Gadolinium-environment in borate–tellurate glass ceramics studied by FTIR and EPR spectroscopy

Modifications of the gadolinium-environment in borate–tellurate glass ceramics system, where 0 ⩽ x ⩽ 30 mol%, under heat treatment have been investigated by FTIR and Electron Paramagnetic Resonance (EPR) spectroscopy. Glass ceramics with a molar composition of xGd2O3·(100 − x)[6TeO2·4B2O3] where 0 ⩽ x ⩽ 30 mol%, was prepared by one-step crystallization heat treatment of the parent glass at temperatures of 400 and 475 °C, respectively. After the heat treatment applied at 400 °C, some structural changes were observed and the B2O3 crystalline phase appeared in the structure of the samples. The crystallization of the B2O3 crystalline phase increases with increasing of the gadolinium concentration because the excess of the oxygen yields the formation of free BO 3 - 3 orthoborate units capable for charge compensation of the gadolinium ions. By increasing of the treatment temperature up to 475 °C, the increase of the capacity of migration inside the glass ceramics network yields the apparition of B2O3 and TeO2 crystalline phases. When a higher Gd2O3 content is introduced, more [BO3] structural units are coupled with gadolinium ions, the Te–O–Te linkages are deformed showing that the Gd2O3 behaves as a glass-former by means of the intercalation of [GdO4] entities in the [TeO4] chain network. The changes produced by devitrification suggest the competition between cations of tellurium and boron with non-bridging oxygens to compensate the positive charge of the gadolinium ions. Thus, gadolinium ions play a dual role of network former (475 °C) and network modifier (400 °C) in the studied samples