Structural phase transition in polyphenyls. VII. A neutron diffraction study of the structural phase transition in biphenyl-d10

Abstract

Recently a soft mode has been observed in biphenylhl0 (hydrogenated) by Raman spectroscopy below T c = 40 K. The structural phase transition in biphenyld l0 (deuterated) has been investigated by neutron diffraction. Surprisingly, superlattice reflections appeared below 40 K, corresponding to a doubling of the b parameter only, and not to the doubling of a and b, as was found for p-terphenyl and p-quaterphenyl. The torsion angle between the planes of the phenyl groups is about 10 °, and is in opposite directions for two neighbouring molecules along b. The intensity of the superlattice reflections shows a linear behaviour with temperature, in agreement with Landau theory.

Organic molecular compounds with modulated crystal structures

“…In [59] the planar conformation of the molecule was interpreted as the result of dynamic disorder with hfi time ¼ 0 and an actual molecular equilibrium geometry of f ¼ 27 (but please note, that also in this description the crystal packing with its intermolecular forces has a flattening effect on the individual molecules). At low temperatures two modulated crystalline phases with non-planar molecules (f 6 ¼ 0 ) [56,60,61] in the crystal structure are found. The inversion symmetry of the molecular structure and the inversion symmetry of the crystal structure are destroyed.…”

“…The structure below T ¼ 20 K was described in a twofold superstructure, i.e. the modulation wave vector component s 2 ¼ 0:46 as can be found in the diffraction pattern was approximated to the rational value s 2 ¼ 1 = 2 [60]. As consequence, in this case the wavelength l of the modulation wave was shortened from l ¼ j Ãj ¼ 2b.…”

“…Also in the superstructure this torsion of the molecule results in a loss of the inversion symmetry of the molecular structure and of the crystal structure. The resulting space group of the superstructure is Pa [60]. The structure refinement of the superstructure however suffered by the fact, that different scale factors had to be applied for the main and for the satellite reflections.…”

Solids modelled by crystal field ab initio methods. 5. The phase transitions in biphenyl from a molecular point of view

“…From measurements of the temperature dependence of librational parameters (Charbonneau & D61ugeard, 1976; and phonon frequencies (Plakida, Belushkin, Natkaniec & Watsiutynski, 1983), the model (~0)time=0 ° emerged as the best description of the hightemperature structure (HT structure) above 40 K. Between 40 and 17 K the HT b axis approximately doubles and the crystal shows incommensurate modulations in both the b* and a* directions (Cailleau, Moussa & Mons, 1979). At 17 K phase III appears in which the modulation in the a* direction has vanished, that along b* remains and the structure tends towards an ordering in space group Pa with Z = 4 but with q~ modulated between + 10 and -10 ° (Cailleau, Baudour & Zeyen, 1979;Baudour & Sanquer, 1983).…”

3-Nitrobiphenyl

“…These systems have also been investigated because of the differences found in the dihedral angle between the phenyl rings (Cailleau & Baudour, 1979;Brock, 1980, and references therein; Sutherland & Rawas, 1983).…”