Raman Spectroscopic Search for Phase Transition in Solid Formic Acid, and Lattice Dynamics

The possibility for solid formic acid to exhibit a "one-dimensional" order-disorder transition in its H-bonding system was investigated by a careful Raman spectroscopic study in the temperature range 10 K to the melting point at 8°C. An intensity anomaly is seen around 220 K near a previously reported thermal anomaly, suggestive of a phase transition. A renewed thermal investigation is needed. Two new low-frequency Raman lines at 27 cm -1 and 35 cm -1 were discovered, probably being connected with the van der Waals interaction between the infinite H-bonded molecular chains. Based on this a new discussion of the lattice dynamics is given.