Th. Woike

Lifetime of small polarons in iron-doped lithium–niobate crystals

Pulsed illumination of lithium–niobate crystals with green light excites electrons from deep traps into the intrinsic defect NbLi5+ (Nb on Li site in the valence state 5+) and creates NbLi4+ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron-doped lithium–niobate crystals with different concentrations of NbLi are investigated for various illumination conditions and temperatures. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation-based model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to...

Raman-spectroscopic and differential scanning calorimetric studies of the light induced metastable states in K2[RuCl5NO]

Abstract In K 2 [RuCl 5 NO] light-induced long-living metastable states can be generated similar to those in Na 2 [Fe(CN) 5 NO] · 2H 2 O and other nitrosylprussides revealing the decisive role of the NO ligand for the formation of the new states. With Raman spectroscopy intramolecular and lattice vibrations were investigated during the population process and in the state of saturation. Only the frequencies of the internal modes are shifted. The thermally induced decay was investigated by differential scanning calorimetry, yielding information about the decay temperature, the activation energy, the frequency factor and the order of reaction.

Infrared-spectroscopic and differential scanning calorimetric studies of the two light-induced metastable states in K2[Ru(NO2)4(OH)(NO)]

Abstract In K2[Ru(NO2)4(OH)(NO)] two light-induced long-living metastable electronic states can be generated similar to those in Na2[Fe(CN)5NO]·2H2O. The trans-effect of the OH ligand raises the decay temperature of state II, which is necessary for the use as information storage elements. The intramolecular vibrations are investigated by means of infrared spectroscopy. The frequencies of the internal modes ν (OH), ν (NO), δ(RuNO), ν (RuN) are shifted to lower energies, whereas the δ(RuOH) vibration is shifted to higher frequencies. The new electronic configuration is studied by the thermal induced decay with differential scanning calorimetry, yielding the decay temperature, the activation energy, the frequency factor, the order of reaction and the energetic position of the new states relative to the ground state. Further, state I can be transferred into state II by irradiation of the crystal with light of a Nd-YAG laser (1064 nm).

New information storage elements on the basis of metastable electronic states

Abstract A new class of photochromic substances is introduced, which are suitable for information storage elements with extremely high capacity on the basis of nearly infinitely long-living metastable electronic states. Their life time is longer than 10 9 s. Absorption spectra show the wavelength region, in which the write- read- and erase-cycle can be performed. The storage velocity is measured with time-dependent transmission spectroscopy. As the change of the absorption is a function of light intensity and time, every transmission value between zero and saturation can be used as an additional parameter for the storage of information.

Instrumental neutron activation and absorption spectroscopy of photorefractive strontium-barium niobate single crystals doped with cerium

Abstract Instrumental neutron activation analysis (INAA) was applied to determine the compositions of SrxBa1−xNb2O6: Ce single crystals with x = 0.61, grown from a melt with concentrations of cerium oxide ranging from 0.1 to 1.6 wt%. A decrease of the Ce-crystal-melt distribution coefficient ( C Ce,crystal C Ce,melt ), from 0.93 to 0.58, with increasing Ce concentrations in the melt was found. Furthermore, an increase of the Ba Sr concentrationratios ( C Ba C Sr ) by (14 ± 1%) from the crystal with the lowest to the sample with the highest Ce content is observed. With the assumption of a constant Nb concentration in all SBN-crystals the experiments reveal an increase of the C Ba C Nb ratio by (4 ± 2)% and a decrease of the C Sr C Nb ratio by (7 ± 2)% with increasing Ce concentration in the melt. This produces a shift of the parameter x in the SBN: Ce-formula from 0.61 to 0.57 ± 0.02. The opposite trend in the C Ba C Nb and the C Sr C Nb ratios with increasing Ce contents of the crystals indicates that Ce occupies Sr positions. The absorption coefficient αe for extraordinarily polarized light increases linearly with the increasing concentration CCe,crystal of Ce in the crystal.

Light-induced structural changes in sodiumnitroprusside (Na2(Fe(CN)5NO)·2D2O) at 80 K

Groundstate and electronic excited state (MSI) of deuterated sodiumnitroprusside (Na2(Fe(CN)5NO)·2D2O) have been investigated by neutron diffraction as well as by optical and Mössbauer techniques. Significant structural changes occur predominantly in the O−N−Fe−C-bond. It has been shown, that the N−O bond-length is not the order parameter, as expected from other studies. We found an increase in the bond lengths Fe−N4 of 0.019(2) Å and N−O of 0.004(4) Å respectively, which is in qualitative agreement with changes determined by Raman spectroscopy and predictions based on diatomic correlations (Badger/Herschbach/Laurie). Additionally we observed a change in the Fe−C1 bond length of 0.012(3) Å in agreement with Raman meaurements.

