Seong M. Cho

Softening and mode crossing of the lowest-frequency A1(transverse-optical) phonon in single-crystal PbTiO3

The polarized Raman spectra of high-quality single-crystal PbTiO3 were measured in the temperature range between 300 and 700 K. The soft-mode behavior of the A1(1TO) (transverse- optical) phonon was observed directly. Above 400 K, however, an intensity transfer between the subpeaks of the A1(1TO) mode was observed, and the line shape of the A1(1TO) mode strongly deviated from the theoretical prediction based on the harmonic approximation in the interatomic potential. In addition, the mode crossing between the A1(1TO) phonon and the E(1LO) (longitudinal optical) phonon was observed. The observed mode crossing behavior clearly supports the assignment of the E(1LO) and A1(1TO) modes, which was originally suggested by Foster and co-workers [Phys. Rev. Lett. 71, 1258 (1993)].

Effects of La doping on the cubic–tetragonal phase transition and short-range ordering in PbTiO3

Effects of cation-site vacancies on the characteristics of the cubic–tetragonal displacive transitions in La-modified PbTiO3 (PLT) were studied using in situ XRD measurements and Raman scattering. For this purpose, two distinct types of PLT series having cation-site vacancies either at the A site (PLT-A) or at the B site (PLT-B) were separately prepared. The PLT-A series exhibited normal ferroelectricity and underwent the cubic-to-tetragonal structural transition for La substitution up to 30 at. %. Contrary to this, the PLT-B became a typical relaxor with the absence of the macroscopic cubic–tetragonal transition. However, for both types of PLTs, Raman-forbidden bands appeared in the high-temperature cubic-phase field at 30 at. % La modification. These observations were attributed to the formation of short-ranged clusters with tetragonal symmetry in a globally cubic matrix and to the reduced dipolar interaction between the clusters in the PLT-B, as compared with the PLT-A.

Polarized Raman scattering of epitaxial PbTiO3 thin film with coexisting c and a domains

Effects of the orientation of ferroelectric domains on the characteristics of polarized Raman spectra were studied using an epitaxially grown PbTiO3 thin film in which the c-axis oriented domains coexist with the a-axis oriented domains on MgO (001). To obtain polarized spectra for both two distinctive c and a domains, we have employed scattering configurations in which the relevant phonon wave vector, k, is perpendicular to the c axis of the tetragonal unit cell. Compared with the mode frequencies of single-crystal PbTiO3, a softening of the E(TO) phonons was evident for both c and a domains, suggesting the presence of a strong tensile film stress. In addition to this, we observed a splitting of the degenerate “silent” mode into two distinctive B1 and E modes in an epitaxially grown film on MgO (001).

Transient Two-Phase Flow in Slide-Gate Nozzle and Mold of Continuous Steel Slab Casting with and without Double-Ruler Electro-Magnetic Braking

Transient mold flow could produce undesirable surface instabilities and slag entrainments, leading to the formation of defects during continuous slab casting of steel. In this work, two Large Eddy Simulations coupled with Discrete Phase Model are run, with and without MagnetoHydroDynamic model, to gain new insights into the surface variations of molten steel-argon gas flow with anisotropic turbulence in the slide-gate nozzle and the mold, with and without double-ruler Electro-Magnetic Braking (EMBr). The model calculations are validated with plant measurements, and applied to investigate the flow variations related to the slide gate on nozzle swirl, jet wobbling, and surface flow variations by quantifying the variations of velocity, horizontal angle, and vertical angle of the transient flow. Transient flow in the slide-gate nozzle bottom is almost always swirling, alternating chaotically between clockwise and counter-clockwise rotation. The clockwise swirl, caused by stronger flow down the same side of the nozzle as the open area near the Outside Radius side of the slide-gate middle plate, produces faster jet flow and higher velocity flow across the top surface of the mold. Counter-clockwise swirl produces slower jet and surface flow, but with more variations. The double-ruler EMBr decreases the asymmetry and duration of velocity variations during nozzle swirl flipping, resulting in less flow variations in the jet and across the surface in the mold.

Subpeak structure of the lowest-frequency E-symmetry transverse optical phonon in Ba-doped PbTiO3 single crystals

Polarized Raman scattering study of Ba-doped PbTiO3 single crystals indicated that, for a limited temperature range between 650 and 750 K, the line shape of the E(1TO) “soft” mode could not be described by a single damped harmonic oscillator but was adequately fitted by two independent oscillators. The line-shape fitting of the E(1TO) spectrum further indicated a gradual intensity inversion between the two subpeaks with increasing temperature. For temperatures below the crossover point, the subpeak with a higher mode frequency dominated the line shape. Contrary to this, the line shape was predominantly described by the subpeak with a lower frequency for temperatures above the crossover point. The subpeak structure becomes progressively evident with increased content of barium (Ba) dopant. We examined possible origins of the observed subpeak structure with the crossover in the integrated intensity and attributed this anomalous behavior to localized defects caused by the Ba impurity.