Vibrational Spectroscopic Properties of a [C(NH₂)₃]₄Cl₂SO₄Ferroelectric Crystal - An Experimental and Theoretical Study

Abstract

Tetraguanidinium dichloride-sulphate crystal, [C(NH2)3]4Cl2SO4, abbreviated as G4Cl2SO4 was investigated. The vibrational infrared spectra of powdered G4Cl2SO4 crystal in Nujol mull were studied in the wide range of temperature, from 298 K to 377 K. This temperature range contains all the phases in the crystal (named III, II, I on heating, respectively). The temperature changes of wavenumbers, centre of gravity, and intensity of the bands were analyzed to clarify the molecular mechanism of the phase transitions. It was shown that in cooling from 377 K to 313 K the phase II is the same as the room temperature phase. Information about hydrogen bonds was obtained. The time dependence of internal vibrations at 356 K was observed and it was connected with slow transition III → II. For more detailed band assignment Raman spectrum at room temperature, at ferroelectric phase was carried out. Theoretical calculations were made based on density functional theory, with the B3LYP method using 6-311 + G(d,p) basic set. Calculated normal vibrational modes of the molecule, their frequencies and intensities were compared with these recorded in experiment. Theoretical description of the molecule including hydrogen bonds were optimized and the bond parameters were obtained. The Mulliken charges population analysis was performed.

Structural Disorder and Molecular Dynamics in Ferroelectric Crystals [C(NH2)3]4Br2SO4 and [C(NH2)3]4Cl2SO4

“…The Raman spectra of G 4 Cl 2 SO 4 and G 4 Br 2 SO 4 measured in the ferroelectric phase at 353 K are compared in Figure . The vibrations were assigned using earlier spectroscopic studies of the materials containing guanidinium and/or sulfate ions. − It is apparent that some of the Raman bands of G 4 Br 2 SO 4 are shifted to lower frequencies, e.g., the external vibration band centered at ∼100 cm –1 is shifted by ∼8 cm –1 when compared with the corresponding band of G 4 Cl 2 SO 4 . Similarly, the frequencies of the symmetric stretching vibrations of SO 4 and CN 3 molecular groups are lower, respectively, by 1 and 3 cm –1 .…”