Modification of the spectroscopic properties of Tb2O3 phosphor under the high-pressure phase transitions sequence

The high-pressure behavior of cubic Tb₂O₃ phase (C-type) has been investigated by in situ photoluminescence and Raman spectroscopy up to 18 GPa and 22 GPa, respectively. The luminescence spectrum of Tb₂O₃ at ambient conditions shows its characteristic sharp-line emission bands with the most intense one corresponding to the ⁵D₄ - ⁷F₅ transition, responsible for the green emission. Excitation and emission spectra measured at low temperature and at different emission and excitation wavelengths suggest that photoluminescence is governed by the Tb³⁺ in the non-centrosymmetric site (C₂). A phase transition between 7.2 and 8.6 GPa is observed by Raman and photoluminescence experiments and is associated with a cubic-to-monoclinic (B-type) structural transformation. From 14 GPa the predominant modes in the Raman spectrum correspond to those from the trigonal phase (A-type), indicating that the pressure-induced structural phase transition sequence of Tb₂O₃ is C → B → A. Upon pressure release, the starting cubic phase is not recovered, but the monoclinic one.