Powder diffraction study on solid ozone

The hitherto unknown crystal structure of ozone was determined from conventional X-ray and high resolution neutron powder diffraction data. Ozone crystallises in the orthorhombic space group Pbca with 8 formula units per unit cell. The lattice parameters at 5 K are: a = 778.88(2) pm, b = 669.73(2) pm, c = 684.13(2) pm. The geometry determined for an ozone molecule in the solid is found in good agreement with the well-known gas phase structure. In particular, there is no significant deviation from ideal C_2v symmetry. The efficient molecular packing is generated by three mutually perpendicular glide planes. Within this packing the orientation of the molecules is such that their polar axes lie roughly parallel or anti-parallel to the c-axis and secondly, neglecting their elevation along z, the atoms of one molecule are almost perfectly aligned with terminal but not central atoms of neighbouring units. This gives rise to parquet type patterns perpendicular to the c-axis. As a consequence solid ozone comprises a number of structural fragments with short intermolecular contacts between the terminal atoms of the O₃ triangles. Of these an ozone dimer with C_i point symmetry is the simplest. The unit cell dimensions of solid ozone have been determined as a function of temperature. The negative coefficient of thermal expansion obtained for the b lattice parameter is explained by structural peculiarities of parquet type patterns.