Stability of the various crystallographic phases of the multiferroic (1-x)BiFeO3-xPbTiO3 system as a function of composition and temperature

In this report, we have studied the stability of various crystallographic phases of (1 -x)BiFeO3-xPbTiO(3) as a function of composition and temperature. The structure of BF-xPT is reconfirmed to be tetragonal and monoclinic in the P4mm and Cc space groups for x>0.31 and 0.10 <= x <= 0.27, whereas the two phases coexist in the morphotropic phase boundary (MPB) region 0.27<0.31. A recent proposition of an orthorhombic phase in the MPB region has been ruled out comprehensively. From the high temperature powder x-ray diffraction (XRD) data it is shown that the MPB in this system is tilted towards the tetragonal side, a unique feature among the family of PbTiO3 based MPB systems. We have established accurately the room temperature phase diagram for this solid solution. By comparing the observed bond lengths between oxygen and other cations, obtained from Rietveld analysis of the room temperature powder XRD data with expected ionic bond lengths, we have shown that the very high c/a ratio in the tetragonal phase of this system is linked with the covalency effects for bonding between both A and B site cations with oxygen. (c) 2010 American Institute of Physics. [doi :10.1063/1.3437396]