C.-S. Tu

Temperature dependent Raman scattering in KTiOPO4 and KTiOAsO4 single crystals

The longitudinal (LO) and transverse (TO) A1 vibrational modes have been measured between 30 and 1200 cm−1 as a function of temperature (30–1240 K) for both KTiOPO4 (KTP) and KTiOAsO4 (KTA). KTP and KTA exhibit an obviously different Raman spectra in the frequency region 400–650 cm−1 (with respect to KTA). This middle‐frequency difference is attributed to the substitution ions in XO4 group (X=P or As) modifing the force constant of crystal. The relative intensities of the low‐frequency bands increase dramatically with increasing temperature due to high mobility of K+ ion. There is no typical soft‐mode like behavior in the measured frequency range. A higher symmetric structure taking place above Tc has been confirmed by the disappearance of the A1g(LO) stretching modes of TiO6 group. Comparison of each frequency belonging to the symmetry A1, A2, B1, and B2 measured along the [110] phonon direction shows complex difference. The vibrational frequencies of various symmetries were also obtained.

Field-cooled-zero-field-heated and zero-field-heated dielectric behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Abstract Temperature and frequency dependent dielectric measurements in the relaxor ferroelectric single crystal (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 (PMN-33%PT) are reported for measured fields along the [001] direction. A sharp phase transition (cuḃic paraelectric phase↔tetragonal ferroelectric phase) was observed near 420 K and 425 K for the zero-field-heated and field-cooled-zero-field-heated runs, respectively. The field-cooled-zero-field-heated dielectric permittivities show that the dc bias field induces a long-range ferroelectric phase transition at 357 K. In addition, the dielectric data ε″c(f,T) taken from the field-cooled-zero-field-heated process, imply the phase coexistence of a short-range ordering (which is attributed to the local structural fluctuations between rhombohedral and tetragonal symmetries) and a long-range ordering due to macro-domain pinning in the region of -330-380 K.

Dielectric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Abstract Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ε′111 (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.

Temperature dependent Raman scattering in RbTiOAsO4 and CsTiOAsO4 single crystals

Abstract The longitudinal (LO) and transverse (TO) A1 vibrational modes have been measured between 30–1200 cm−1 as a function of temperature (30–1240 K) for both RbTiOAsO4 (RTA) and CsTiOAsO4 (CTA). Both crystals exhibit similar Raman spectra in the frequency region above 400 cm−1 with all corresponding Raman components of CTA shifting to lower frequencies. The relative intensities of the low frequency bands increase noticeably with increasing temperature perhaps due to the high mobility of Rb+ (or Cs+) ion. Some of the LO and TO modes show a possible softening as temperature increases. However, there is no typical soft-mode like behavior observed in the measured frequency range. A transformation to higher symmetry crystallographic structure above Tc has been suggested by the group analysis and noticing the disappearance of the LO v1 (A 1g) stretching modes of TiO6 group in both RTA and CTA, and by the splitting of the TO broad band near 240cm−1 in CTA. Comparison of each frequency below 400 cm−1 belongin...

Hypersonic anomaly of Beta-BaB2O4 single crystal

Abstract The Brillouin light back-scattering spectra along the [001] phonon direction have been measured as a function of temperature (25–960 °C) for a BetaBaB2O4 (BBO) single crystal. As temperature increases, the longitudinal acoustic (LA) phonon frequency of BBO shows an obvious softening with a weak modification of slope near 500 °C. We propose that the structural transition (α-phase→β-phase) of BBO occurs near 500 °C and is first-order type. In addition, a broad damping evolution was observed and can be attributed to the dynamic order-parameter fluctuations. With the previous result, the LA[001] sound velocity VLA was also calculated at room temperature.

Temperature Dependent Raman Scattering in CsTiOAsO 4 Single Crystal

The longitudinal (LO) and transverse (TO) A 1 vibrational modes have been measured between 30-1200 cm −1 as a function of temperature (30–1240 K) for CsTiOAsO 4 (CTA). The frequencies for all corresponding Raman components shifted to lower frequencies on increasing the temperature, however, there is no typical soft-mode like behavior observed in the measured frequency range. The relative intensities of the low frequency bands increase dramatically with increasing temperature due to high mobility of Cs + ion. A higher symmetry structure taking place above 940K has been confirmed by changes in the phonon spectra.

Raman Studies on Potential substrate Materials of Sr(Al 1/2 Ta 1/2 )O 3 and Sr(Al 1/2 Nb 1/2 )O 3 For Htsc

Complex oxide perovskites, namely strontium aluminum tantalum oxide (SAT) and strontium aluminum niobium oxide (SAN) were recently investigated to be potential substrate materials for HTSC films in microwave applications. Single crystals were prepared by laser heated pedestal growth technique (LHPG). We report Raman vibrational spectrum studies on them for the first time. Order-disorder effects of (Al, Ta) and (Al, Nb) sites were studied with particular interests by comparing Raman spectra of single-crystal samples with ceramic samples. Influences of B sites (Ta and Nb) on O-O modes are discussed in relation to their dielectric properties.