Incorporation of Europium(III) into Scheelite-Related Host Matrices ABO4 (A = Ca2+, Sr2+, Ba2+; B = W6+, Mo6+): Role of A- and B- Sites on the Dopant Site-Distribution and Photoluminescence


Incorporation of Europium(III) into Scheelite-Related Host Matrices ABO4 (A = Ca2+, Sr2+, Ba2+; B = W6+, Mo6+): Role of A- and B- Sites on the Dopant Site-Distribution and Photoluminescence

Xiao, B.; Schmidt, M.

Calcium orthotungstates and -molybdates are naturally occurring minerals that have been studied extensively. The minerals are named scheelite (CaWO4) and powellite (CaMoO4), respectively. Scheelite is the most important economic W mineral. Powellite is actively studied in the nuclear waste management field. Powellite is one of the primary Mo crystalline phases expected to form in high-level nuclear waste (HLW) borosilicate glasses during waste processing.
Both scheelite and powellite have a large number of synthetic derivatives that are based on a general formula ABO4 (A = Ca2+, Sr2+, Ba2+; B = W6+, Mo6+). Recently, much of the interest in study of scheelite-type materials arises from their exceptional compositional variability. In the context of nuclear waste disposal, this compositional variability offers a potential pathway for the effective retention of highly radiotoxic actinides like Pu and Am in a powellite secondary phase. However, the thermodynamic stability of these solid solutions will depend on their structural deviation from the stoichiometric phases.
Investigations have shown that the presence of excess positive charge in scheelite-typed ABO4 materials upon incorporation of each trivalent ion is compensated via coupled substitution with a monovalent alkali cation. Single-crystal X-ray measurements demonstrate that the crystal structure of the resulting solid solutions is disordered, that is, the trivalent dopant and monovalent charge-compensating cation statistically occupy the same divalent A2+ site in ABO4 structure. However, the structural details behind such disordered substitution, such as specific ionic environment around dopants, number of non-equivalent doping species as well as spatial accommodation of doping centers, are difficult parameters to characterize from the crystallographic data, especially when the dopant is present at trace concentration levels.
Polarization-dependent site-selective time resolved laser-induced fluorescence spectroscopy (p-TRLFS) is unique in its capability to characterize the local environment of a fluorescent probe, here Eu3+, in a multi-species system with point-group accuracy at trace concentration levels. This work aims to clarify the impact of site effect on the local symmetry distortion from the bulk crystallographic site symmetry in scheelite-type ABO4 single crystals. This will improve our understanding of the formation of solid solutions on the molecular scale.

Keywords: TRLFS

  • Lecture (Conference)
    GeoBremen, 24.-29.09.2017, Bremen, Germany

Permalink: https://www.hzdr.de/publications/Publ-26801
Publ.-Id: 26801