Abstract
The results of mid infrared absorption experiments on the phase transitions in KNO 3 are described and compared with the behaviour of NaNO 3. The first order phase II-Phase I transition is observed in KNO 3 upon heating to 401 K through the appearance of an additional peak next to the ν 2 peak. Upon cooling phase I, the ferroelectric I–III transition is observed by discontinuities in the frequency and linewidth of the ν 2 mode. However, the actual change in these properties at the phase transition is very small (particularly for the frequency shift which is only about 0.5 cm −1), and hence it is suggested that phases I and III have very similar structures as far as the local symmetry of the nitrate group is concerned. These results agree with earlier suggestions that phase I consists of ferroelectric microdomains which order to produce a macroscopic dipole moment at the I–III transition. The sudden increase in the linewidth of the ν 2 mode at T c is interpreted as being due to solitonic excitations which result in domain growth and reorganisation.
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