G. A. Kourouklis

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

The pressure dependences of the Raman active modes in yttrium vanadate (YVO4) crystallizing in the zircon-type structure D4h19 (I41amd), have been studied using a diamond anvil cell up to 15 GPa. At room temperature the zircon-type structure transforms to the scheelite-type structure C4h6(I41a) with a = 5.04 A and c = 11.24 A near 7.5 GPa. The density changes from 4.24 to 4.74 gcm3 in the transition, and the corresponding volume decrease is ΔV = 5.07 cm3mole. The high-pressure phase is retained on release of pressure. The optical absorption edge shifts from 3.87 to 2.77 eV at the transition, presumably due to charge transfer, or due to V-O bond length change of the VO4 tetrahedra. The internal mode frequencies of the VO4 tetrahedra also decrease abruptly at the transition. The V-O bond stretching frequency decreases from 891 to 828 cm−1 in the scheelite-type phase, which suggests a fundamental change in the VO4 unit. The general question of the stability of the zircon-type structure under high pressure is discussed in the light of the high-pressure behavior of a large number of rare earth vanadates and arsenates.

A high pressure Raman study of calcium molybdate

Abstract In this work we present a Raman study of CaMoO4 as a function of hydrostatic pressure up softening in the scheelite structure, while in the high pressure phases its slope becomes positive. The v1v3 stretching modes of the MoO2−4 group exhibit drastic changes in their pressure dependencies at both phase transitions. This behavior is connected with the increase in the coordination number of the Mo ion in the high pressure phases. These characteristics are compared with the high pressure behavior of other molybdates and tungstates.

Pressure screening in the interior of primary shells in double-wall carbon nanotubes

The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to

New approach in the monitoring and characterization of titanium nitride thin films

10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}

Lattice dynamical properties of the rare earth aluminum garnets (RE3Al5O12)

. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to

Temperature and pressure dependence of Raman-active phonons of CaMoO4: an anharmonicity study

9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}

Laser two-photon ionization of organic molecules in dielectric liquids

, exhibiting a behavior similar (but less pronounced) to that of single-wall carbon nanotubes, which undergo a shape distortion at higher pressures. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. All findings lead to the conclusion that the outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application.

High-pressure induced metastable phase in tetragonal 2D polymeric C60.

In situ and ex situ spectroscopic ellipsometry (SE), Raman spectroscopy (RS), x-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) have been used to study the stoichiometry and characterize TiN x thin films deposited by magnetron sputtering at various stoichiometries. In situ SE can provide parameters, such as the plasma energy, that can be utilized for monitoring of the film stoichiometry. Besides plasma energy, optical phonon position in RS was also found to be a sensitive probe of TiN x stoichiometry as detected by RS, XPS, and ex situ SE. Under these conditions, AES faces difficulties for reliable film characterization, and the complementary use of other techniques is required for determining the exact film stoichiometry.

A high pressure raman study of ThO2 to 40 GPa and pressure-induced phase transition from fluorite structure

This paper reports detailed lattice dynamics studies involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE 3 Al 5 O 12 ). The studies are fairly involved as these garnets have complex crystal structure with 80 atoms/primitive cell. Our calculations have provided a theoretical understanding of the mode eigenvectors, phonon dispersion relations, density of states, and effective charges of these materials. The calculated Raman mode eigenvectors reveal that they correspond to mixtures of molecular modes of the basic polyhedra, implying thus strongly coupled polyhedra. The assignment of the Raman and infrared active modes has been elucidated and the frequencies of some modes, which do not appear in the vibrational spectrum, have been calculated. Our calculations show a differentiation of the effective charges at the various symmetry sites, which leads to a mostly covalent and to an almost ionic character for the tetrahedral Al-O and the dodecahedral RE-O bonds, respectively. Finally, the dispersion curves along the [100] direction of the Brillouin zone as well as the one and two-phonon density of states have been calculated and discussed.

A high pressure Raman study of KTP and pressure induced phase transitions

The Raman spectra of tetragonal CaMoO4 (scheelite structure, C4h6 space group) have been measured in the temperature range 12–1300 K. At high temperatures, the frequency and linewidth of all Raman-active phonons vary almost linearly with temperature, indicating that the three-phonon decay processes are dominant over the four-phonon ones. All Raman phonons display normal negative (∂ω/∂T) slopes throughout the temperature range, except for the lowest-frequency Bg phonon at 111.5 cm−1 which is almost temperature independent in the region 12–400 K, but then for T > 400 K it shows a slight (normal) softening with temperature. The reduced (∂ω/∂T)/ω slopes of the internal modes (vibrations of atoms within the tetrahedral MoO42− units) are an order of magnitude smaller than the respective slopes of the external ones (pure lattice modes). There are no discontinuities or sharp changes of slope in the ω(T) plots, implying that CaMoO4 remains stable over the entire temperature range. Combining the temperature-dependent Raman data of this work and the previously reported pressure-dependent Raman data on this crystal (Christofilos D, Kourouklis G A and Ves S 1995 J. Phys. Chem. Solids 56 1125), as well as the thermal expansion coefficient β(T) and compressibility κ(T) data, it has been possible to separate and evaluate quantitatively the volume (expansion) Δωvol and the pure temperature (anharmonic) Δωanh contributions to the total Δωtot shift of phonons with temperature. It has been found that for most phonons at high temperatures, the volume effect is greater than the pure temperature one, thus indicating that most of the bonds in CaMoO4 are predominantly ionic in character.