Detailed study of the phase transition in [Ni(H₂O) ₆](NO₃)₂·(15-crown-5)·H ₂O and analysis in terms of mean-field theory

The transition between 190 and 200 K in [Ni(H₂O) ₆](NO₃)₂·(15-crown-5)·H ₂O has been followed by determining the structure at 22 temperatures in the range 90-273 K. The structural change is a zone-boundary transition with a critical point at (1/2, 0, 1/2) in the Brillouin zone of the high-temperature phase; both phases have space-group symmetry P21 but the volume of the unit cell is halved when a crystal is heated through the transition. The only obvious disorder in the high-temperature phase is of the lattice water molecule, which occupies two sites; some disorder persists below the transition. The greatest changes in the structure below the transition are the rotations of one of the two 15-crown-5 molecules and of one of the two nitrate ions; above the transition the two molecules are related by symmetry as are the two ions. Below the transition these two rotation angles evolve linearly with one another, and can thus be associated with a single order parameter that describes the structural evolution. The evolution of the spontaneous strain arising from the transition does not, however, follow the same evolution as the structural order parameter. This observation indicates that the transition cannot be described in terms of a Landau-type expansion that is characterized by a single order parameter, perhaps because the potential-energy surface for this essentially molecular crystal is more complicated than for the inorganic and framework structures in which such simple behaviour is observed.