Essentially identical pretransitional smectic A behavior is reported in both normal and reentrant nematic phases of mixtures of octyloxycyanobiphenyl (80CB) and hexyloxycyanobiphenyl (60CB). A phenomenological model which assumes an optimum density for smectic interactions explains quantitatively our correlation length measurements for six different mixtures.PACS numbers: 61.30.EbOne of the most interesting phenomena exhibited by liquid crystals is the reentrant nematic-smectic A behavior discovered by Guillon, Cladis, and Stamatoff} In certain bilayer materials such as octyloxycyanobiphenyl (80CB) this behavior is readily induced either by the application of modest pressure or by forming mixtures with a second species such as hexyloxycyanobiphenyl (60CB). In spite of extensive research the physics underlying this phenomenon is still not understood. Concomitantly, the nematic-smectic A transition which is perhaps the simplest example of melting in nature, has also eluded a satisfactory theoretical explanation. In this paper we report a highresolution x-ray study of the smectic A fluctuation in the nematic phases of mixtures of 80CB and 60CB with emphasis on the region of concentration when the smectic phase barely forms or becomes unstable before the correlation length has truly diverged. We find a number of unusual phenomena in this region including dramatically increased bare lengths for the smectic correlation, critical exponents which may be as large as twice those found in pure 80CB, and dramatic crossover effects. We have constructed a phenomenological theory which is an extension of the Landau model of Pershan and Prost 2 ; this latter model rests on the concept of an optimum density for the smectic A phase, with reentrant behavior the result of thermal expansion varying the density through its optimum value. Our model is able to account quantitatively for all of the unusual effects observed in our x-ray studies including the doubling of the critical exponents.The experiments were carried out with a 12-kW Rigaku rotating-anode x-ray source with Cu Ka x rays. Very high resolution was achieved with the use of the (111) reflections from perfect Si crystals for collimation; this gave a longitudinal resolution, half width at half maximum, 1.3 x 10" 4 A"* 1 along the director (g n ). The samples were oriented in a magnetic field of ~ 4.0 kOe and director mosaicity ranged from 0.05° to 0.2°, giving a maximum in-plane resolution from 2X10" 4 to 8xl0~4 A" 1 . Slits were used to determine resolution from 4.7 X10" 2 to 9.1 X10" 2 A" 1 normal to the scattering plane. The 80CB and 60CB materials were used as obtained from British Drug Houses and sealed in cells with 0.01-in. Be windows. The illuminated sample volume was 1.5 x 2.0x 6.0 mm 3 . A two-stage oven provided temperature stability of 2 mK.Reciprocal space scans along #,, and q ± were PHASE DIAGRAM so| , , j , , , , , , i Nematic I j I jj -60 l ^^""^^""^^ o *^^ b 40-IA < I Smectic-A -J W \ JrAW \ o_ y\\ | Reentrant Nematic 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35...