X-rays investigations of the smectic A1 - smectic A2 transition

We confirm by X-rays studies on aligned samples the existence in a binary system of a smectic A - smectic A transition. We find that the smectic layer periodicity is doubled from the higher phase S_A1 to the lower one S_A2. The observation of diffuse scattering planes in nematic and S_A1 indicates the existence of pretransitional S_A2 effects and we suggest an analogy between the S_A1–S_A2 transition and the classical para-antiferroelectric transition in crystals.