${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclear quadrupole resonance measurements around the order-disorder ferroelectric transition in HCl and HCl-DCl mixed crystals are analyzed for possible evidence of pretransitional clusters of biased ferroelectric dipoles in the matrix of the paraelectric phase. It is shown that the hypothesis of an extrinsic mechanism related to local freezing-in around the DCl molecules does not fit the temperature dependence of the concentration of clusters, or the effect of sizable deuteration. The hypothesis of an intrinsic mechanism related to heterophase fluctuations expected in the framework of the thermodynamical droplet model is supported by the negligible change of shape and volume at the transition, which reduces the coherency strain energy that is usually thought to suppress heterophase fluctuations at first-order phase transitions. However, in such a model, the temperature dependence of the concentration of clusters can only be justified by assuming a temperature dependence for the average size of the clusters different from that expected for the correlation length in the mean-field description of critical phenomena.

• Received 27 August 1980

DOI:https://doi.org/10.1103/PhysRevB.24.4925