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

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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