Structural characterisation of tin fluorophosphate glasses doped with   Er2O3
  EXAFS and confocal Raman microscopy have been used to study 50SnF2.(20-x)SnO.30P2O5.xEr2O3 (x=0, 0.1, 0.25) glasses. EXAFS data reveal an average coordination of Sn to O of 1.5 in both undoped glass and Er-doped glass samples. The first coordination sphere of Er in glasses doped with Er2O3 was found to have 9 F atom neighbours at an average bond length of 2.292+/-0.005 {\AA}, indicating that Er preferentially bonds with F. Raman spectra clearly show the emergence of orthophosphate Q0 units on addition of Er2O3. The saturation of Er solubility is found to be between 0.25 and 0.5 mol$\%$ Er2O3. An increase of the glass transition temperature from $\sim$80{\deg}C in undoped glass to $\sim$87{\deg}C in samples doped with 0.25 mol$\%$ Er2O3 was also observed. This process is accompanied by an increase in the difference between the crystallisation and glass transition temperatures, which is usually associated with improved thermal stability of the glass. 
J. Trimble, R. Golovchak, J. Oelgoetz, C. Brennan, A. Kovalskiy
cond-mat.mtrl-sci