74 research outputs found
Far infrared properties of the rare-earth scandate DyScO3
We present reflectance measurements in the infrared region on a single
crystal the rare earth scandate DyScO3. Measurements performed between room
temperature and 10 K allow to determine the frequency of the infrared-active
phonons, never investigated experimentally, and to get information on their
temperature dependence. A comparison with the phonon peak frequency resulting
from ab-initio computations is also provided. We finally report detailed data
on the frequency dependence of the complex refractive index of DyScO3 in the
terahertz region, which is important in the analysis of terahertz measurements
on thin films deposited on DyScO3
Raman scattering of perovskite DyScO3 and GdScO3 single crystals
We report an investigation of DyScO3 and GdScO3 single crystals by Raman
scattering in various scattering configurations and at various wavelengths. The
Raman spectra are well-defined and the reported spectral signature together
with the mode assignment sets the basis for the use of Raman scattering for the
investigation of RE-scandates. The observed positions of Raman modes for DyScO3
are for most bands in reasonable agreement with recent theoretical ab initio
predictions of the vibrational spectrum for the same material. Further to the
phonon signature, a luminescence signal is observed for both scandates. While
the luminescence is weak for DyScO3, it is very intense for GdScO3 when using a
488 or 514 nm excitation line, which in turn inhibits full analysis of the
phonon spectrum. We show that a meaningful phonon Raman analysis of GdScO3
samples can be done by using a 633 nm excitation
Raman scattering of perovskite SmScO3 and NdScO3 single crystals
We report an investigation of perovskite-type SmScO3 and NdScO3 single
crystals by Raman scattering in various scattering configurations and at
different wavelengths. The reported Raman spectra, together with the phonon
mode assignment, set the basis for the use of Raman scattering for the
structural investigation of RE-scandates. Further to the phonon signature, a
fluorescence signal is observed for both scandates and is particularly intense
for NdScO3 when using a 488 or 514 nm excitation line. A comparison of Raman
spectra of RE-scandates with literature Raman data on orthorhombic perovskites
shows that the frequency of particular modes scales with the orthorhombic
distortion in terms of the rotation (or tilt) angle of the ScO6 octahedr
Temperature-dependent Raman scattering of DyScO3 and GdScO3 single crystals
We report a temperature-dependent Raman scattering investigation of DyScO3
and GdScO3 single crystals from room temperature up to 1200 {\deg}C. With
increasing temperature, all modes decrease monotonously in wavenumber without
anomaly, which attests the absence of a structural phase transition. The high
temperature spectral signature and extrapolation of band positions to higher
temperatures suggest a decreasing orthorhombic distortion towards the ideal
cubic structure. Our study indicates that this orthorhombic-to-cubic phase
transition is close to or higher than the melting point of both rare-earth
scandates (\approx 2100 {\deg}C), which might exclude the possibility of the
experimental observation of such a phase transition before melting. The
temperature-dependent shift of Raman phonons is also discussed in the context
of thermal expansion
Strain analysis of multiferroic BiFeO3-CoFe2O4 nanostructures by Raman scattering
We report a Raman scattering investigation of columnar BiFeO3-CoFe2O4
(BFO-CFO) epitaxial thin film nanostructures, where BFO pillars are embedded in
a CFO matrix. The feasibility of a strain analysis is illustrated through an
investigation of two nanostructures with different BFO-CFO ratios. We show that
the CFO matrix presents the same strain state in both nanostructures, while the
strain state of the BFO pillars depends on the BFO/CFO ratio with an increasing
tensile strain along the out-of-plane direction with decreasing BFO content.
