Temperature-driven structural transformations in Pb-based perovskite-type relaxors are studied by using polarized Raman spectroscopy, high-resolution powder, and synchrotron single-crystal x-ray diffraction applied to PbSc 0.5 Ta 0.5 O 3 ͑PST͒ and Pb 0.78 Ba 0.22 Sc 0.5 Ta 0.5 O 3 ͑PBST͒. The two compounds were chosen as model systems because PST is a relaxor that exhibits ferroelectric long-range order on cooling, whereas PBST shows canonical relaxor behavior. The temperature evolution of phonon anomalies and the pseudocubic unit-cell parameter for both PST and PBST reveals the existence of a characteristic temperature T ء between the Burns temperature T B and the temperature of the dielectric-permittivity maximum T m. T ء is associated with the coupling of initially nucleated small polar clusters and their aggregation into larger polar clusters. The temperature range between T B and T ء is characterized by a coupling between adjacent off-centered BO 6 octahedra to form initial polar clusters, while the range between T ء and T m is characterized by a coupling between off-centered B cations from adjacent polar clusters. Off-centered Pb atoms exist even above the Burns temperature and their length of coherence governs the coupling between polar regions comprising B-cation offcenter shifts and, consequently, directs the formation of the ferroelectric state to a normal, long-range ordered or nonergodic relaxor state.
Variation of a bond angle can tune the reactivity of a chemical compound. To exemplify this concept, the nature of the siloxane linkage (Si-O-Si), the most abundant chemical bond in the earth's crust, was examined using theoretical calculations on the molecular model compounds H(3)SiOSiH(3), (H(3)Si)(2)OHOH, and (H(3)Si)(2)OHOSiH(3) and high-resolution synchrotron X-ray diffraction experiments on 5-dimethylhydroxysilyl-1,3-dihydro-1,1,3,3-tetramethyl-2,1,3-benzoxadisilole (1), a molecular compound that gives rise to the formation of very rare intermolecular hydrogen bonds between the silanol groups and the siloxane linkages. For theoretical calculations and experiment, electronic descriptors were derived from a topological analysis of the electron density (ED) distribution and the electron localization function (ELF). The topological analysis of an experimentally obtained ELF is a newly developed methodology. These descriptors reveal that the Si-O bond character and the basicity of the siloxane linkage strongly depend on the Si-O-Si angle. While the ionic bond character is dominant for Si-O bonds, covalent bond contributions become more significant and the basicity increases when the Si-O-Si angle is reduced from linearity to values near the tetrahedral angle. Thus, the existence of the exceptional intermolecular hydrogen bond observed for 1 can be explained by its very small strained Si-O-Si angle that adopts nearly a tetrahedral angle.
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.
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