Competitive role of the tellurium and gadolinium cations in structural aspects of the gadolinium-phosphate-tellurate glasses

Glasses and glass ceramics in the ternary xGd2O3(100 - x)[7TeO2.3P2O5] systems with 0 ≤ x ≤ 70 mol.% have been prepared from melt quenching method. Main results of the quantum-chemical calculation of the structural model for the 7TeO2.3P2O5 glass network show that there is a charge transfer between the tellurium atoms coordinated +3 and +4 and between the tellurium atoms coordinated +4 with the phosphate network. Presence of the multiple cations of gadolinium and tellurium in the glasses to attract the [PO4] structural units for compensation of charge yield a competition between these cations showing the drastic reduction of the characteristic features corresponding to the [PO4] structural units in bandwidth, position and intensity. After the heat treatment applied at 500 deg. C for 24 h, two crystalline phases appear, namely the Te4P2O13 and GdPO4. The Te4P2O13 crystalline phase is characteristically of the host glass ceramic. The strong affinity of the Gd3+ ions towards the phosphorus groups containing non-bridging oxygen is responsible for the disappearance of Te4P2O13 crystalline phase. The addition of higher Gd2O3 content yields the gradual depolymerization of the phosphate chains and the formation of the GdPO4 crystalline phase.