Temperature evolution of Raman spectrum of iron phosphate glass

Iron phosphate glass that has glass transition Tg ∼ 782 K, has been investigated between 81 and 873 K using Raman spectroscopy. After correcting for the thermal population factor, the reduced spectra are analyzed to obtain the mode wavenumbers and their line widths. A total of nine modes were followed as a function of temperature. The temperature dependences of mode wavenumbers of nearly all the modes are found to show anomalies near Tg. Below Tg, the asymmetric stretching of Q1 tetrahedra of [P2O7]4− units and Q0 tetrahedra of [PO4]3− units at 1090 and 946 cm−1, respectively, exhibit conventional anharmonic behavior, whereas the network vibration, iron polyhedral and PO4 network bending modes at 170, 306 and 420 cm−1, respectively, show hardening suggesting stiffening of the force constants arising from structural relaxations. In addition, the 170 cm−1 mode is also found to become sharp at elevated temperatures. As the reduced spectra represent the vibrational density of states, the changes in the intensities of the 750 and 946 cm−1 mode also suggest redistribution of density of states due to structural relaxations.