Showing papers by "Ruyan Guo published in 1996"

Temperature dependent Raman scattering in KTiOPO4 and KTiOAsO4 single crystals

Abstract: 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.

Temperature dependent Raman spectroscopic studies on microwave dielectrics Sr(AL1/2Ta1/2)O3 and Sr(Al1/2Nb1/2)O3

Abstract: 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 (disordered phase) were prepared by laser heated pedestal growth technique (LHPG) and ordered ceramics samples were prepared by conventional sintering technique. We report Raman vibrational spectrum studies on them for the first time. Order-disorder effects of (A1, Ta) and (Al, Nb) sites were studied 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.

Temperature‐Dependent Raman Studies of Ba(Mg 1/3Ta2/3)O3

Abstract: Microprobe Raman spectroscopy has been used to study ceramics and single-crystal fibres of BaMg1/3Ta2/3O3. According to x-ray diffraction, ceramics exhibit 1:2 order on the B sites whereas fibres remain disordered. The Raman spectra, however, are almost identical in both cases and are mostly consistent with 1:1 short-range order in nanoscale regions. Some extra lines of weak intensity originate from distortions in microstructures, probably providing charge compensation. Raman scattering measurements did not reveal the existence of 1:2 ordering in the nanoscale region in ceramics or single crystals.

Dielectric properties of CsTiOAsO4 single crystal

Abstract: The dielectric properties of CsTiOAsO4 single crystals have been measured in the temperature range from -170°C to 200°C at different frequencies. The material exhibits a clear dielectric relaxation process. The dielectric relaxation process is well described by the Cole-Cole plots with a relaxation time distribution parameter of 0.17. The temperature dependence of optimum relaxation time of the relaxation process obeys the Arrhenius relationship reasonably well with an activation energy of 0.4eV. Based on the above results, the mechanism of the relaxation process is discussed using the polarizability and movement characteristics of the Cs+ ion in the open crystallographic structure.

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...

Raman scattering in CsTiOAsO4, single crystal

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 CsTiOAsO4 (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 substantially with increasing temperature due to high mobility of Cs+ ion. Transition to higher symmetry structure was noticed above 940K and the corresponding changes in the phonon spectra were observed.

Single crystal fibers of rare earth ortho-niobate: Growth, ferroelastic property, and domain studies

Abstract: One dimensional single crystals of LaNbO/sub 4/, NdNbO/sub 4/ and CeNbO/sub 4/ have been grown successfully by a laser heated pedestal growth technique. Some physical properties and the ferroic phase transitions as a function of temperature are studied over the range room temperature to 800/spl deg/C by various thermal, electric and optical analytical techniques. Twin walls are observed, and their relations with the ferroelastic phase transition are discussed.

The effect of annealing temperature on the formation of SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) thin films

Abstract: In this paper we report on synthesis of SBT thin-films by sol-gel processing. Sr metal, Bi, 2-ethyl hexanoate and Ta-ethoxide were used as precursors. Thin-films with nominal composition SrBi/sub 2/Ta/sub 2/O/sub 9/ and SBT +10% excess Bi content were made. Films were annealed at various temperatures to study the microstructure, crystallization temperature and the polarization values. Good crystallization of SBT was obtained by annealing at 700/spl deg/C-2 hrs, independent of the Bi content in the films. Films annealed in oxygen atmosphere at 800/spl deg/C-2 hrs did not show any significant change in the polarization value. Crack free films were made with film thicknesses of 0.4 /spl mu/m. Films annealed at 800/spl deg/C-2 hrs showed a grain size of /spl sim/0.2 /spl mu/m, and reasonably good polarization values of 5 /spl mu/C/cm/sup 2/. In contrast, films prepared with 10% excess Bi showed a very fine grain size <0.1 /spl mu/m with a lower polarization values of 1.5 /spl mu/C/cm/sup 2/.

Crystal structure analysis of ferroelectric tetragonal tungsten bronze Pb/sub 0.596/Ba/sub 0.404/Nb/sub 2.037/O/sub 6/

Abstract: Lead barium niobate single crystals of the composition Pb/sub 0.596/Ba/sub 0.404/Nb/sub 2.037/O/sub 6/ near the tetragonal orthorhombic morphotropic phase boundary with tetragonal symmetry at room temperature, were the subject of this study. For the first time, the crystallographic details, including the ionic coordination, the site occupancies and the thermal anisotropy of a lead-containing tetragonal tungsten bronze crystal are determined. This paper reports the structural refinement results, the site-preference by cations and vacancies, the macroscopic polarization properties derived from the cation displacement, and the apparent valences determined by the bonding distance. Unlike other non-lead-containing ferroelectric tungsten bronzes, significant polarization contribution can be attributed to cations (Pb2/sup +/ and Ba/sup 2/+) at the A2-site. Large anisotropic thermal coefficients found for cations at the A2-site further strengthens the local polarization fluctuation model and suggests the local polarization components are perpendicular to the minor plane along the direction.