Crystal structure and relaxor-type transition in SrBi2Ta2O9 doped with praseodymium

The effects of Pr substitution in the structure and ferroelectric response for the Sr(1-x)PrxBi(2)Ta(2)O(9) ( SBT- Pr) compound have been studied. Rietveld refinement of the x- ray diffraction patterns indicates that the Pr ion progressively replaces the Sr site in the A2(1)am space group structure. The solubility of Pr in solid solution is around 15%. The replacement induces a change in the crystal structure and, as a consequence, the dielectric properties are affected. The ferroelectric transition at T(m) similar to 558 K is shifted to lower temperatures, T(m) similar to 413 K for x = 0.15 composition. Apparently, the tilt angle (alpha) associated with the c- axis does not play an important role since it remains essentially constant. However, the rotation in the ab- plane ( beta) as well as the octahedral distortion observed are strongly related to the coupling between T(m) and x. A relaxor- type transition is observed as Pr is increased, leading to polar microregions above the nominal ferroelectric transition. The local disorder induced by the Pr ion is confirmed by the continuous increase in the diffuseness coefficient according to Isupov's model. These facts hinder the displacement of the TaO(6) octahedra with respect to Bi(2)O(2) along the polarization axis, decreasing the polarization values.