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

Scheelite- and powellite-related materials doped with trivalent lanthanides or actinides have been the subject of extensive research due to their important role in mineralogical, technological, and environmental implications. Detailed structural knowledge of these solid solutions is essential for understanding their physicochemical properties and predicting material properties. In this work, we conduct a comprehensive spectroscopic analysis by means of polarization-dependent site-selective time-resolved laser-induced fluorescence spectroscopy, to delineate the influence of the host phase cations for a series of scheelite-type matrixes based on a general formula of ABO(4) (A = Ca2+, Sr2+, Ba2+; B = W6+, Mo6+) on the local environment of the Eu3+ dopant. Eu3+ has been used as a luminescent probe to access the local structural environment of crystalline substitutional sites suitable for trivalent lanthanide or actinide occupation. Our results show that the lattice distortion is overall minor, but increases with increasing mismatch of host and guest cation size. We observe a linear dependence of Eu3+'s excitation energy on the host cation size and the A-O bond distance, which can be used to determine the hitherto unknown Eu-O bond distance in NaEu(WO4)(2). A value of 2.510 angstrom was determined, somewhat larger than a previously reported number for NaEu(MoO4)(2). The results also show clear evidence that the local coordination environment of Eu3+ in WO42- materials is more symmetrical than in their isostructural MoO42- counterparts. The detailed spectroscopic interpretation conducted in this study resolves the relation between local distortion around a dopant and the host phase cations in structural disordered materials and may give novel insights with respect to rational design and tailoring of functional materials.