Thermal and structural study of glasses in the binary system TeO _2-Pb(PO_3)_2

摘要

The main objective of this work has been to prepare and characterize the thermal and structural properties of glasses in the pseudo binary system TeO_2-Pb(PO_3)_2 with respect to the composition. Homogeneous and transparent glass samples were obtained by the melt-quenching method in a large glass forming range in the pseudo binary system (100 - x)TeO_2-xPb(PO_3)_2 with x varying from 5 to 100. Thermal properties investigated by DSC pointed out an increase of the glass transition temperature from x = 5 to x = 40 and further decrease of Tg for higher Pb(PO_3)_2 concentrations. A similar tendency has been observed for the thermal stability against devitrification measured using the stability parameter Tx-Tg. FTIR together with Raman spectroscopies allowed building a structural model for these glasses with the contribution of distinct phosphate and tellurite units depending on the composition. Identification of crystalline phases obtained after the glasses heat-treatments obtained by X-ray diffraction support the structural evolution suggested by vibrational spectroscopies. These data suggest that incorporation of TeO_2 in the lead metaphosphate glass results in tellurium conversion from TeO_4 seesaw geometry to TeO_3 trigonal pyramids and consequent conversion of well-known metaphosphate units Q~2 to modified pyrophosphate units Q~2 _(1Te) in which the phosphorus PO_4 tetrahedron is linked to another PO_4 unit and one TeO_3 pyramid. These tellurite trigonal units cross-link the modified metaphosphate chains with a resulting increase of the glass network connectivity. For high TeO_2 concentrations, all Q~2 were converted to Q~2 _(1Te) and the additional tellurium atoms are incorporated in the glass network as TeO_4 seesaw units with a glass network built from a tridimensional network of TeO_3, TeO_4 and Q~2 _(1Te). Finally, this work pointed out the possibility to use this pseudo-binary system for the preparation of phosphate glass-ceramics containing a TeO_2 crystalline phase or tellurite glass-ceramics containing a pyrophosphate or metaphosphate crystalline phase.