  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.
