Structural phase transition in d-benzil characterised by capacitance measurements and neutron powder diffraction

The ferroelectric phase transition in deuterated benzil, C14H10O2, has been studied using capacitance measurements and neutron powder diffraction. Hydrogenous benzil shows a phase transition at 83.5 K from a high temperature P3121 phase to a cell-doubled P21 phase. The phase transition in d-benzil occurs at 88.1 K, a small isotope effect. Neutron powder diffraction was consistent with a low temperature phase of space group P21. Upon deuteration the transition remained first-order and the dynamics of the phenyl ring dominated the behaviour. The isotope effect can be attributed to the difference in mass and moment of inertia between C6H5 and C6D5.