Phase Transitions in Rod‐Like Liquid Crystals

Guillon, Daniel

doi:10.1002/9783527620555.ch2

Cited by 1 publication

(1 citation statement)

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“…For molecular crystals, the transition from a complete orientational and translational disorder ( isotropic liquid phase) toward complete order may be achieved in several stages in such a way that each phase transition involves a stepwise reduction of the disorder. Depending on whether orientational or translational orders appear when cooling the isotropic liquid, one is dealing with liquid crystals or plastic crystals, respectively. , While the former are generally found in molecular materials formed by elongated molecules, plastic crystals are formed by substances whose molecular shapes are symmetrical about one or more axes and, particularly, those where the molecular shape is somewhat globular. Well-known examples of such molecular materials representative of plastic crystals (i.e., displaying orientationally disordered, OD, phases) are the tert- butyl compounds, (CH 3 ) 3 CX, X = Cl, Br, NO 2 , CN, SH, COOH ..., because they have molecular shapes sufficiently close to spherical (due to the comparable size of the X groups to the methyl group) to enable orientational disorder in the solid state. − …”

Section: Introductionmentioning

confidence: 99%

Structure of Phase III and Polymorphism of (CH₃)₃CBr

Négrier

Barrio

Tamarit

et al. 2010

Crystal Growth & Design

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The polymorphism of 2-bromo-2-methyl-propane ((CH 3) 3 CBr) has been investigated by both thermal and X-ray powder diffraction experiments. The long time unknown structure of the low-temperature phase III has been determined by X-ray powder diffraction as orthorhombic Pmn2 1 , with lattice parameters a = 19.1350(4) Å , b = 5.7378(1) Å , and c = 10.9626(2) Å at 180 K, Z = 8 and with an asymmetric unit containing one molecule in a general position and two additional molecules placed in a mirror (Z 0 = 1 þ 1/2 þ 1/2). The phase transitions between the different phases have been characterized at normal pressure as well as at high pressures (up to 300 MPa). From the p T slopes of the two-phase coexistence lines and density measurements as a function of pressure, the volume variations at the transition points have been obtained and compared with those obtained by means of X-ray powder diffraction at normal pressure. The packing as well as the anisotropy of the intermolecular interactions have been analyzed by means of the thermal-expansion tensor and compared with tert-butyl ((CH 3) 3 CX, X = Cl, CN) related compounds.

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