Mössbauer spectroscopy of europium-containing glasses: Optical activator study for x-ray image plates.

A fluorozirconate glass (ZBLAN) containing BaCl2 nanocrystals doped with divalent Eu is a promising material for x-ray image plates for medical diagnosis. Since it is known that Eu2+ readily oxidizes to Eu3+, which reduces fluorescence efficiency of the image plates, 151Eu Mössbauer spectroscopy was used in this work to monitor the Eu oxidation state of the samples during degradation over time in the presence of ambient humidity. In addition, Mössbauer spectroscopic experiments show that the oxidation state has already changed during the glass melt: The sample made from 5 mol% EuCl2 contained 78% EuCl2 + 22% EuCl3 deduced from the relative areas of the absorption lines. The sample made from 2.5 mol% EuCl2 + 2.5 mol% EuCl2 contained 37% EuCl2 + 63% EuCl3, i.e. 26% of the original EuCl2 was oxidized to EuCl3.

Crystallization studies on rare-earth co-doped fluorozirconate-based glasses.

This work focuses on the structural changes of barium chloride (BaCl2) nanoparticles in fluorochlorozirconate-based glass ceramics when doped with two different luminescent activators, in this case rare-earth (RE) ions, and thermally processed using a differential scanning calorimeter. In a first step, only europium in its divalent and trivalent oxidation states, Eu2+ and Eu3+, is investigated, which shows no significant influence on the crystallization of hexagonal phase BaCl2. However, higher amounts of Eu2+ increase the activation energy of the phase transition to an orthorhombic crystal structure. In a second step, nucleation and nanocrystal growth are influenced by changing the structural environment of the glasses by co-doping with Eu2+ and trivalent Gd3+, Nd3+, Yb3+, or Tb3+, due to the different atomic radii and electro-negativity of the co-dopants.

Mössbauer spectroscopy of europium-doped fluorochlorozirconate glasses and glass ceramics: optimization of storage phosphors in computed radiography.

Eu(2+)-doped fluorochlorozirconate (FCZ) glasses and glass ceramics, which are being developed for medical and photovoltaic applications, have been analysed by Mössbauer spectroscopy. The oxidation state and chemical environment of the europium ions, which are important for the performance of these materials, were investigated. Routes for maximizing the divalent europium content were also investigated. By using EuCl2 instead of EuF2 in the starting material a fraction of about 90% of the europium was maintained in the Eu(2+) state as opposed to about 70% when using EuF2. The glass ceramics produced by subsequent thermal processing contain BaCl2 nanocrystals in which Eu(2+) is incorporated, as shown by the narrower linewidth in the Mössbauer spectrum. Debye temperatures of 147 K and 186 K for Eu(2+) and Eu(3+), respectively, were determined from temperature dependent Mössbauer measurements. The f-factors were used to obtain the Eu(2+)/Eu(3+) ratio from the area ratio of the corresponding absorption lines.

The oxidation state of europium in halide glasses.

The luminescent properties of divalent europium ions can be exploited to produce storage phosphors for x-ray imaging applications. The relatively high cost and limited availability of divalent europium halides makes it desirable to synthesize them from the readily available trivalent salts. In this work, samples of pure EuCl(3) and fluoride glass melts doped with EuCl(3) were processed at 700-800 °C in an inert atmosphere furnace. The Eu oxidation state in the resulting materials was determined using fluorescence and Mössbauer spectroscopy. Heat treatment of pure EuCl(3) for 10 min at 710 °C resulted in a material comprising approximately equal amounts of Eu(2+) and Eu(3+). Glasses made using mixtures of EuCl(2) and EuCl(3) in the starting material contained both oxidation states. This paper describes the sample preparation and analysis and discusses the results in the context of chemical equilibria in the melts.

Crystallization behavior of rare-earth doped fluorochlorozirconate glasses.

A series of fluorochlorozirconate (FCZ) glasses, each doped with a different rare-earth, was prepared and examined to determine thermal stability and activation energy, Ea , of the dopant dependent BaCl2 crystallization. Non-isothermal differential scanning calorimetry (DSC) measurements were done to investigate the endothermic and exothermic reactions upon heat treatment of the glass samples. In comparison to the rare-earth free FCZ glass, significant changes in the Hruby constant, Hr , and Ea were found due to the addition of a rare-earth and also between the individual dopants.

Structural properties of fluorozirconate-based glass ceramics doped with multivalent europium.

The structure/property relationships of fluorochlorozirconate glass ceramics as a function of divalent and trivalent europium (Eu) co-doping and thermal processing have been investigated; the influence of doping ratio on the formation of barium chloride (BaCl(2)) nanocrystals therein was elucidated. X-ray absorption near-edge structure spectroscopy shows that the post-thermal annealing changes the Eu valence of the as-poured glass slightly, but during the melting process Eu(3+) is more strongly reduced to Eu(2+), in particular, when doped as a chloride instead of fluoride compound. The Eu(2+)-to-Eu(3+) doping ratio also plays a significant role in chemical equilibrium in the melt. X-ray diffraction measurements indicate that a higher Eu(2+) fraction leads to a BaCl(2) phase transition from hexagonal to orthorhombic structure at a lower temperature.

Differential scanning calorimetry investigations on Eu-doped fluorozirconate-based glass ceramics.

The properties of Eu-doped fluorochlorozirconate (FCZ) glass ceramics upon thermal processing and the influence of Eu-doping on the formation of BaCl(2) nanocrystals therein have been investigated. Differential scanning calorimetry indicates that higher Eu-doping shifts the crystallization peak of the nanocrystals in the glass to lower temperatures, while the glass transition temperature remains constant. The activation energy and the thermal stability parameters for the BaCl(2) crystallization are determined.

Eu oxidation state in fluorozirconate-based glass ceramics.

The influence of InF(3) doping and remelting on Eu-doped fluorozirconate-based glass ceramics was investigated using near-edge x-ray absorption and optical spectroscopy. It was found that the addition of InF(3) to the melt decreases the Eu(2+)Eu(3+) mole ratio, while remelting leads to a significant change in the Eu(2+)Eu(3+) ratio in favor of Eu(2+). Photoluminescence spectroscopy shows that additional annealing steps lead to the formation of BaCl(2) nanoparticles in the glass. In as-made glass ceramics containing InF(3), a phase transition of the nanoparticles from hexagonal to orthorhombic structure is observed. This phase transition is not observed in the remelted glasses studied here.