Abstract

Whenever a phase transition is described an immediate question that is asked is whether it is first order, involving the coexistence of two phases or second order, where one phase changes uniformly and continuously into another. Here we investigate the tetragonal-to-orthorhombic transition in SrFe2As2, an end member compound of the recently discovered iron-pnictide superconductors. Like many similar phase transitions, there is controversy over the nature of the phase transition. It is our opinion that much of this controversy results from the macroscopic nature of the measuring techniques so instead we measured the order parameter on a local scale using large-angle convergent-beam electron diffraction. This immediately showed that the phase transition was first order. The measurements allowed us to go far beyond this simple characterisation, however, and we are able to describe the mechanism by which the atoms are rearranged to give the new phase. The transition is martensitic and proceeds by partial dislocations travelling across material, forming needle-shaped twins of the new phase in their wake. This mechanism gives an insight into how some of the x-ray diffraction experiments used by other authors to characterise the transition gave apparently contradictory results.