Raman scattering study of phase transitions in NH4SCN

Abstract

The Raman spectra (30–3300 cm−1) of a single crystal and of polycrystalline forms of NH4SCN are presented in the temperature range 135 to −175 °C. The polarization behavior of the Raman bands reveals the antipolar arrangements of SCN− ions in the room temperature phase. Above 86.5 °C the spectra do not show polarization behavior due to the formation of polycrystalline domains. Other changes observed in the entire spectrum at this phase transition are attributed to reorientation of SCN− ions between two equivalent configurations. At the next phase transition around 112 °C the lattice modes of SCN− ions are affected. The characteristics are those of a second-order displacive phase transition. A new phase transition observed at −70 °C is characterized by the strengthening of the hydrogen-bond networks in the lattice.