High-resolution x-ray study of the nematic - smectic-A and smectic-A - smectic-C transitions in 8barS5-aerosil gels

The effects of dispersed aerosil nanoparticles on two of the phase transitions of the thermotropic liquid crystal material 4-n-pentylphenylthiol-4'-n-octyloxybenzoate 8barS5 have been studied using high-resolution x-ray diffraction techniques. The aerosils hydrogen bond together to form a gel which imposes a weak quenched disorder on the liquid crystal. The smectic-A fluctuations are well characterized by a two-component line shape representing thermal and random-field contributions. An elaboration on this line shape is required to describe the fluctuations in the smectic-C phase; specifically the effect of the tilt on the wave-vector dependence of the thermal fluctuations must be explicitly taken into account. Both the magnitude and the temperature dependence of the smectic-C tilt order parameter are observed to be unaffected by the disorder. This may be a consequence of the large bare smectic correlation length in the direction of modulation for this transition. These results show that the understanding developed for the nematic to smectic-A transition for octylcyanobiphenyl (8CB) and octyloxycyanobiphenyl (8OCB) liquid crystals with quenched disorder can be extended to quite different materials and transitions.Comment: 7 pages, 8 figure

TRANSFORMATION BEHAVIOR OF Ti50Ni47Fe3 ALLOY :I. INCOMMENSURATE AND COMMENSURATE PHASES

The transformation behavior of TiNiFe alloys of nominal composition Ti50Ni47Fe3 has been studied by transmission electron microscopy and diffraction, and electrical resistance measurements, between room temperature and -196°C. Based on the present results and complementary neutron and x-ray diffraction data, charge density wave (CDW) phenomena and associated phase transitions were found to be involved in the "premartensitic" behavior of the Ti50Ni47Fe3 alloys. The crystal structure of the high temperature parent phase was confirmed to be the CsCl(B2) structure. Upon cooling below room temperature, 1/3(110) and 1/3(111) superlattice reflections appeared, coincident with the onset of an electrical resistance anomaly (increase). The "1/3" superlattice reflections were observed to intensify with decreasing temperature, and close inspection showed that such reflections deviate slightly from the exact 1/3 positions relative to the parent CsCl structure, suggesting the formation of an incommensurate superlattice over a 12°C temperature range. The appearance of the "1/3" superlattice reflections is interpreted to be associated with the formation of CDW's. A second "premartensitic" effect was found to occur approximately 12°C below the onset of the initial "normal-to-incommendurate" transition. This subsequent phase change involves a structural transition from a "distorted" CsCl parent to a rhombohedral product, during which the "1/3" superlattice reflections are shifted to precise 1/3 positions. The rhombohedral distortion is effected by a homogeneous expansion along directions, allowing the lattice and CDW's to lock-in and become commensurate

Neutron scattering study of premartensitic phases of TiNi(Fe)

The premartensitic phases of TiNi(Fe) have been studied by means of neutron diffraction, specific heat, and electron microscopy. Evidence for an incommensurate charge density wave has been found, with a wavevector near, but not at, (h/3, k/3, 0) positions. A lock-in transition occurs at lower temperatures, accompanied by the formation of needle-shaped domains oriented along (110) directions