Structure determination and phase transition behaviour of dimethyl sulfate

The crystal structures of phase I and phase II of dimethyl sulfate, (CH 3 O) 2 SO 2 , have been determined using complementary high‐resolution neutron powder and single‐crystal X‐ray diffraction techniques. Below its melting point of 241 K dimethyl sulfate crystallizes in an orthorhombic structure (I) in the space group Fdd2 . On cooling below ∼175 K the crystal transforms to a monoclinic structure (II) in the space group I 2/ a . The molecule is located on a twofold axis ( Z ′ = 1/2) in both structures. The phase transition is of first order with strong hysteresis. The phase transition results in changes to both the intra‐ and the intermolecular coordination environment.