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.

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In their crystalilne phases, the conformation of non-rigid polyphenyl molecules results from a delicate balance between competing intramolecular and intermolecular forces. At low temperature, the polyphenyl crystals undergo structural phase transitions associated with stabilization of a nonplanar conformation, with respect to a torsional angle between the phenyl rings. An interesting feature is the different natures of these phase transitions: displacive in biphenyl and order-disorder in p-terphenyl. In order to illustrate this point we consider the following features: the thermal motion in the high-temperature phases, the influence of hydrostatic pressure on the phase transition in p-terphenyl, the transitional heat capacities, the observation of the critical slowing down of fluctuations in p-terphenyl and the existence of incommensurate phases in biphenyl.

We report on elastic neutron scattering results showing that the intermediate phase of quartz existing in a 1.3 K range between the α and β phases is in fact incommensurate. Six satellites are observed around most Bragg peaks along the three [100] directions of reciprocal space. The modulation corresponds to the superposition of sinusoidal plane waves as no higher order harmonics are observed. With decreasing temperature the modulation vector decreases from 0.033 to 0.029 reciprocal units. Premonitory diffuse scattering is observed in the β phase a few degrees above Ti = 847.2 K, the incommensurate transition temperature. The results are discussed in relation with the current theories of incommensurate phases and are in many respect analogous to the situation of NaNO2

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