Resonance Raman scattering (RRS) of two model perovskite-type $\left({\mathrm{ABO}}_3\right)$ relaxor compounds ${\mathrm{PbSc}}_{0.5}{\mathrm{Ta}}_{0.5}{\mathrm{O}}_3$ (PST) and ${\mathrm{PbSc}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$ (PSN) excited with a laser wavelength of 325 nm (3.8 eV) is studied at different temperatures and ambient pressure as well as at high pressures and room temperature (for PST). The origin of the observed RRS is reinspected by applying group-theory analysis of phonons compatible with symmetry-allowed electron transitions in cubic and possible polar and nonpolar rhombohedral ferroic structures. It is shown that the simultaneous enhancement of first- and second-order RRS generated by antisymmetric ${\mathrm{BO}}_6$ bending and stretching modes under resonance conditions when the photon energy is slightly above the energy gap $E_g\sim 3.2\text{eV}$ results exclusively from spatial regions with coherent polar structural distortions. Upon cooling RRS appears in the vicinity of the characteristic temperature $T^{*}$, and its total intensity significantly increases upon further temperature decrease. The predominate type of ${\mathrm{BO}}_6$ polarity changes from related to difference in B-O bonds to related to distorted O-B-O bond angles. At room temperature and high pressures RRS drops in intensity above the critical pressure of development of long-range antiphase octahedral tilting. However it persists up to 8.3 GPa, which is the highest pressure reached in the experiment, indicating that the high-pressure phase is polar due to the slight ${\mathrm{BO}}_6$ distortions accompanying the tilt order.

• Received 1 May 2014
• Revised 19 July 2014

DOI:https://doi.org/10.1103/PhysRevB.90.064107