Dimitrina Petrova

Effect of doping on Raman spectra of Bi12SiO20

Abstract The effect of doping of Bi12SiO20 with various ions on the Raman spectra is investigated. It is shown that the dopants modify in a different way the structure of BSO.

Octahedral tilting in Pb‐based relaxor ferroelectrics at high pressure

We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc(0.5)Ta(0.5)O(3) (PST) and Pb(0.78)Ba(0.22)Sc(0.5)Ta(0.5)O(3) (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is Fm3m as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the 111 cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either R3c or R3 symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a(-)a(-)a(-) that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3 GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35 GPa.

Absorption and MCD study of photochromism in Bi12SiO20 doped with 4D or 5D transition metal ions

We present the first spectroscopic investigation concerning the role of four ions (Ru, Rh, Os, Re) of the platinum group on the photochromic behavior of the photorefractive crystal Bi12SiO20. Absorption and magnetic circular dichroism (MCD) spectra were taken in the thermally bleached state and after various illuminations so as to determine the coloration thresholds. Ruthenium and Osmium are demonstrated to enter the crystal, whereas the answer is not yet certain in the case of rhodium and rhenium. We believe that the doping occurs at the pseudo-octahedral Bi site.

Optical properties of single Pb(Sc12Ta12)O3 crystals

Single ferroelectric Pb(Sc12Ta12)O3 crystals of perovskite structural type were obtained by the high temperature solution growth (HTSG) method. The refractive index was measured by the prism method at 10 selected wavelengths and the dispersion in the visible spectral range was calculated by Sellmeiers formula. Optical transmission and reflection spectra were measured and the absorption coefficient α was determined in the visible range. Using the calculated α(hν) dependence, the α12(hν) function was plotted and it was used to determine the bandgap width and characterize the fundamental absorption edge.

Dielectric and optical properties of Bi12SiO20 crystals doped with Zn and Cd and Co-doped with P

Abstract Single crystals of Bi 12 SiO 20 (BSO) Zn- and Cd-doped and co-doped with P were grown by the Czochralski technique and segregation coefficients were determined. The dielectric permeability e and dielectric loss tan δ were measured at room temperature in two frequency ranges: 10 3 –10 7 and 10 3 –10 5 Hz. Dopants were found to decrease the dielectric permeability in the whole frequency range and P, Zn+P and Cd+P doping increased dielectric loss in the frequency range 10 3 –10 5 Hz. Transmission and reflection spectra were taken in the range 0.38–0.85 μm and absorption was calculated. P, Zn+P and Cd+P doping shifted transmission spectra to the shorter wavelengths and drastically reduced the transmission shoulder region as well as the absorption coefficients.

Optical absorption properties of PB5GEO4(VO4)2 single crystals

Abstract Thermal and pulling conditions have been optimized in a laboratory Czochralski apparatus to yield optically homogeneous crystals of hexagonal Pb 5 GeO 4 (VO 4 ) 2 . The wavelength dependence of absorption coefficient was calculated from transmission and reflection measurements in the visible region. A hypothesis regarding the zone structure in connection to the fundamental absorption edge was raised and the bandgap width corresponding to indirect transition at room temperature was determined from the α 1 2 -hv curve.