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.

Alpha-U has been observed to undergo the following sequence of transformations at T 0 < 43K (lower limit of stability of the structure) a transition involving modes for which q CDW = < q x , q y , q z >; ortho.: Cmcm T 0 = 43K > mono.: C2/m11 (q x , q y , q z) T 1 = 37K > mono.: P211 (1/2, q y , q z). The phonon dispersion was measured by neutron inelastic scattering in the range (200, 43K) of existence of the high-temperature orthorhombic phase and in the range of the phase transformations at 43, 37 and 22K. Soft branches were associated with the normal-to-incommen-surate transitions in Brillouin zone: (201). The main component of the displacement pattern is consistent with the symmetry for a Σ 4 phonon mode. The static displacements associated with the displacive transition are produced by low-frequency and damped phonons at positions q s [(q x , q y , q z)] which on approaching the second-order phase transition (T 0) soften more than those with q c = [1/2, 0, 0], but not totally. Increasing the energy resolution by using cold neutrons on the three axis spectrometer IN14 near (101), we have seen in the range T-T 0 = 7K a small deviation from the linear law of Curie. The experimental phonon softening [1] which is accompanied by large changes in cell parameters at T 0 , is dependent on q y (T), q z (T) contrary to predictions of the Yamada theory. At T 0 = 43 K the modulation wave vector of the incommensurate low-temperature condensing soft mode is q min = [0.497 (1), 0.13 (1), 0.21 (1)] (q CDW. The electronic instability which causes Kohn anomaly also triggers the displacive (Peierls) transition. Relaxors are special class of ferroelectrics that exhibit a broad, diffuse phase transition over a temperature range and a strong frequency dependence of the dielectric constant as a function of temperature. Near room temperature they exhibit very high dielectric permittivity, electrostrictive and electrooptical coefficients , which determine relaxors as multi-functional materials, with a wide range of technological applications, including non-volatile memory devices. The global, average structure of relaxors, detectable by diffraction methods, remains pseudo-cubic even at liquid helium temperatures, whereas their nanoscale structure is rather complex. Near the Curie range and under zero-field conditions the ferroic clusters are sized only a few unit-cell parameters and they create and annihilate within 10-5-10-6 s. Thus, because of its length-and timescale sensitivity, inelastic light scattering is vital for gaining structural information. The mechanism of paraelectric-to-relaxor ferroelectric phase transition is still not clarified. To better understand the local structural phenomena occurring in relaxors we have applied Raman scattering and X-ray diffraction on single crystals of stoichiometric PbSc 0.5 Ta 0.5 O 3 (PST), solid solutions of type PbSc 0.5 Ta .5 O 3-PbSc 0.5 Nb .5 O 3 and PbSc 0.5 Ta .5 O 3-PbSnO 3 , A-site mixed (Pb 1-x A'' x)Sc 0.5 Ta 0.5 O 3 (A'' = Ba,), as well as Ru-doped PbSc 0.5 Ta 0.5 O 3. The ...

We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc(0.5)Ta(0.5)O(3) (PST) and Pb(0.78)Ba(0.22)Sc(0.5)Ta(0.5)O(3) (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is Fm3m as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the 111 cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either R3c or R3 symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a(-)a(-)a(-) that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3 GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35 GPa.

The effect of A-site incorporated Ba 2+ and Bi 3+ on the pressure-driven structural transformations in Pb-based perovskite-type relaxor ferroelectrics has been studied with in situ x-ray diffraction and 2+ for Pb 2+ represents the case in which A-site divalent cations with stereochemically active lone-pair electrons are replaced by isovalent cations with a larger ionic radius and no active lone pairs, leading to formation of strong local elastic fields. In contrast, substitution of Bi 3+ for Pb 2+ involves the replacement of divalent A-site cations with active lone-pair electrons by aliovalent cations with nearly the same ionic radius and active lone pairs so it induces local electric fields but not strong elastic fields. The two types of dopants have rather distinct effects on the changes in the atomic structure under pressure. The embedding of Ba 2+ and associated elastic fields hinders the development of pressure-induced ferroic ordering and thus smears out the phase transition. The addition of Bi 3+ enlarges the fraction of spatial regions with a pressure-induced ferroic distortion, resulting in a more pronounced phase transition of the average structure, i.e., the preserved lone-pair order and the absence of strong local elastic fields enhances the development of the ferroic phase at high pressure. For all compounds studied, the high-pressure structure exhibits glide-plane pseudosymmetry associated with a specific octahedral tilt configuration.

The structural alteration induced by the substitution of three-valent cations with an isotropic electronic outermost shell for Pb 2+ in perovskite-type relaxors was investigated in the solid solutions Pb 1-x La x Sc (1+x)/2 Ta (1-x)/2 O 3 , x = 0.08 (PST-La) and Pb 1-. In order to distinguish the "charge" effects from "strain" effects associated with the incorporation of La 3+ in the structure, Srcontaining PbSc 0.5 Nb 0.5 O 3 was characterized as well. The structure of the compounds was analyzed by in-situ Raman spectroscopy, single-crystal x-ray diffraction and powder neutron diffraction at different temperatures or pressures. It is shown that the embedding of La 3+ strongly affects the ferroic structural species due to "strain" effects through a disturbance of the system of lone-pair electrons associated with Pb 2+ and a decrease in the tolerance factor. La doping suppresses the dynamical coupling between off-centered Pb and B-site cations and enhances anti-phase BO 6 octahedral tilting which, depending on the level of doping, may lead to long-range order of anti-phase BO 6 tilts at ambient conditions and frustrated anti-ferroelectric order of Pb ions at low temperatures.