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SUMMARY:Characterisation of the nature of the tetragonal to orthorhombic p
 hase transition in SrFe2As2 by measuring the local order parameter using t
 ransmission electron microscopy - James Loudon (Department Materials Scien
 ce and Metallurgy\, University of Cambridge)
DTSTART:20091026T160000Z
DTEND:20091026T170000Z
UID:TALK20500@talks.cam.ac.uk
CONTACT:Ian Farnan
DESCRIPTION:Whenever a phase transition is described an immediate\nquestio
 n that is asked is whether it is first order\, involving the\ncoexistence 
 of two phases or second order\, where one phase changes uniformly and cont
 inuously 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\, ther
 e is controversy over the nature of the phase transition. It is our opinio
 n that much of this controversy results from the macroscopic nature of the
  measuring techniques so instead we measured the order parameter on a loca
 l scale using large-angle convergent-beam electron diffraction. This immed
 iately showed that the phase transition was first order. The measurements 
 allowed us to go far beyond this simple characterisation\, however\, and w
 e 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 t
 he 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.
LOCATION:Harker 2 lecture room\, Department of Earth Sciences\, Downing Si
 te
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