Powdered samples of the type Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb, are synthesized over the range 0≤x≤0.5 starting from nitrate solutions of the rare earths. X‐ray diffraction and Raman scattering are used to analyze the samples. These compounds, at least in the low doping regime and for strictly trivalent dopants, form solid solutions that maintain the fluorite structure of CeO2 with a change in lattice constant that is approximately proportional to the dopant ionic radius. The single allowed Raman mode, which occurs at 465 cm−1 in pure CeO2, is observed to shift to lower frequency with increasing doping level for all the rare earths. However, after correcting for the Gruneisen shift from the lattice expansion, the frequency shift is actually positive for all the strictly trivalent ions. In addition, the Raman line broadens and becomes asymmetric with a low frequency tail, and a new broad feature appears in the spectrum at ∼570 cm−1. These changes in the Raman spectrum are attributed to O va...

Raman and x‐ray studies of Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb

Titanium dioxide (TiO2 ) nanocrystals are prepared by a hydrolysis process of tetrabutyl titanate. Nanocrystal samples with various sizes of 6.8-27.9 nm are obtained after annealing from 100 to 650 °C. The crystal structures and the average particle sizes are examined using x-ray diffraction. Raman scattering was employed to investigate the evolution of the anatase phase in the nanocrystals during annealing. Phonon confinement and non-stoichiometry effects are responsible for the blueshift and broadening of the lowest-frequency Eg Raman mode. The influence of interfacial vibrations on the Raman linewidth is also discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/33/8/305/pdf

Raman scattering study on anatase TiO2 nanocrystals

The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.

Perovskite‐Type Oxides—the New Approach to High—Tc Superconductivity

Luminescence of inorganic solids: From isolated centres to concentrated systems

The physical properties of a topologically disordered amorphous material (glass), such as heat capacity and thermal conductivity, are markedly different from those of its ordered crystalline counterpart. The understanding of these phenomena is a notoriously complex problem. One of the universal features of disordered glasses is the 'boson peak', which is observed in neutron and Raman scattering experiments. The boson peak is typically ascribed to an excess density of vibrational states. Here, we study the nature of the boson peak, using numerical simulations of several glass-forming systems. We discovered evidence suggestive of the equality of the boson peak frequency to the Ioffe-Regel limit for 'transverse' phonons, above which transverse phonons no longer propagate. Our results indicate a possibility that the origin of the boson peak is transverse vibrational modes associated with defective soft structures in the disordered state. Furthermore, we suggest a possible link between slow structural relaxation and fast boson peak dynamics in glass-forming systems.

Universal link between the boson peak and transverse phonons in glass.

We have studied the effect of isotopic substitution on the superconducting T/sub c/ in the 90-K superconductors Ba/sub 2/YCu/sub 3/O/sub 7/ and Ba/sub 2/EuCu/sub 3/O/sub 7/ by replacing /sup 16/O with the heavier isotope /sup 18/O. Samples with approximately 75% of the oxygen /sup 16/O replaced by /sup 18/O were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected --4%. The transition temperatures, however, are found to change by less than 0.2%. This change in T/sub c/ is much less than that expected for strongly coupled phonon-mediated superconductivity.

Isotope effect in the high-Tc superconductors Ba2YCu3O7 and Ba2EuCu3O7.

We present Raman and infrared absorption data for ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$ with 7gxg6 for several samples with well-characterized oxygen content. Over this range the material has an oxygen-deficient tripled perovskite structure whose superconductivity properties depend critically on x. An orthorhombic-to-tetragonal transition occurs for x\ensuremath{\approxeq}6.5. We have examined both phases for the frequency range \ensuremath{\omega}g400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which contains the Cu-O bond-stretching vibrations. A force-constant model has been developed which allows us to assign the features in the spectra. Our results are consistent with the linear Cu-O-Cu-O chains in ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$ being absent in the nonsuperconducting ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{6}$.

Cu-O vibrations of Ba 2 YCu 3 O x

Cerium dioxide (${\mathrm{CeO}}_{2}$), which crystallizes in the fluorite structure, has been studied by high-pressure Raman spectroscopy with use of a diamond anvil cell, up to 35 GPa. The pressure shift of the first-order fluorite Raman peak (${F}_{2g}$) and the shifts of several second-order features were measured. The Gr\"uneisen parameter for the ${F}_{2g}$ mode (465 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) is obtained as \ensuremath{\gamma}=1.44 and for the ${F}_{1u}$ (${\ensuremath{\omega}}_{\mathrm{LO}=585}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) as \ensuremath{\gamma}=1.5; the latter from the 2${\ensuremath{\omega}}_{\mathrm{LO}}$ shift in the second-order spectrum. Near 31 GPa, a phase transition occurs and six new Raman peaks appear. By comparison with the high-pressure phases of alkaline-earth difluorides, the high-pressure phase is believed to be of the ${\mathrm{PbCl}}_{2}$-type structure. The high-pressure phase is found to be metastable over a 20-GPa interval, but completely reverses on releasing pressure.

G. A. Kourouklis

Papers

Isotope effect in the high-Tc superconductors Ba2YCu3O7 and Ba2EuCu3O7.

Cu-O vibrations of Ba 2 YCu 3 O x

High-pressure Raman study of CeO 2 to 35 GPa and pressure-induced phase transformation from the fluorite structure

A high-pressure Raman study of yttrium vanadate (YVO4) and the pressure-induced transition from the zircon-type to the scheelite-type structure

A high pressure Raman study of calcium molybdate