Pressure-Induced Phase Transition inPbSc0.5Ta0.5O3as a Model Pb-Based Perovksite-Type Relaxor Ferroelectric

Abstract

We report pressure-induced structural changes in PbSc(0.5)Ta(0.5)O3 studied by single-crystal x-ray diffraction and Raman scattering. The appearance of a soft mode, a change in the volume compressibility, broadening of the diffraction peaks, and suppression of the x-ray diffuse scattering show that a phase transition occurs near pc approximately 1.9 GPa. The critical pressure is associated with a decoupling of the displacements of the B site and Pb cations in the existing polar nanoregions, leading to the suppression of B-cation off-center shifts and enhancement of the ferroic distortion in the Pb-O system.

Role of random electric fields in relaxors

“…A monotonic increase of the diffuse scattering on cooling is observed in all pseudorhombohedral compositions of PMN-xPT and PZN-xPT (25,36). Other neutron and X-ray studies have demonstrated that relaxor diffuse scattering is strongly affected by external electric fields (37,38) and vanishes at high pressures (27,39). For these reasons many researchers identify the diffuse scattering with static, nanometer-scale regions of shortrange polar order known as polar nanoregions that condense from the soft TO mode on cooling from high temperatures well above T max (23,30).…”

“…To answer these questions we compared the nanoscale structures of PMN and PZT using neutron-based measurements of the elastic diffuse scattering, which is arguably the single most interesting and intensely examined property of lead-oxide relaxors to date. Strong, temperaturedependent and highly anisotropic diffuse scattering has been documented in many different lead-oxide perovskite relaxors using X-ray and neutron-scattering methods (20)(21)(22)(23)(24)(25)(26)(27). Fig.…”

Structural state of relaxor ferroelectrics PbSc0.5Ta0.5O3and PbS

“…12,42 According to our previous highpressure studies, the critical pressure p c1 of the continuous cubic-to-rhombohedral phase transition involving a − a − a − tilting is 1.9 GPa for PST and 4.1 GPa for PSN, while the preceding pressure p is 1.2 GPa for PST and 2.5 for PSN. [28][29][30][31][32] The change in the bulk modulus K at p c1 is 44.5 GPa for PST and 15.6 GPa for PSN. The jump of K at p c1 is a combined result of the actual structural difference between the two phases as well as the material volume involved in the phase transition.…”

“…28 The Raman data up to 30 GPa show that the splitting increases with pressure [see Figs. 3 and 5(a)].…”

“…For this purpose we have chosen to study pure PST and PSN in the pressure regime up to 30 GPa, since these two compounds have been thoroughly studied in the low-pressure regime by single-crystal in-house and synchrotron XRD, neutron powder diffraction, and singlecrystal Raman spectroscopy. [28][29][30][31][32] We applied high-pressure single-crystal synchrotron XRD and simultaneous off-beam Raman spectroscopy to the same specimens, using He as a pressure-transmitting medium, which is hydrostatic up to 20 GPa 27 and between 20 and 30 GPa exhibits a negligible degree of deviation from hydrostaticity as compared to the other commonly used pressure-transmitting media. 27 In addition, we performed density functional theory (DFT) calculations for PST and PSN to determine the energetically preferred octahedral tilt patterns at high pressures and to check whether polar or nonpolar structures are favored by high pressure.…”