[en] Light-induced metastable electronic states as observed e.g. in sodium nitroprusside are of fundamental interest for data storage and optical computing. Structural functionality and therefore the light-induced structure itself is of basic interest in such systems. As neutrons are sensitive to the position of the nucleus and non-destructive with respect to the metastable electronic states, neutron photocrystallography provides a useful method to determine light-induced structural changes. A photocrystallographic experimental setup has been built-up at SINQ and has been successfully used at the single-crystal instrument TriCS and the test facility TOPSI (now called MORPHEUS)

[en] The temperature dependence of the spontaneous polarization in strontium-barium-niobate single crystals with varying Sr/Ba ratio is presented. The temperature of the relaxor phase-transition is determined from the inflexion point of the curve of the ferroelectric polarization versus temperature. The influence of the Sr/Ba ratio in the crystal on the transition temperature is discussed within an effective field approach. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] The structures of the ground state and the light-induced metastable state SI of Na₂[Fe(CN)₅NO]2H₂O were investigated by single-crystal neutron diffraction. We propose that a small but significant change in the Fe-N bond length (≅0.05A) and in the C-Fe-N angle (≅3.5 deg.) are the only rearrangements in the structure connected with the excitation of the metastable state SI. This result is consistently found for two data sets with different populations (22% and 44%) of the metastable state SI

[en] Two metastable nitrosyl linkage isomers SI and SII are generated by light irradiation in the spectral range 370-500 nm in the two diamagnetic compounds [RuNO(NH₃)₅][Co(CN)₆] and [RuNO(NH₃)₅]₂[ZrF₆]₃ as well as in the paramagnetic compound [RuNO(NH₃)₅][Cr(CN)₆]. The frequencies of the ν(NO) stretching vibrations of SI and SII identify SI as the isonitrosyl Ru-O-N isomer and SII as the side-on η² isomer of NO. The population, i.e., the number of generated linkage isomers, is determined from the decrease of the area of the fundamental ν(NO) and of the higher harmonic 2 . ν(NO) of the ν(NO) stretching vibration of the ground state. Using differential scanning calorimetry (DSC) the heat release during the thermal decay of the metastable linkage isomers is determined. The activation energies, frequency factors, and the energetic position of the metastable linkage isomers are determined from the DSC and infrared spectroscopic experiments. It is found that the exchange of the counter ion significantly influences the energetic positions of the linkage isomers, while the activation energy and frequency factor are much less affected

[en] A method is presented to acquire the absorption cross sections of dopants in photorefractive lithium niobate crystals utilizing doubly doped samples. The absorption cross section of one dopant must be well known. By illumination with ultraviolet light, electrons are transferred from one centre to the other. From the changes of the absorption spectra, the absorption cross section of the centre under investigation is deduced. For a wavelength of λ = 577 nm the absorption cross section of Mn³⁺ is determined by this method to be σ_{Mn³⁺,577nm}^o = (9.2 ± 1.3) x 10^-19 cm² for ordinarily polarized light. The described method can be adapted to other dopants and host materials. (letter to the editor)

[en] The structure of the light-induced metastable state SII of Na₂[Fe(CN)₅NO]·2H₂O was investigated by single-crystal neutron diffraction. The population of SII of 16(3)% was determined by an in situ transmission measurement. The final structural refinement yields a side-on bonded configuration of NO in the metastable state SII with distances Fe-O, Fe-N, and N-O of 2.01(5) A, 1.96(5) A, and 1.11(8) A, respectively. The angle Fe-N-O is 77(4) deg. The population determined from the refinement is 12(2)%, in good agreement with the value of 16(3)% determined from the independent in situ transmission measurement

[en] We report high-resolution neutron powder diffraction on Sr_0.61Ba_0.39Nb₂O₆ in the temperature range 15-500 K. The results indicate that the low-temperature anomalies (T≤100 K) 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 T_c≅350 K start already at 200 K, explaining the anisotropic thermal expansion in the temperature range 200-300 K observed in a recent x-ray diffraction study

[en] Thermodynamic measurements on irradiated Na₂[Fe(China)₅NO]-bar 2H₂O reveal the existence of four light-induced long-lived metastable states, which decay exponentially in time. The activation energies and frequency factors of the four independent states are determined from isothermal calorimetric and differential scanning calorimetric measurements

[en] An experimental study of intensity dependent properties of photo-induced scattering (beam fanning) in Ce-doped Sr_0.61Ba_0.39Nb₂O₆, both field-free and under the action of external electric fields, is reported. A new effect, the spontaneous local reorientation of the scattering pattern, is found and is attributed to optically induced local switching of the polar structure. It is demonstrated that features of scattering observed at high intensities are governed by changes of ferroelectric and photorefractive properties of SBN in the beam area during heating up to the vicinity of the phase transition by coherent illumination

[en] The oxidation state of Fe in highly doped lithium niobate crystals (2 wt% Fe₂O₃) in the as-grown, reduced, and oxidized states is determined by the combination of differentiation and integration methods applied to their Fe K-edge XANES (x-ray absorption near edge spectroscopy) spectra. The obtained valences are confirmed further by absorption measurements in the IR/vis (infrared and visible) spectral range. It is shown that reduction and thermo-electric oxidization of as-grown Fe:LiNbO₃, respectively, leads to a noticeable lowering of the number of Fe³⁺ and to complete oxidization of all Fe²⁺ to Fe³⁺, respectively. It is found that Fe in highly iron-doped lithium niobate samples (0.5, 2, and 4 wt% Fe₂O₃) is incorporated onto the Li site comparing the experimental XANES spectra to the FEFF8 calculations. The difference of the XANES spectrum of the 4 wt% Fe₂O₃ doped sample as compared to those of the 0.5 and 2 wt% doped samples is explained by a co-phase formation in the former one. The results obtained can help to tailor such crystals for non-linear optical applications as well as for photorefraction