Structural phase transition in deuterated benzil C14 D10 O2: Neutron inelastic scattering

Neutron inelastic scattering has been used to examine the structural phase transition in deuterated benzil C14 D10 O2. The transition in benzil, in which the unit cell goes from a trigonal P 31 21 unit cell above TC to a cell doubled P 21 unit cell below TC, leads to the emergence of a Bragg peak at the M -point of the high temperature Brillouin zone. It has previously been suggested that the softening of a transverse optic phonon at the Γ -point leads to the triggering of an instability at the M -point causing the transition to occur. This suggestion has been investigated by measuring the phonon spectrum at the M -point for a range of temperatures above TC and the phonon dispersion relation along the Γ-M direction just above TC. It is found that the transverse acoustic phonon at the M -point is of lower energy than the Γ -point optic mode and has a softening with temperature as T approaches TC from above that is much faster than that of the Γ -point optic mode. This behavior is inconsistent with the view that the Γ -point mode is responsible for triggering the phase transition. Rather the structural phase transition in benzil appears to be driven by a conventional soft TA mode at the M -point.