287 research outputs found
Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films
Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser
deposition, of thicknesses down to 40 nm, were studied using infrared
transmission and reflection spectroscopy. The complex dielectric responses of
polar phonon modes, particularly ferroelectric soft mode, in the films were
determined quantitatively. The compressed epitaxial STO films on (100)
La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened
phonon responses, whereas the soft mode in polycrystalline film on (0001)
sapphire substrate shows a strong broadening due to grain boundaries and/or
other inhomogeneities and defects. The stiffened soft mode is responsible for a
much lower static permittivity in the plane of the compressed film than in the
bulk samples.Comment: 11 page
Origin of the "Waterfall" Effect in Phonon Dispersion of Relaxor Perovskites
Inelastic neutron scattering study of the perovskite relaxor ferroelectric
PZN:8%PT elucidates the origin of the previously reported unusual kink on the
low frequency transverse phonon dispersion curve (known as "waterfall" effect).
We show that its position depends on the choice of the Brillouin zone and that
the relation of its position to the size of the polar nanoregions is highly
improbable. The observation is explained in the framework of a simple model of
coupled damped harmonic oscillators representing the acoustic and optic phonon
branches.Comment: 4 pages, 6 figures, LaTe
Central mode and soft mode behavior in PbMg1/Nb2/3O3 relaxor ferroelectric
The relaxor ferroelectric PbMg1/Nb2/3O3 was investigated by means of
broad-band dielectric and Fourier Transform Infrared (FTIR) transmission
spectroscopy in the frequency range from 1 MHz to 15 THz at temperatures
between 20 and 900 K using PMN films on infrared transparent sapphire
substrates. While thin film relaxors display reduced dielectric permittivity at
low frequencies, their high frequency intrinsic or lattice response is shown to
be the same as single crystal/ceramic specemins. It was observed that in
contrast to the results of inelastic neutron scattering, the optic soft mode
was underdamped at all temperatures. On heating, the TO1 soft phonon followed
the Cochran law with an extrapolated critical temperature equal to the Burns
temperature of 670 K and softened down to 50 cm-1. Above 450 K the soft mode
frequency leveled off and slightly increased above the Burns temperature. A
central mode, describing the dynamics of polar nanoclusters appeared below the
Burns temperature at frequencies near the optic soft mode and dramatically
slowed down below 1 MHz on cooling below room temperature. It broadened on
cooling, giving rise to frequency independent losses in microwave and lower
frequency range below the freezing temperature of 200 K. In addition, a new
heavily damped mode appeared in the FTIR spectra below the soft mode frequency
at room temperature and below. The origin of this mode as well as the
discrepancy between the soft mode damping in neutron and infrared spectra is
discussed.Comment: 7 pages with 7 figures, submitted to Phys. Rev.
Ferroelectric and Incipient Ferroelectric Properties of a Novel Sr_(9-x)PbxCe2Ti2O36 (x=0-9) Ceramic System
Sr_(9-x)PbxCe2Ti12O36 system is derived from the perovskite SrTiO3 and its
chemical formula can be written as (Sr_(1-y)Pby)0.75Ce0.167TiO3. We
investigated dielectric response of Sr_(9-x)PbxCe2Ti12O36 ceramics (x = 0-9)
between 100 Hz and 100 THz at temperatures from 10 to 700 K using low- and
high-frequency dielectric, microwave (MW), THz and infrared spectroscopy. We
revealed that Sr9Ce2Ti12O36 is an incipient ferroelectric with the R-3c
trigonal structure whose relative permittivity e' increases from 167 at 300 K
and saturates near 240 below 30 K. The subsequent substitution of Sr by Pb
enhances e' to several thousands and induces a ferroelectric phase transition
to monoclinic Cc phase for x>=3. Its critical temperature Tc linearly depends
on the Pb concentration and reaches 550 K for x=9. The phase transition is of
displacive type. The soft mode frequency follows the Barrett formula in samples
with x=3.
The MW dispersion is lacking and quality factor Q is high in samples with low
Pb concentration, although the permittivity is very high in some cases.
However, due to the lattice softening, the temperature coefficient of the
permittivity is rather high. The best MW quality factor was observed for x=1:
Q*f=5800 GHz and e'=250. Concluding, the dielectric properties of Sr_(9-
x)PbxCe2Ti12O36 are similar to those of Ba_(1-x)SrxTiO3 so that this system can
be presumably used as an alternative for MW devices or capacitors.Comment: subm. to Chem. Mate
Anisotropic dielectric function in polar nano-regions of relaxor ferroelectrics
The paper suggests to treat the infrared reflectivity spectra of single
crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally
frozen polarization makes the dielectric function strongly anisotropic in the
phonon frequency range and the random orientation of the polarization at
nano-scopic scale requires to take into account the inhomogeneous
depolarization field. Employing a simple effective medium approximation
(Bruggeman symmetrical formula) to dielectric function describing the polar
optic modes as damped harmonic oscillators turns out to be sufficient for
reproducing all principal features of room temperature reflectivity of PMN. One
of the reflectivity bands is identified as a geometrical resonance entirely
related to the nanoscale polarization inhomogeneity. The approach provides a
general guide for systematic determination of the polar mode frequencies split
by the inhomogeneous polarization at nanometer scale.Comment: 5 pages, 2 figure
High-frequency dielectric spectroscopy of batio3 core - silica shell nanocomposites: Problem of interdiffusion
Three types of BaTiO3 core - amorphous nano-shell composite ceramics were
processed from the same core-shell powder by standard sintering, spark-plasma
sintering and two-step sintering techniques and characterized by XRD, HRSEM and
broad-band dielectric spectroscopy in the frequency range 10^3 - 10^13 Hz
including the THz and IR range. The samples differed by porosity and by the
amount of interdiffusion from the cores to shells, in correlation with their
increasing porosity. The dielectric spectra were also calculated using suitable
models based on effective medium approximation. The measurements revealed a
strong dielectric dispersion below the THz range, which cannot be explained by
the modeling, and whose strength was in correlation with the degree of
interdiffusion. We assigned it to an effect of the interdiffusion layers,
giving rise to a strong interfacial polarization. It appears that the
high-frequency dielectric spectroscopy is an extremely sensitive tool for
detection of any gradient layers and sample inhomogeneities even in dielectric
materials with negligible conductivity
Xenon prevents cellular damage in differentiated PC-12 cells exposed to hypoxia
BACKGROUND: The neuroprotective effect of xenon has been demonstrated for glutamatergic neurons. In the present study it is investigated if dopaminergic neurons, i.e. nerve-growth-factor differentiated PC-12 cells, are protected as well against hypoxia-induced cell damage in the presence of xenon. RESULTS: Pheochromocytoma cells differentiated by addition of nerve growth factor were placed in a N(2)-saturated atmosphere, a treatment that induced release of dopamine, reaching a maximum after 30 min. By determining extracellular lactate dehydrogenase concentration as marker for concomitant cellular damage, a substantial increase of enzymatic activity was found for N(2)-treated cells. Replacement of N(2 )by xenon in such a hypoxic atmosphere resulted in complete protection against cellular damage and prevention of hypoxia-induced dopamine release. Intracellular buffering of Ca(2+ )using the Ca-chelator 1, 2-bis(2-Aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester (BAPTA) reduced the neuroprotective effect of xenon indicating the essential participation of intracellular Ca(2+)-ions in the process of xenon-induced neuroprotection. CONCLUSIONS: The results presented demonstrate the outstanding property of xenon to protect neuron-like cells in a hypoxic situation
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