Characterization of the smectic-A-hexatic-B transition in 65OBC by adiabatic scanning calorimetry

Abstract

We investigated the smectic-A-hexatic-B (SmA-HexB) transition in the liquid-crystal n-hexyl-4'-n-pentyloxybiphenyl-4-carboxilate (650BC) with adiabatic scanning calorimetry. We were able to prove in a direct way that this transition is indeed very weakly first order, as was already suggested in the literature. The latent heat at the transition was determined to be deltaHL = 0.04 +/- 0.02 J/g. Our experiments confirm the high value for the heat capacity critical exponent earlier reported, yielding alpha = 0.64 +/- 0.05.

Structural studies of the bond-orientational order and hexatic–smectic transition in liquid crystals of various compositions

“…The temperature width of the region where the BO order fluctuations are really strong has not been evaluated in the theory [39]. From the X-ray experiments it can be estimated as few degrees, in accordance with the range of the divergent behavior of the specific-heat at the Hex-B -Sm-A phase transition [16][17][18][19].…”

“…These include the coupling of the BO order with the short-range herringbone molecular packings [34], interaction of BO order with in-plane positional strains [17], and coupling between the hexatic order and layer displacement fluctuations [43]. The precise adiabatic scanning calorimetry study of the hexatic compound 65OBC by Van Roie et al [18] have proved that the Hex-B -Sm-A transition is indeed very weakly first order [17,19]. The latent heat at the transition was found to be as small as ∆H ≈ 0.04 J/g.…”

“…9 and Table III). In a similar way higher order coupling invariants of the type C 3 6 C 18 , C 4 6 C 24 , and corresponding quadratic terms can be added in the Landau expansion (18). We neglect here the higher order coupling terms of the type C 3 12 C 2 18 and so on.…”

“…As a result the singular part of the heat capacity represents a sum of two terms: the first with a small critical exponent α ≈ −0.01 characteristic of the 3D XY universality class and the second term with the exponent α ≈ 1 which originates from the interaction of the BO order parameter with density fluctuations. Thus, the heat capacity critical exponent α = 0.6 − 0.65 [16][17][18][19] is based on a single, but not universal, power-law fitting of the experimental data, which reflects a crossover behavior between two regimes described above [59].…”

Theory of the anomalous critical behavior for the smectic-A–hexatic transition

“…In this work we examine properties of a liquid crystal near the continuous smectic-A-hexatic phase transition. Such transitions are observed in a variety of materials [3,4,6,[10][11][12][13][14][15][16][17][18]. We use the Landau approach introducing phenomenologically the hexatic orientational order parameter [19].…”