AbstractAt temperatures above the monoclinic‐orthorhombicP21/c‐Pcmnphase transition at 180°C in terbium pentaphosphate, the presence of a third phase was observed spectroscopically, identified as aC2/cmonoclinic structure. Two lines at 12 and 17 cm−1were 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 thePcmnphase, made Raman‐active by the doubling of the unit cell along thec‐axis in theC2/cphase. The interpretation of these lines resolves a question concerning the asymmetry (‘shoulder’) of the soft optic mode at theP21/c‐Pcmnferroelastic transitio