Peculiar changes in the ferroelectric hysteresis loops of single crystals of triglycine sulfate result when the crystals are subjected to ionizing radiations (x-rays and electrons). Fully polarized crystals develop hysteresis loops biased along the field axis while partially polarized crystals develop double-loop patterns, each subsidiary loop being biased to the same extent but in opposite directions. The biased loops retain their rectangularity during the early stages of the bombardment. The bias field builds up steadily during the bombardment while the reversible polarization shows no change at first but eventually, at a fairly well-defined value of the bias field, it begins to decay in a fairly rapid manner.

Unusual field- and temperature-dependent changes in the shapes of the "damaged" hysteresis loops are observed. Under certain conditions, a double-loop pattern will "anneal" into a normal single loop simply as a result of applying the ac field; but when the field is removed, the crystal relaxes back to its double-loop form. Such cycles can be repeated, apparently indefinitely, provided that the field is not removed for too long a period. If this happens, the double-loop pattern fails to respond to further field treatments though it can be returned to the more unstable state by gentle heat treatment of the crystal.

The experiments rule out explanations of the effects based on the setting up of space charge fields or on nonuniform damaging of the crystal. It seems most likely that the effects are caused by the gradual building up of a more or less uniform strain in the crystal. The results are discussed in terms of the conventional double-minimum potential energy curve used for describing ferroelectric mechanisms. On the basis of some pyroelectric studies, however, it appears that a rather more complicated potential energy curve is needed to describe ferroelectricity in triglycine sulfate.

It is pointed out that, as the x-ray dosages required to produce large changes in the ferroelectric properties are small compared with those usually used in crystallographic structure determination, it seems most unlikely that the structure of an undamaged triglycine sulfate crystal has yet been determined, and one wonders how true this may be of certain other structure determinations for radiation-sensitive organic crystals.

• Received 18 August 1958

DOI:https://doi.org/10.1103/PhysRev.113.159