Raman spectroscopy of coexisting phases in TbP 5 O 14

At temperatures above the monoclinic‐orthorhombic P 2 1 / c‐Pcmn phase transition at 180°C in terbium pentaphosphate, the presence of a third phase was observed spectroscopically, identified as a C 2/ c monoclinic structure. Two lines at 12 and 17 cm −1 were observed to grow in intensity as the temperature was raised from 180 to 240°C; they would be described as acoustic modes at the Brillouin zone boundary in the Pcmn phase, made Raman‐active by the doubling of the unit cell along the c ‐axis in the C 2/ c phase. The interpretation of these lines resolves a question concerning the asymmetry (‘shoulder’) of the soft optic mode at the P 2 1 / c ‐ Pcmn ferroelastic transition raised long ago.