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The local atomic environment of Yb 3+ ions doped into Yb:Er:SrFCl, Yb:Er:SrFBr, and Yb:Er:BaFCl nanocrystals was probed using Yb L 2 edge EXAFS spectroscopy. A structural model derived from substitution of Yb 3+ for Sr 2+ or Ba 2+ in the fluorohalide lattice failed to provide a crystallochemically meaningful description of the first coordination shell of Yb 3+ . On this basis, the presence of Yb 3+ coordinated as YbF 4 Cl 5 or YbF 4 Br 5 capped square antiprisms of C 4 v symmetry was ruled out. Two alternative models were evaluated. The first model was inspired by YbF 9 capped square antiprisms that make up the crystal structure of orthorhombic YbF 3 . The second model was based on YbO 4 F 3 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 Yb 3+ . From a compositional standpoint, extra fluoride anions and/or oxide anions appear to be incorporated in the vicinity of Yb 3+ dopants. From a structural standpoint, the local symmetry around Yb 3+ ( C s or C 1 ) 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. 

A new synthetic route to access pristine and rare-earth-doped BaFBr nanocrystals is described. Central to this route is an organic–inorganic hybrid precursor of formula Ba 5 (CF 2 BrCOO) 10 (H 2 O) 7 that serves as a dual-halogen source. Thermolysis of this precursor in a mixture of high-boiling point organic solvents yields spherical BaFBr nanocrystals (≈20 nm in diameter). Yb:Er:BaFBr nanocuboids (≈26 nm in length) are obtained following the same route. Rare-earth-doped nanocrystals display NIR-to-visible photon upconversion under 980 nm excitation. The temperature-dependence of the green emission from Er 3+ may be exploited for optical temperature sensing between 150 and 450 K, achieving a sensitivity of 1.1 × 10 −2 K −1 and a mean calculated temperature of 300.9 ± 1.5 K at 300 K. The synthetic route presented herein not only enables access to unexplored upconverting materials but also, and more importantly, creates the opportunity to develop solution-processable photostimulable phosphors based on BaFBr.