthanide levels with respect to the host bands are reviewed and the latest developments are addressed.RST/Radiation, Science and TechnologyApplied Science

Mechanism of Persistent Luminescence in Eu2+ and Dy3+ Codoped Aluminate and Silicate Compounds

A mechanism of persistent luminescence that was proposed in 1996 for SrAl2O4:Eu2+;Dy3+ has been widely adopted to explain afterglow in many Eu2+ and Dy3+ codoped aluminates and silicates. The mechanism involves the thermally activated release of a hole from Eu2+ in its excited 5d state to the valence band which is subsequently trapped by Dy3+. In this work the location of the lanthanide energy levels relative to the valence and conduction band of various compounds is presented. It is shown that the mechanism of persistent luminescence cannot be correct. An alternative model that involves the ionization of the 5d electron to conduction band states and subsequent trapping by Dy3+ is proposed. The level schemes are consistent, both qualitatively and quantitatively, with many observations regarding persistent luminescence. They also provide insight into the mechanism of thermal quenching of Eu2+ 5d-4f emission.RRR/Radiation, Radionuclides and ReactorsApplied Science

Thermal quenching of lanthanide luminescence via charge transfer states in inorganic materials

There are various routes of luminescence quenching such as multi-phonon relaxation from excited states to lower energy states, energy migration to killer sites, and radiation less relaxation to the ground state via the crossing point in a configurational coordinate diagram. In this work, we will consider and review quenching of lanthanide luminescence by means of charge carrier transfer to the valence band or the conduction band of the host compound. We will focus on 4fn-4fn emission quenching due to thermally activated electron transfer from the Pr3+ 3P0 level and the Tb3+ 5D4 level to the conduction band, and due to thermally activated hole transfer from the Eu3+ 5D0 level to the valence band. In addition, we will consider the quenching of the 4fn−15d-4fn emission of Eu2+ and Ce3+ which often (if not always) proceeds by electron transfer to the conduction band. Since all the above quenching routes involve reduction or oxidation of lanthanides, the location of the lanthanide charge transition levels with respect to the host bands is crucial. In other words, we need to know the location of the ground and excited states in the band gap or equivalently the vacuum referred binding energies (VRBE) in the lanthanide states as can be established using the (refined) chemical shift model. A clear correlation between the temperature T50 at which luminescence intensity or luminescence decay time has dropped by 50% and thermal quenching activation energies ΔE derived from VRBE schemes will be demonstrated. Since T50 typically changes 400-800 K with a 1 eV change in ΔE, and since VRBE energies may contain 0.3-0.5 eV error, it will be clear that the accurate prediction of quenching temperatures from the VRBE data is not yet feasible. Nevertheless, one may derive trends and provide guidelines on how to improve the thermal stability of luminescence.RST/Luminescence Material

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