@article {L.M.Fortes2014,
	title = {EXAFS study of Er,Yb doped hollow and dense SiO2microspheres},
	journal = {Journal of non-crystalline solids},
	volume = {402},
	year = {2014},
	pages = {244–251},
	publisher = {Elsevier},
	address = {New York ;},
	abstract = {Hollow and dense SiO2microspheres doped with Er,Yb were characterized by extended X-ray absorptionfine structure (EXAFS) on the Er,Yb LIIIedges and photoluminescence (PL)measurements. The samples were synthe-sizedby the sol-gelmethod andin situEr,Ybdoped, by introducingfluoride compounds (ErF3,YbF3) in the sol. Er, Yb doped transparent glass-ceramic, formed by NaYF4nanocrystals dispersed in SiO2matrix, was taken as the reference. The EXAFS results illustrate the Er,Yb local environment in all studied SiO2matrices: in hollow-microspheres, crystalline Er,Ybfluoride was formed while in dense-microspheres, crystalline Er,Yb oxide developed. PL measurements of the Er 3+ emission at 1.54?m were performed, on hollow and dense SiO2microspheres, for different Er 3+ /Yb 3+ ratios, ranging from 1:3 to 1:10 mol%. Upconversion was observed inall samples, ordered by efficiency for the studied Er,Yb environments as: glass-ceramic (NaYF4environment)>>hollow SiO2micro-spheres (Er,Ybfluoride crystalline environment)Ndense SiO2microspheres (Er,Yb oxide crystalline environ-ment). X-ray excited optical luminescence (XEOL) confirmed the Er green upconversion results},
	issn = {0022-3093},
	doi = {10.1016/j.jnoncrysol.2014.06.004},
	author = {Fortes, L.M. and Goncalves, M.C. and Pereira, J.C. and D'Acapito, F.}
}
