46 research outputs found
First-order Raman spectra of double perovskites AB''{1/2}O3
First principles computations of Raman intensities were performed for
perovskite-family compound CaAlNbO (CAN). This compound
features 1:1 (NaCl-type) ordering of Al and Nb superimposed onto the
octahedral tilting. Raman tensor for CAN was computed using the package for
first-principles computations ABINIT (URL \underline {http://www.abinit.org}).
Computations performed for both untilted cubic () and tilted
monoclinic () CAN structures showed that the strongest Raman lines are
associated with the ordering of Al and Nb. The computed spectrum agreed
qualitatively with the experimental data measured on powder (CAN is available
in polycrystalline form only). The effect of cation disorder on the Raman
intensities was considered using phenomenological theory of light scattering in
the vicinity of a phase transition. We suggest that, for certain modes, the
corresponding Raman intensities depend primarily on the average long range
order while, for other modes, the intensities are determined by fluctuations of
the order parameter.Comment: 4 figures, submitte
Direct observation of the formation of polar nanoregions in Pb(MgNb)O using neutron pair distribution function analysis
Using neutron pair distribution function (PDF) analysis over the temperature
range from 1000 K to 15 K, we demonstrate the existence of local polarization
and the formation of medium-range, polar nanoregions (PNRs) with local
rhombohedral order in a prototypical relaxor ferroelectric
Pb(MgNb)O. We estimate the volume fraction of the PNRs as a
function of temperature and show that this fraction steadily increases from 0 %
to a maximum of 30% as the temperature decreases from 650 K to 15 K.
Below T200 K the PNRs start to overlap as their volume fraction reaches
the percolation threshold. We propose that percolating PNRs and their
concomitant overlap play a significant role in the relaxor behavior of
Pb(MgNb)O.Comment: 4 pages, 3 figure
Quasi-Elastic Scattering, Random Fields and phonon-coupling effects in PbMg1/3Nb2/3O3
The low-energy part of the vibration spectrum in PbMgNbO
(PMN) relaxor ferroelectric has been studied by neutron scattering above and
below the Burns temperature, T. The transverse acoustic and the lowest
transverse optic phonons are strongly coupled and we have obtained a model for
this coupling. We observe that the lowest optic branch is always underdamped. A
resolution-limited central peak and quasi-elastic scattering appear in the
vicinity of the Burns temperature. It is shown that it is unlikely that the
quasi-elastic scattering originates from the combined effects of coupling
between TA and TO phonons with an increase of the damping of the TO phonon
below T. The quasi-elastic scattering has a peak as a function of
temperature close to the peak in the dielectric constant while the intensity of
the central peak scattering increases strongly below this temperature. These
results are discussed in terms of a random field model for relaxors
Probing of 1:2 Ordering in Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3 Ceramics by XRD and Raman Spectroscopy
On the effect of ordering on vibrational spectrum of relaxor ferroelectric PbSc1/2Ta1/2O3 in paraphase
Behavior of optical phonons near the diffuse phase transition in relaxor ferroelectric Pb
COTTON-MOUTON BIREFRINGENCE IN FERRIMAGNETIC GARNETS
Nous donnons une description phénoménologique de la biréfringence magnétique à l'aide de la polarisabilité électronique d'un cristal liée au spin. Nous discutons plus en détail la biréfringence d'un cristal cubique magnétique ; dans lequel l'effet peut être décrit par trois coefficients magnéto-optiques ρ11, ρl2 et ρ44. Nous avons étudié expérimentalement la biréfringence magnétique dans les ferrites grenats d'ythium et de terres rares. Le rapport a = (2 ρ44) (ρ l1-ρ12)-1 est introduit pour la description des résultats. La valeur de a caractérise l'anisotropie magnéto-optique du cristal.A phenomenological description of magnetic birefringence is given in terms of spin-dependent electronic polarizability of the crystals. In more details is discussed the birefringence in cubic magnetic crystals, where the effect can be described by three magneto-optical coefficients ρ11, ρl2 et ρ44. The experimental study of magnetic birefringence has been performed in yttrium and rare-earth iron garnets. For the description of the results the ratio a = (2 ρ44(ρ 11-ρ12)-1 was introduced. The value of a characterizes magneto-optical anisotropy of the crystal