The effect of pressure on vibrational modes in Li₃PO₄

We have studied the effect of pressure on the stretching and bending modes of (PO 4 ) 3- molecular groups in undoped Li 3 PO 4 and (MnO 4 ) 3- groups in Mn 5+ -doped Li 3 PO 4 using Raman spectroscopy and luminescence. The high-pressure Raman spectroscopy study confirmed an irreversible phase transition from the high-temperature phase to the low-temperature phase, observed in our previous high-pressure luminescence study (Riedener T, Shen Y R, Smith R J and Bray K L 2000 Chem. Phys. Lett. 321 445) and further characterized the rate and irreversibility of the phase transition. We observed and analysed vibronic transitions occurring in the 1 E emission of Mn 5+ in both phases. A stronger vibronic transition associated with the bending mode is interpreted in terms of an E⊗e Jahn-Teller coupling. Bulk and local compressibilities were predicted from variations of the energies of the (PO 4 ) 3- and (MnO 4 ) 3- stretching modes with pressure.