The low-temperature phase III structure and phase transition behaviour of cyclohexanone

The crystal structure of phase III of perdeuterocyclohexanone, C ₆D₁₀O, has been determined at 5 K using high-resolution neutron powder diffraction. Below its melting point of 245 K cyclohexanone forms a plastic crystal in the space group Fm3̄m. On cooling below 225 K the crystal transforms to the monoclinic phase III structure in the space group P2₁/n. The orthorhombic phase II structure exists under high pressure, but the triple point for all three phases is close to atmospheric pressure. Details of the phase II structure are also reported at 4.8 kbar (273 K) and ambient pressure. The phase behaviour of the compound and isotope effects are discussed.