4y .| = 0 is used here for simplicity and is not essen-Recently, there has been considerable interest in the study of the tricritical point, where a line of phase transitions changes from first to second order. This interest is largely motivated by the fact that the conventional scaling approach, which has been so highly successful in describing second-order transitions, breaks down near a tricritical point. 1 Examples of systems having tricritical points include the metamagnetic-antiferromagnetic transitions in dysprosium aluminum garnet, 2 FeCl 2 , s and Ni(No s ) 2 -2H 2 0, 4 the A transition in He 8 -He 4 mixtures, 5 and the orientational order-disorder transitions in NH 4 C1 6 and ND 4 C1 7 crystals. It is the primary purpose of this Letter to report the discovery of a new type of tricritical point which occurs at higher pressures for the transition between the smectic-.A and cholesteric liquid-crystalline phases of cholesteryl oleyl carbonate (COC).Until recently, all liquid-crystal transitions were believed to be first order. It was originally suggested by McMillan, 8 based upon mean-field calculations for a specific interaction model, that the transition between the smectic-yl and the nematic or cholesteric phases might be second order under certain conditions. In particular, he predicted that for a homologous series of liquid crystals, the discontinuity in the order parameter at the transition should decrease with chain length and eventually vanish for sufficiently short tial to present results. If we assume adiabatic compression, Eq. (2) remains essentially the same so long as \k x 2 \»\k z 2 \. . 18 Damped surface eigenmodes have been identified in laboratory 0-pinch experiments (W. Grossman and J. A. Tataronis, to be published). molecules, thus rendering the transition second order. This trend was subsequently verified experimentally by Doane et al., 9 who found one instance of a nematic -smectic -A transition which was second order or at least very nearly second order.It seemed quite plausible that the behavior observed by varying chain length could also be induced for molecules of a given size through changes in the density. Therefore, we began an investigation of the phase transitions of the liquid crystal COC at elevated pressures. This substance was chosen primarily because of its ready availability and convenient temperature ranges for the smectic-^4 and cholesteric phases. To the best of our knowledge, the only other liquid crystal which has been investigated at high pressures is £-azoxyanisole, 10 which exhibits only the nematic mesophase.The COC used in this study was obtained from Eastman Organic Chemicals and was not further purified. A sample of approximately \ mm thickness was sandwiched between two cylindrical glass windows and contained at the perimeter by a tightly stretched piece of fluran tubing. This arrangement was placed in a high-pressure optical cell filled with Octoil S, which was used as the pressurizing fluid. High pressures were generated by means of a reciprocating hand pum...

We report and discuss the pressure–temperature phase diagrams for seven materials having liquid crystal phases: N- (p-methoxybenzylidene) -p-butylaniline (MBBA), N- (p-ethoxybenzylidene) -p-butylaniline (EBBA), N- (p-methylbenzylidene) -p-butylaniline (MBA), cholesteryl oleyl carbonate (COC), 4,4′-bis (heptyloxy) -azoxybenzene (HAB), cholesteryl nonanoate (CN), and N- (p-cyanobenzylidene) -p-octyloxyaniline (CBOOA).

The thermal conductivity~of solid natural neon has been measured by the linear-flow method for specimens isobarically frozen from the dense-fluid phase within a high-strength steel cell. By crossing the fusion curve at several different pressures to as high as 7 &(10' bars, crystalline samples with molar volumes between 11.16 and 13.35 cm'/mole were grown, and for each the isochoric variation of~vs T was determined at a set of temperatures in the experimentally accessible range between 5 and 40 K. The large magnitude of~and reproducibility of the results demonstrate that the present method of preparation and manipulation of solid neon consistently yields good-quality specimens. An analysis utilizing the zero-degree limit of the Debye temperature So to account for all volume dependence indicates that the array of data, when expressed in terms of the resistive mean free path, may be rendered into a single function of the inverse reduced temperature 80/T. The exponential variation of this common curve over more than 2 orders of magnitude is characteristic of three-phonon umklapp scattering and thereby gives support to Peierls s model of heat transport in dielectric crystals, More recent first-principles theoretical calculations are in qualitative agreement with experiment but yield conductivities somewhat below the observed values.

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