Anisotropic holographic scattering in centrosymmetric sodium nitroprusside

Anisotropic holographic light scattering has been discovered in centrosymmetric single crystals of sodiumnitroprusside, Na2[Fe(CN)5NO]⋅2H2O, in which the photorefractive response is based on the excitation of metastable electronic states. Besides a bright homogeneous scattering corona generated by isotropic scattering, a sharp scattering ring builds up with light polarized orthogonal to that of the pump beam and the corona. This anisotropic scattering resulting from interference of scattered waves with the pump beam occurs although there is no linear electro-optic effect in centrosymmetric crystals.

Ferroelectric phenomena in holographic properties of strontium-barium niobate crystals doped with rare-earth elements

Abstract We report on a relation between the two-beam coupling gain factor Γ in Sr0·61Ba0·39Nb2O6 (SBN:0·6) crystals and their ferroelectric properties. In SBN:0·6 doped with rare-earth-metal impurities Ce and Tm we found a hysteresis dependence of Γ versus the external dc-field E applied after recording which is due to the ferroelectric P-E hysteresis. Particularly, Γ-E hysteresis manifests itself in an alteration of the sign of Γ, i.e., of the direction of the energy transfer between the coupling beams, under applying of E reversing Ps. The value of Γ at room temperature essentially increases with impurity concentrations. It is caused by an increase of the dielectric permittivity, which in its turn is due to a significant lowering of the phase-transition temperature Tc produced by a rare-earth doping.

Nitroprussides, a new group of materials for holographic information storage on the basis of metastable electronic states

Abstract A new group of materials for holographic information storage is introduced. The storage process is based on the excitation of long living metastable electronic states. The measured efficiency in the write- and erase mode indicates the existence of mixed phase- and amplitude gratings in single crystals of Na2[Fe(CN)5NO]·2H2O. The angle dependence of the efficiency determines an upper limit of the hologram thickness up to 75 μm. In the write mode (λ=476.5 nm) the maximum of the diffraction efficiency reaches 0.65% with an energy flux of about 15 J/cm2. In the erase mode (λ=632.8 nm) the efficiency increases to 1.3% with an energy flux of 2 J/cm2.

Bimetallic cyanide-bridged complexes based on the photochromic nitroprusside anion and paramagnetic metal complexes

The synthesis, crystal structure, and physical characterization of the coordination compounds [Ni(en)2]4[Fe(CN)5NO]2[Fe(CN)6]x5H2O (1), [Ni(en)2][Fe(CN)5NO]x3H2O (2), [Mn(3-MeOsalen)(H2O)]2[Fe(CN)5NO] (3), and [Mn(5-Brsalen)]2[Fe(CN)5NO] (4) are presented. 1 crystallizes in the monoclinic space group P2(1)/n (a = 7.407(4) A, b = 28.963(6) A, c = 14.744(5) A, alpha = 90 degrees, beta = 103.26(4) degrees, gamma = 90 degrees, Z = 2). Its structure consists of branched linear chains formed by cis-[Ni(en)2]2+ cations and ferrocyanide and nitroprusside anions. The presence of two kinds of iron(II) sites has been demonstrated by Mössbauer spectroscopy. 2 crystallizes in the monoclinic space group P2(1)/c (a = 11.076(3) A, b = 10.983(2) A, c = 17.018(5) A, alpha = 90 degrees, beta = 107.25(2) degrees, gamma = 90 degrees, Z = 4). Its structure consists of zigzag chains formed by an alternated array of cis-[Ni(en)2]2+ cations and nitroprusside anions. 3 crystallizes in the triclinic space group P1 (a = 8.896(5) A, b = 10.430(5) A, c = 12.699(5) A, alpha = 71.110(5) degrees, beta = 79.990(5) degrees, gamma = 89.470(5) degrees, Z = 1). Its structure comprises neutral trinuclear bimetallic complexes in which a central [Fe(CN)5NO]2- anion is linked to two [Mn(3-MeOsalen)]+ cations. 4 crystallizes in the tetragonal space group P4/ncc (a = 13.630(5) A, c = 21.420(8) A, Z = 4). Its structure shows an extended 2D neutral network formed by cyclic octameric [-Mn-NC-Fe-CN-]4 units. The magnetic properties of these compounds indicate the presence of quasi-isolated paramagnetic Ni2+ and Mn3+. Irradiated samples of the four compounds have been studied by differential scanning calorimetry to detect the existence of the long-lived metastable states of nitroprusside.