  Despite the technological significance of the tetragonal PbTiO3 for the
piezoelectric transducer industry, its high pressure behaviour is quite
controversial as two entirely different scenarios, involving pressure induced
(1) morphotropic phase boundary (MPB) like structural transition with
concomitant rotation of the ferroelectric polarization vector and (2)
antiferrodistortive (AFD) phase transition followed by emergence of a reentrant
ferroelectric phase, have been proposed in recent theoretical and experimental
studies. We have attempted to address these controversies through a high
resolution synchrotron x-ray diffraction study of pressure induced phase
transitions in the tetragonal phase of a modified PbTiO3 composition containing
50% BiFeO3, where BiFeO3 was added to enhance the AFD instability of PbTiO3. We
present here the first experimental evidence for the presence of the
characteristic superlattice reflections due to an AFD transition at a moderate
pressure pc1 ~2.15 GPa in broad agreement with scenario (2), but the high
pressure ferroelectric phase belongs to the monoclinic space group Cc, and not
the tetragonal space group I4cm predicted under scenario (2), which permits the
rotation of the ferroelectric polarization vector as per scenario (1). We show
that the monoclinic distortion angle and ferroelectric polarization of the Cc
phase initially decrease with increasing pressure for p < 7 GPa, but start
increasing above pc2 ~ 7 GPa due to an isostructural Cc-I to Cc-II transition
reminiscent of MA(apc > bpc ~ cpc) to MB(apc< bpc ~ cpc) transition predicted
for MPB systems. We also show that octahedral tilting provides an efficient
mechanism for accommodating pressure induced volume reduction for the
stabilisation of the reentrant ferroelectric phase Cc-II.
