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]⋅\cdot2H2O

The structure of the light-induced metastable state SII of Na2[Fe(CN)5NO]$\cdot$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 $\rm Na_2[Fe(CN)_5NO]2H/D_2O$ 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 $90^\circ$ 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 $180^\circ$ 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, $\rm MS_I$ and $\rm MS_{II}$) 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 $\mathrm{Sr_{0.61}Ba_{0.39}Nb_{2}O_{6}{:}Ce^{3+}}$ (SBN61:Ce) has been measured as a function of temperature within the range of $78 \leq T \leq 850 \mathrm{K}$. Large tails have been observed above the ferroelectric phase transition temperatures $T_{\mathrm{c}} = 350$, 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 $\mathrm{Ce}^{3+}$ 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