Negative ENDOR study of an irradiated single crystal of triglycine sulfate and its ferroelectric phase transition

With the use of the negative ENDOR technique an x-ray irradiated crystal of triglycine sulfate was studied over the temperature range from 170 to 420 K (T_c=322 K). Within the 11-17 MHz frequency range, signals from more than four different types of protons were detected. Each of these proton signals including the one from H₇ is a singlet when T≳T_c, while the signal splits into a double when T<T_c. Although the splitting depends on temperature, no evidence for line broadening was detected. This temperature behavior was explained in terms of a simple quantum-tunneling process, the exchange integral (Γ) of which was estimated to be 10² cm^-1 in order of magnitude.