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SUMMARY:The Nature of the Structural Phase Transition in SrFe2As2 Determin
 ed by Measurement of the Local Order Parameter using Transmission Electron
  Microscopy - Dr James Loudon\, Dept of Materials Science &amp\; Metallurg
 y\, University of Cambridge
DTSTART:20091201T150000Z
DTEND:20091201T160000Z
UID:TALK21156@talks.cam.ac.uk
CONTACT:Dr Jonathan Barnard
DESCRIPTION:Whenever a phase transition is described an immediate\nquestio
 n that is asked is whether it is first order\, involving the coexistence o
 f two phases or second order\, where one phase changes uniformly and conti
 nuously into another. Here we investigate the tetragonal-to-orthorhombic t
 ransition 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 immedi
 ately showed that the phase transition was first order. The measurements a
 llowed us to go far beyond this simple characterisation\, however\, and we
  are able to describe the mechanism by which the atoms are rearranged to g
 ive the new phase. The transition is martensitic and proceeds by partial d
 islocations travelling across material\, forming needle-shaped twins of th
 e new phase in their wake. This mechanism gives an insight into how some o
 f the x-ray diffraction experiments used by other authors to characterise 
 the transition gave apparently contradictory results.\n\n
LOCATION:T001 [Tower Seminar Room]\, Materials Science and Metallurgy\, De
 partment of
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