Specific-heat measurements near the nematic-smectic-^4. transitions of the octyloxy, nonyloxy, and decyloxy members of the 4-w-pentyl-phenylthiol-4'-alkoxybenzoate (rcS5) homologous series reveal the presence of critical-tricritical crossover behavior upon increasing n f with n t ~9. The results are consistent with Alben's extension of de Gennes's superfluid analogy to include superf!uid-He 3 -He 4 -like tricritical behavior.The question of the existence of a nematicsmectic-^4 tricritical point has motivated interest among experimentalists for several years. Since the theoretical discovery by Kobayashi 1 and McMillan 2 that coupling between the nematic and smectic order parameters may drive this transition first order, there have been several experiments that strongly suggest such a point does exist on phase diagrams of temperature versus pressure 3 or concentration. 4 * 5 On the theoretical side, de Gennes 6 has written a Landau theory exhibiting coupling between the smectic order parameter and the nematic director. Based on this analogy, Halperin, Lubensky, and Ma 7 ' 8 have made calculations which predicted that the nematic-smectic-A transition should always be first order, thus ruling out conventional tricritical behavior. In this Letter we report specificheat and birefringence measurements which indicate that coupling to the nematic order parameter, in the manner of Kobayashi 1 and McMillan, 2 appears to produce a conventional tricritical point. Specifically, we have made measurements on three members of the nS5 homologous series, 9 namely, SS5, 5S5, and TUS5.The specific-heat data were taken on a highresolution ac microcalorimeter described elsewhere. 10 ' 11 The BS5 data were published previously. 10 The data for all three homologs are shown in Fig. 1, where the most striking feature is seen to be the profound increase in the strength of the anomaly as n increases from B to IU. This may be understood as a consequence of the decreasing nematic temperature range, as seen in Fig. 2, and the consequent increase in contribution from coupling between the nematic and smectic order parameters. In the microscopic theory this coupling leads to first-order behavior for T NA /T NI >0.87. 2 Experimentally 4 this occurs at a somewhat higher value of T NA /T NI (i.e., for a 500 a O 100 r F .

High-resolution ac microcalorimetry studies of octyloxycyanobiphenyl, 80CB, reveal a nematic-smetic-A transition' consistent with the superfluid analogy as regards the specific-heat exponents, namely a = a' -O(log). However, the amplitude ratio, A/A' = 1.35, is inconsistent with the superfluid analogy. This may be related to two complicating features of the nematicsmectic-A transition of the 80CB studied. First, the transition appears to be weakly first order and slightly broadened by impurities. The twophase region is -0.04 K wide and an analysis of it based on weak-solution theory gives a transition entropy in the range 1.3 &( 10 R, (AS (3.1 &( 10 R, . Secondly, the specific-heat exponent exhibits sharp crossover above T, from a & 0.3 to a -O(log) as the asymptotic limit is approached.

Differential-scanning-calorimetry measurements and optical-microscopy observations are reported on mixtures of two stable liquid crystalline compounds, one of which has the nematic, smectic-A and smectic-, C phases, while the other has only nematic and smectic C. The phase diagram for this system is found to have the following features: (i) a line of second-order nematicsmectic-A transitions with large pretransitional effects, (ii) a line of continuous smectic-Asmectic-C transitions, (iii) a line of first-order nematicsmectic-C transitions with very weak pretransitional effects, and (iv) a multicritical point where these three lines meet. A phenornenological Landau-like theory is presented, which qualitatively describes the observed phase diagram. On the basis of this work it is argued that the nematicsmectic-C transition is always of first order.

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When the nematic-smectic-A-smectic C (NAC) multicritical point was first suggested, the two proposed explanations of it were very different. Chu and McMillan 1 (CM) proposed a model in which smectic-C has the dipolar order parameter of McMillan's theory, 2 while the smectic-A order parameter was the one-dimensional density wave of Kobayashi, 3 McMillan, 4 and deGennes. 5 Tilt of the director away from the layer normal entered the model somewhat incidentally through a gradient term coupling the two order parameters. Chen and Lubensky 6 (CL) found an NAC point in a model where the only order parameter was the one-dimensional density wave. Tilt of the director relative to the layer normal is a central feature of the model and occurs when the coefficient of the transverse gradient term becomes negative. From the experimental point of view the differences between the two models are primarily twofold, First, in the CL model the NAC point is a type of Lifshitz point, 7 which leads to the predic-1033 (1979). 9 G.We did not, however, compensate for the intensity reduction by scattering. 14 G. Northrop, to be published. 15 J. Callaway, Phys. Rev. LL3, 1046LL3, (1959.tion that x-ray scattering in the nematic phase near the NAC point falls off in the transverse direction as k ± '* 9 rather than the usual k ± ' 2 predicted by the CM model. Secondly, the nematic-smectic-C (NC) transition entropy is zero in the CM model but finite in the CL model because of the Brazovskii 8 " 10 effect. The NC entropy was found to be finite when the NAC point was discovered experimentally. 11 In this Letter evidence is presented, based on the topology of the phase diagram and the nematicsmectic-C transition entropy, that for the system studied, which is the same as the one studied previously, 11 the NAC point is likely not either of the above kinds of multicritical points. Instead it conforms well with a phenomenological Landau expansion suggested recently by Benguigui, 12 for which is offered a physical interpretation.In earlier work on mixtures of octyl-and heptyloxy-£'-pentylphenyl thiolbenzoate (7S5-8S5), 11 gross features of the phase diagram were deter-A study of the phase diagram near the nematic-smectic-v4. -smectic-C multicritical point and of nematic-smectic-C transition entropies leads to the conclusion that this is not a Lifshitz point. It is shown that a biaxial second-rank tensor order parameter may be needed. 664