36 research outputs found
Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K to 500 K investigated by high-resolution neutron powder diffraction and specific heat measurements
We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6,
SBN61, in the temperature range 15-500 K. The results indicate that the
low-temperature anomalies (T<100K) observed in the dielectric dispersion are
due to small changes in the incommensurate modulation of the NbO6-octahedra, as
no structural phase transition of the average structure was observed. This
interpretation is supported by specific heat measurements, which show no latent
heat, but a glass-like behavior at low temperatures. Furthermore we find that
the structural changes connected with the ferroelectric phase transition at Tc
approx. 350K start already at 200K, explaining the anisotropic thermal
expansion in the temperature range 200-300K observed in a recent x-ray
diffraction study.Comment: Accepted by PRB (2006
M\"ossbauer, nuclear inelastic scattering and density functional studies on the second metastable state of Na2[Fe(CN)5NO]2H2O
The structure of the light-induced metastable state SII of
Na2[Fe(CN)5NO]2H2O 14 was investigated by transmission M\"ossbauer
spectroscopy (TMS) in the temperature range 15 between 85 and 135 K, nuclear
inelastic scattering (NIS) at 98 K using synchrotron 16 radiation and density
functional theory (DFT) calculations. The DFT and TMS results 17 strongly
support the view that the NO group in SII takes a side-on molecular orientation
18 and, further, is dynamically displaced from one eclipsed, via a staggered,
to a second 19 eclipsed orientation. The population conditions for generating
SII are optimal for 20 measurements by TMS, yet they are modest for
accumulating NIS spectra. Optimization 21 of population conditions for NIS
measurements is discussed and new NIS experiments on 22 SII are proposed
Discontinuous domain wall motion in the relaxor ferroelectric
The relaxor ferroelectric strontium-barium-niobate is
characterized by internal random electric fields at the
origin of its relaxor behavior. The electric poling cycle
induces an irreversible transition into a ferroelectric
state accompanied by a large flow of mobile charges. Bipolar
fields applied in the poled state yield no acoustic
emissions for half-cycles in poling direction. In contrast,
discontinuous domain wall movement is observed under reverse
fields. The data cannot be explained by homogeneous clouds
or gradients of space charge, but necessitate charge cluster
formation to a certain degree. The irreversibility of the
poling process necessitates charge carrier activation by
large electric fields
Possible reaction coordinates in the metastable states of sodiumnitroprusside N
The metastable states of sodiumnitroprusside
are
extremely stable at temperatures below 200 K.
It is possible to allocate structural changes measured by neutron diffraction to measured spectroscopic parameters, but the amount of the
structural change is relatively small for a reaction co-ordinate as the metastable states have
an extremely long lifetime. New hypotheses for related systems try to explain such a
phenomena in two ways: The first way is a bending of the NO-bond in the metastable
state, the second one an exchange of the oxygen and nitrogen atoms in the NO-bond
(which can be regarded as an bending). As such changes would be
possible also from
our density functional calculations,
we re-investigated our neutron diffraction data using
the new models. However, our results are
not compatible with one of these models. On the contrary, the neutron diffraction data show partially opposite tendencies.
We compare both models with EXAFS measurements, with vibrational spectroscopic
results and the data found by Mössbauer spectroscopy.
We propose a potential
scheme for all three states (GS, and ) extracted from absorption and
thermodynamic data to explain the electronic and
energetic rearrangement, and the population
dynamics
Phase transitions in Sr
The linear birefringence (LB) of
(SBN61:Ce) has been measured as a function of temperature within the range of
. Large tails have been observed above
the ferroelectric phase transition temperatures , 328, 320
and 291 K for the concentrations x = 0, 0.0066, 0.0113 and 0.0207,
respectively. Within an Ornstein-Zernike analysis the critical exponents γ,
ν and β are determined. It suggests that pure SBN61 belongs to the 3D Ising
universality class. Doping with ions, which seem to act as
random fields, enhances the relaxor properties. The critical exponents γ
and ν of SBN61:Ce shift against those of the three-dimensional
random-field Ising model
Holographic recording with orthogonally polarized waves in centrosymmetric
Recording of holographic gratings by excitation of long-living
metastable electronic states in centrosymmetric sodium
nitroprusside can be performed with mutually orthogonal polarized
waves. Two different cases have to be distinguished: 1) In a
b-cut the optimum diffraction efficiency of 15% is achieved if
the mutually orthogonal polarized writing waves form an angle of
45
\un{^\circ} with respect to the a- and c-axis. This, together with
the fact that the polarization state is not changed in the
diffraction process, indicates that the gratings originate from
the presence of light-induced linear dichroism. 2) For a c-cut
the polarization of the diffracted wave turns out to be orthogonal
to the one of the incident beam. The efficiency of this so-called
anisotropic diffraction reaches a maximum of up to 90% for waves
mutually orthogonal polarized along the a- and b-axis
Holographic scattering in the ultraviolet spectral range in iron-doped lithium niobate
Photo-induced light scattering in iron-doped lithium niobate
crystals is investigated at different wavelengths in the near
ultraviolet spectral range. We observed a remarkable difference
of the angular distribution of scattered light when changing the
pump wavelength. These findings can be explained by the spectral
dependence of the competing recording mechanisms photovoltaic
effect and diffusion. We found that the magnitude of the
photovoltaic effect is decreasing from 364\un{nm} to 334\un{nm},
and diffusion becomes the dominant charge transport mechanism for
photorefraction