Temperature-dependent Raman study of ammonium perchlorate single crystals: The orientational dynamics of the NH+4 ions and phase transitions

Abstract

A detailed temperature-dependent study of the Raman spectra of oriented single crystals of NH4ClO4 is reported in the spectral regions of lattice modes and internal vibrations of the ClO−4 and NH+4 ions between 10 and 300 K. The internal modes of the ClO−4 ions show splitting into several components due to site and correlation field effects. The linewidth, frequency shift, and intensities of some of the internal modes of the ClO−4 and NH+4 ions and the frequency shift of a few lattice modes show anomalous temperature dependence around 180 and at 40 K. These anomalies have been explained in terms of phase transformations associated with the changes in hydrogen bonding strength and reorientational freedom of the NH+4 ions in the lattice. The low temperature transition at 40 K exhibits a sharp and discontinuous anomaly in some of the spectral parameters measured in this study which is associated with order–disorder-type transition. The measured linewidth of the ν′1 mode in the diagonal scattering configuration can be understood in terms of vibrational dephasing of the ν1 excited state due to vibrational–librational coupling. The estimated activation energies in the 50–160 K and 10–40 K temperature ranges are found to be 141 and 51 cm−1, respectively, which correspond to the observed NH+4 librational frequencies.