Raman spectra and improper ferroelastic phase transition in single crystal

Abstract

Polarized Raman spectra of lithium ammonium sulphate (LAS) single crystals in the temperature range 40-290 K have been studied and compared with the available data. Careful analysis of their temperature evolution it made possible to propose a comprehensive assignment of the observed Raman active phonon modes and to gain deeper insight into the lattice dynamics of this crystal in the low-temperature ferroelastic phase III. In particular, numerous new modes connected with doubling of the unit cell were unambiguously identified in agreement with group-theoretical predictions. The peculiar temperature evolution of the spectra below the transition temperature (284 K) is explained by coexistence of both phases, with metastable polar clusters and heterophase fluctuations persisting in a wide temperature range below . This interpretation accounts also for other anomalies observed in macroscopic properties of LAS in this temperature interval.

Raman and X-ray Investigations of Ferroelectric Phase Transition in NH4HSO4

“…Here, we recorded the Raman spectra of NH 4 HSO 4 in the range 350–3600 cm –1 at room temperature, which is shown in Figure . All the Raman active modes in NH 4 HSO 4 at 298 K are assigned based on the information available in the literature. − The internal modes in these regions are divided into five groups of frequencies: (i) S–O–H, O–S–O bending (400–850 cm –1 ), (ii) symmetric S–OH stretching (850–900 cm –1 ), (iii) symmetric and asymmetric S–O stretching (950–1350 cm –1 ), (iv) symmetric and asymmetric N–H bending (1350–1800 cm –1 ), and (v) symmetric and asymmetric N–H stretching (3000–3200 cm –1 ). Along with these above fundamental group frequencies, we also observed frequencies that are arising from O–H···O hydrogen bonds as well as combination modes and overtones of the S–O and N–H vibrations.…”