Order-disorder octahedral tilting transitions in SrSnO3 perovskite

High-temperature powder neutron diffraction has been used to characterize two structural phase transitions in the perovskite SrSnO3. A continuous orthorhombic Pnma to Imma symmetry transition occurs at 636°C, and a first-order transition to a tetragonal I4 mcm phase occurs at 800°C. Such octahedral tilting transitions are very common in perovskite oxides and are generally assumed to be displacive. However, a displacive description of the transitions in SrSnO3 gives physically unrealistic results, and a considerable dynamic order-disorder component is found. A more realistic order-disorder modeling shows that the population difference between majority and minority tilt states is an appropriate order parameter, and the onset of tilting fluctuations ∼300°C below the Pnma to Imma transition leads to an unusual divergence of the lattice parameters on warming towards the transition.