Cubic-tetragonal-orthorhombic-rhombohedral ferroelectric transitions in perovskite potassium niobate

Cubic-tetragonal-orthorhombic-rhombohedral ferroelectric transitions in perovskite potassium niobate  :
neutron powder profile refinement of the structures

Abstract.  The orthorhombic and rhombohedral structures of  ferroelectric potassium niobate have been determined by neutron diffraction measurements  on powdered crystals, using the profile refinement technique to  analyse  the  data.  In the orthorhombic  phase at 22 "C  the atomic
displacements  along [Oll]  from  the  cubic perovskite positions  are 6(K) = -0.006  + SA, 6(Nb) = 0,070A, S(0,)  =  -0.138  + 6A and 6(0,,)  6(0,,,) =  -0,125  + 4A. In the rhombohedral  phase  at  230K,  the  displacements  are  along  [lll];  6(K) = -0,013  i  32A, 6(Nb) = 0.078A  and  6(0,) = 6(0,,) = S(O,,,)  =  -0,140  + 4A. As  a  check  on  the  profile technique, the tetragonal structure was redetermined, and good agreement obtained with the previous  single-crystal  work,  with  displacements  along  [OOl]  of  S(K) = -0,014  + 26A, 6(Nb) = 0.063A,  S(0,)  = -0.117  + 4A  and  6(0,,) = 6(0,,,) = 0099 + 3A. All  of  these transitions  result from the condensation at q  = 0 of soft modes in which almost rigid oxygen octahedra vibrate  against  the potassium  and  niobium  atoms. Our results  account  for  the observed changes  in  the spontaneous polarization  of  potassium  niobate,  and demonstrate the  power  of  the  profile  refinement  technique  for  the  investigation  of  structural phase transitions.


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