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

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Abstract
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 (Tc=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≳Tc, while the signal splits into a double when T<Tc. 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.