Nematic-smectic-A-smectic-Cpolycritical point: Experimental evidence and a Landau theory

Abstract

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.

Phase behaviour of novel liquid crystalline mixtures based on thiobenzoates†

“…The phase transition temperatures thus obtained agreed among the methods of detection within the accuracy of 0.1 K. During the POM and TLI measurements, the sample was contained in a Linkam 5 mm-cell with planar orientation. [29] and Siguard et al [30] in binary liquid crystal mixture 7OS5/8OS5. In the mixtures 7OS5/9OS5 and 8OS5/9OS5, the NAC point was reached by D. Brisbin et al [17], and in the 7OS5/8OCB system (OCB being 4-n-octyloxycyanbiphenyl) -by Martinez-Miranda et al [31].…”

Multicriticality in smectic liquid crystals

“…The points SmA-SmC-SmF and SmA-HexB-SmI were identified as ordinary triple points with three first order phase transition lines [29,30]. However, based on symmetry considerations, it is possible that SmA-SmC-SmF and SmA-HexB-SmI points may be multicritical points [1,10,25]. Pyzuk et al [14] observed the SmC-SmF-HexB and SmA-SmC-HexB multicritical points in binary mixtures of two liquid crystal compounds.…”

“…An interesting feature of multicritical points is universality of generic phase diagrams. One of the most studied multicritical point is the nematic-smectic-A-smectic-C (N-SmA-SmC) point [1,2,6]. Musevic et al [4] demonstrated a Lifshitz point in field and temperature phase diagrams of the ferroelectric smectic-C * (SmC * ) liquid crystalline phase.…”

“…The multicritical points in liquid crystalline systems have been the subject of extensive experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and theoretical [17][18][19][20][21][22][23][24][25][26][27][28] studies. The multicritical points in various liquid crystal phase transitions have been observed by varying the concentration in binary mixtures, increasing the pressure and varying the electric field.…”

Landau theory of the nematic-smectic a tricritical phase transition in a binary mixture of liquid crystals

“…80.3 + 20.3 (Johnson et al, 1975); C6 + C7 (Achard et al, 1976); C7 + C8 (Achard ef al., 1976); 7S5 + 8S5 (Johnson et al, 1977;Safinya et al, 1981Safinya et al, ,1983Witanachchi et al, 1983;DeHoff et al, 1982) 60 CB + 80 CB ; 9CB + 10 CB (Thoen eral., 1984); 8CB + lOCB (Marynissen et al, 1985); 608 + 6010 (Anisimov et al, 1988) ; 7S5 + 80 CB (Huster et al, 1987); 40.6 + 60.4 (Rananavare et al, 1987); 6012 + 6010 (Anisimov et al, 1987;1990); 608 + 6010; (Anisimov, 1988); 608 + 6010 + 6012 (Anisimov, 1988); 40.8 + 60.8 (Stine and Garland, 1989); 508 + 608 (Thoen and Parret, 1989).…”