Our results demonstrate that Raman scattering allows monitoring strain states
in complex 3D multiferroic pillar/matrix composites.Comment: revised version submitted to Appl. Phys. Let
Phase transition close to room temperature in BiFeO3 thin films
BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be
mapped by appropriately substrate-induced strain in epitaxial films. By using
Raman spectroscopy, we conclusively show that films of the so-called
supertetragonal T-BFO phase, stabilized under compressive strain, displays a
reversible temperature-induced phase transition at about 100\circ, thus close
to room temperature.Comment: accepted in J. Phys.: Condens. Matter (Fast Track Communication
Enthalpy of formation of yeâelimite and ternesite
Calcium sulfoaluminate clinkers containing yeâelimite (Ca4Al6O12(SO4)) and ternesite (Ca5(SiO4)2SO4) are being widely investigated as components of calcium sulfoaluminate cement clinkers. These may become low energy replacements for Portland cement. Conditional thermodynamic data for yeâelimite and ternesite (enthalpy of formation) have been determined experimentally using a combination of techniques: isothermal conduction calorimetry, X-ray powder diffraction and thermogravimetric analysis. The enthalpies of formation of yeâelimite and ternesite at 25 °C were determined to be â 8523 and â 5993 kJ molâ1, respectively
Raman scattering from Ti3SiC2 single crystals
International audienceThe lack of single crystalline Ti3SiC2 samples is currently limiting the accurate measurement of its basic properties as its layered crystalline structure presents a very strong anisotropy. In this letter, we report the growth of pure Ti3SiC2 single crystals after a careful study of the Ti3SiC2 liquidus surface extent through thermodynamical calculations. From a Raman scattering study on those single crystals, an unambiguous assignment of most of the phonon modes has been established, giving an answer to the discrepancies existing in the literature. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3558919
Oxidation resistance of decorative (Ti,Mg)N coatings deposited by hybrid cathodic arc evaporation-magnetron sputtering process
International audienceTitanium-magnesium nitride coatings (Ti,Mg)N were deposited on steels and silicon substrates by hybrid reactive arc evaporation-magnetron sputtering process from cathodic Ti and sputter Mg targets in an argon/nitrogen gas mixture. X-ray diffraction analyses (XRD) of as-deposited coatings with various Mg/Ti atomic ratios gave evidence of a fcc TiN-like structure strongly oriented in the [111] direction. The TiN lattice parameter increases with the addition of Mg resulting from the substitution of Ti atoms by Mg ones. Optical investigations by spectrophotometry revealed that Mg addition to TiN leads to a change in colour from golden through coppery and violet to grey. Nanoindentation measurements showed that increasing Mg content does not alter the hardness of coatings. As-deposited films were annealed in air from 450 to 750 degrees C with a 100 degrees C step. XRD and Raman analyses revealed the formation of rutile TiO(2) and MgTiO(3) phases. Secondary neutral mass spectrometry measurements were performed to study the elemental depth profiles after air annealing. A diffusion of Mg atoms towards the film surface was evidenced above 650 degrees C. leading to the formation of the MgTiO(3) phase. However, thermogravimetric measurements showed that this oxide phase did not protect the films against high temperature oxidation. On the contrary, below 650 degrees C Mg affords to TiN a beneficial protective effect, able to reduce the oxidation kinetics by half. (C) 2011 Elsevier B.V. All rights reserved
Phase transformations and selective growth in YMnO3 films
International audienceAn irreversible phase transformation from amorphous to crystalline orthorhombic YMnO3 (o-YMO) phase takes place in Y-Mn-O films deposited by pulsed-injection metal organic chemical vapor deposition (MOCVD) on Si(100) substrate. This phase transformation was studied through ex-situ and in-situ thermal annealings and the corresponding structural changes were analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. It was shown that it takes place at an almost constant temperature (similar to 700 degrees C) and in a short period of time (similar to min) and that the o-YMO phase thus obtained is stable at least up to 900 degrees C. We also demonstrate that the selective growth of amorphous, orthorhombic or hexagonal films is made possible by adapting the MOCVD temperature according to the temperature stability region of the different phases. (C) 2014 Elsevier Inc. All rights reserved
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