Local atomic environment of Yb3+ in alkaline-earth fluorohalide nanocrystals

Abstract

The local atomic environment of Yb³⁺ ions doped into Yb:Er:SrFCl, Yb:Er:SrFBr, and Yb:Er:BaFCl nanocrystals was probed using Yb L2 edge EXAFS spectroscopy. A structural model derived from substitution of Yb³⁺ for Sr²⁺ or Ba²⁺ in the fluorohalide lattice failed to provide a crystallochemically meaningful description of the first coordination shell of Yb³⁺. On this basis, the presence of Yb³⁺ coordinated as YbF₄Cl₅ or YbF₄Br₅ capped square antiprisms of C_4v symmetry was ruled out. Two alternative models were evaluated. The first model was inspired by YbF₉ capped square antiprisms that make up the crystal structure of orthorhombic YbF₃. The second model was based on YbO₄F₃ capped trigonal prisms encountered in monoclinic YbOF. Both models correctly reproduced radial structure functions and yielded chemically meaningful ytterbium–fluorine and ytterbium–oxygen distances. Results from EXAFS studies indicate that compositional and structural heterogeneities appear in the fluorohalide lattice upon aliovalent doping with Yb³⁺. From a compositional standpoint, extra fluoride anions and/or oxide anions appear to be incorporated in the vicinity of Yb³⁺ dopants. From a structural standpoint, the local symmetry around Yb³⁺ (C_s or C₁) is lower than that of the crystallographic sites occupied by alkaline-earth cations. These conclusions hold for three different fluorohalide host compositions and rare-earth doping levels spanning one order of magnitude.