Optical Study of X-Ray Irradiation Effect on the Phase Transition in Triglycine Sulfate and Diglycine Nitrate

Abstract

Infrared spectra are measured at various temperatures on ferroelectric triglycine sulfate (Tc=48°C) and diglycine nitrate (Tc=−67°C) and nonferroelectric diglycine hydrochloride, each of which includes a short hydrogen bond connecting two glycine molecules. Almost all of the spectra of these substances are only slightly changed as the temperature passes through the respective Curie points. In contrast, a band at 575 cm−1 in triglycine sulfate is very characteristically changed, and its behavior is strongly influenced by x-ray irradiation. Further, a detailed inspection of the spectra shows that above and below the respective Curie points one of the two glycine molecules is in zwitterion form in diglycine nitrate and not in triglycine sulfate, and that the potential for the proton is of symmetrical single-minimum type in the latter and of asymmetrical double-minimum type in the former. Based on these facts, it is suggested that the origin of the phase transition, in triglycine sulfate, is not attributable to the hydrogen bond but to the remarkable intensity behavior of the 575-cm−1 band. On the other hand, in diglycine nitrate it is ascribed to a collective motion leading to the interchanging of the hydrogens between two possible equilibrium positions on the hydrogen bonds.