Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 127
Effect of Er +3 doping
Fig. (4.19) shows DTA traces of the Er +3 containing oxyfluoride glasses (MOF015 to 18),
where Er2O3 and ErF3 was added in various amounts to the base ternary composition
70TeO2-10Na2O-20ZnF2 (mol. %): 69.3TeO2-9.9Na2O-19.8ZnF2-1Er2O3, 69.93TeO2-
9.99Na2O-19.98ZnF2-0.1Er2O3, 69.86TeO2-9.98Na2O-19.96ZnF2-0.2ErF3 and
69.98TeO2-10Na2O-20ZnF2-0.02ErF3 mol. %. All compositions showed a lower
temperature crystallisation exotherm in addition to the single peak the undoped ternary
70TeO2-10Na2O-20ZnF2 (mol. %) exhibited (see fig. (4.14)), except the composition
containing 0.02 mol. % ErF3 (MOF018). Erbium (III) is similar in size to tellurium (89
and 97 pm, respectively), however ErF3 is 8-coordinate bicapped trigonal prismatic [54].
Er 3+ in oxide tellurite glasses is 6- or 7-coordinated [56], and has been shown to enhance
the stability of TeO2-ZnO-Na2O glasses [22]. Therefore, this instability when added to
fluorotellurite compositions could arise from the increase in coordination of erbium (III)
in the presence of fluorine, making the glass network less tolerant to its addition, only
resulting in stable glass for low dopant levels. The lower temperature crystallisation peak
is likely to be an erbium rich phase (see section 5), indicating these compositions could
be used to produce nano-glass-ceramics for photonic devices, with further compositional
development. Hirano et al. [57] found that in the ternary glass 70TeO2-15K2O-15Nb2O5,
when doped with around 1 mol. % rare-earth (Eu +3 , Gd +3 , Er +3 , Tm +3 and Yb +3 ) and heat
treated after forming a glass from quenching, a rare-earth rich phase formed in nano-
crystals (diameter = 20 nm). This phase resulted in an additional lower temperature