Abstract
Tellurite-based glasses of the chemical formula xLa
2
O
3
-(100 – x) TeO
2
, (2.5 ≤ x ≤ 12.5 mol%) have been prepared and studied by FTIR spectroscopy and the ultrasonic technique. The FTIR analysis indicated that La
2
O
3
transformed the TeO
3
into TeO
4
structural units in the range 2.5 ≤ x ≤ 5, while in the range 5 ≤ x ≤ 7.5 mol%; there is a coordination transformation of Te atoms in [TeO
4
] groups into
[
TeO
3
−
]
groups associated with the formation of non-bridging oxygens. Beyond x = 7.5 mol%, the structural units
[
TeO
3
−
]
(tp) were destroyed and transformed into TeO
4
structural units that acted as a former and tightened the tellurite glass network more. The stretching force constant of the tellurite structural units was affected with the increase of La
2
O
3
content. The structural variations of the structural units of TeO
2
and La
2
O
3
affected the increase of density, the behaviour of the ultrasonic velocities, the elastic moduli, and both of the optical and acoustical Debye temperature with increasing La
2
O
3
content. The ratio of optical to acoustic Debye temperature represented the three-dimensional nature of tellurite-based glasses.