Neutron inelastic scattering has been used to examine the structural phase transition in deuterated benzil ${\mathrm{C}}_{14}{\mathrm{D}}_{10}{\mathrm{O}}_2$. The transition in benzil, in which the unit cell goes from a trigonal $P{3}_{1}21$ unit cell above $T_C$ to a cell doubled $P{2}_1$ unit cell below $T_C$, 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 $\Gamma$-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 $T_C$ and the phonon dispersion relation along the $\Gamma \text{−}M$ direction just above $T_C$. It is found that the transverse acoustic phonon at the $M$-point is of lower energy than the $\Gamma$-point optic mode and has a softening with temperature as $T$ approaches $T_C$ from above that is much faster than that of the $\Gamma$-point optic mode. This behavior is inconsistent with the view that the $\Gamma$-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.

• Received 9 January 2006

DOI:https://doi.org/10.1103/PhysRevB.73.134116