Ferroic materials play an increasingly important role in novel (nano)electronic devices. Recently, research on domain walls (DWs) receives a big boost by the discovery of DW conductivity (DWC) in BiFeO3 and Pb(ZrxTi1‐x)O3 ferroic thin films. Here, it is demonstrated that DWC is not restricted to thin films, but equally applies to millimeter‐thick wide‐bandgap, ferroic single crystals, such as LiNbO3. In this material transport along DWs can be switched by super‐bandgap illumination and tuned by engineering the tilting angle of DWs with respect to the polar axis. The results are consistently obtained using conductive atomic force microscopy to locally map the DWC and macroscopic contacts, thereby in addition investigating the temperature dependence, DW transport activation energies, and relaxation behavior.

Two light-induced metastable NO linkage isomers with oxygen-bound (SI) and side-on configuration (SII) of NO are generated in trans-[RuCl(py)(4)(NO)][PF(6)](2).(1/2)H(2)O. Irradiation by light in the blue-green spectral range (450-530 nm) leads to the population of SI. A further irradiation by near infrared light (920-1100 nm) transfers SI into SII at temperatures below 150 K. The heat release during the thermal decay of the linkage isomers shows that SI and SII are separated from the ground state (GS) by potential barriers of E(A)(SI) = 0.70(3) eV and E(A)(SII) = 0.38(3) eV, and are energetically situated at 1.42(6) eV and 1.07(7) eV above the ground state, respectively. Maximum populations of 76% for SI and of 56% for SII can be generated, as determined by the decrease of the nu(NO) stretching absorption band of the ground state. The nu(NO) stretching vibration shifts to lower energies by 143 cm(-1) in SI and by 300 cm(-1) in SII, indicating that the linkage isomers are of the same type as found in other octahedrally coordinated transition-metal nitrosyl complexes. The experimental observations are in agreement with results from calculations by the density functional theory, which predict that the metastable states correspond to a side-on bonded (SII) and an isonitrosyl (SI) configuration of the NO ligand. The calculations provide the energy minima of the ground state and the metastable states SI and SII as well as the saddle points along the reaction coordinate Q. This reaction coordinate corresponds to a rotation of the NO ligand by about 90 degrees (SII) and 180 degrees (SI), and therefore allows the comparison between observed and calculated activation energies.

The structure of a crystal of Sr0.61Ba0.39Nb2O6 has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(\,pp1/2,p - p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) Å, modulation vectors q 1 = 0.3075 (6) (a* + b*), q 2 = 0.3075 (6) (a* − b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 Å, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary.

In mononitrosyl complexes of transition metals two long-lived metastable states corresponding to linkage isomers of the nitrosyl ligand can be induced by irradiation with appropriate wavelengths. Upon irradiation, the N-bound nitrosyl ligand (ground state, GS) turns into two different conformations: isonitrosyl O bound for the metastable state 1 (MS1) and a side-on nitrosyl conformation for the metastable state 2 (MS2). Structural and spectroscopic investigations on [RuCl(NO)py(4)](PF(6))(2)·1/2H(2)O (py = pyridine) reveal a nearly 100% conversion from GS to MS1. In order to identify the factors which lead to this outstanding photochromic response we study in this work the influence of counteranions, trans ligands to the NO and equatorial ligands on the conversion efficiency: [RuX(NO)py(4)]Y(2)·nH(2)O (X = Cl and Y = PF(6)(-) (1), BF(4)(-) (2), Br(-)(3), Cl(-) (4); X = Br and Y = PF(6)(-) (5), BF(4)(-) (6), Br(-)(7)) and [RuCl(NO)bpy(2)](PF(6))(2) (8), [RuCl(2)(NO)tpy](PF(6)) (9), and [Ru(H(2)O)(NO)bpy(2)](PF(6))(3) (10) (bpy = 2,2'-bipyridine; tpy = 2,2':6',2"-terpyridine). Structural and infrared spectroscopic investigations show that the shorter the distance between the counterion and the NO ligand the higher the population of the photoinduced metastable linkage isomers. DFT calculations have been performed to confirm the influence of the counterions. Additionally, we found that the lower the donating character of the ligand trans to NO the higher the photoconversion yield.

Structure analysis of ground state (GS) and two light-induced (SI and SII) metastable linkage NO isomers of [Ru(py)4Cl(NO)](PF6)2.0.5H2O is presented. Illumination of the crystal by a laser with lambda = 473 nm at T = 80 K transfers around 92% of the NO ligands from Ru-N-O into the isomeric configuration Ru-O-N (SI). A subsequent irradiation with lambda = 980 nm generates about 48% of the side-on configuration Ru<(N)(O) (SII). Heating to temperatures above 200 K or irradiation with light in the red spectral range transfers both metastable isomers reversibly back to the GS. Photodifference maps clearly show the N-O configurations for both isomers and they could be used to find a proper starting model for subsequent refinements. Both metastable isomers have slightly but significantly different cell parameters with respect to GS. The main structural changes besides the Ru-O-N and RU<(N)(O) linkage are shortenings of the trans Ru-Cl bonds and the equatorial Ru-N bonds. The experimental results are compared with solid-state calculations based on density functional theory (DFT), which reproduce the observed structures with high accuracy concerning bond lengths and angles. The problem of how the different occupancies of SI and GS could affect refinement results was solved by a simulation procedure using the DFT data as starting values.

We investigate the fundamental necessary conditions for optical generation of nitrosyl linkage isomers in ML(5)NO compounds (M = transition metal, L arbitrary ligand) on the examples of K(3)[Mn(CN)(5)NO].2H(2)O and Na(2)[Fe(CN)(5)NO].2H(2)O. We show that the NO linkage isomers of the side-on bonded type (SII, 90 degrees rotation of NO) and of the isonitrosyl type M-ON, where NO is O-bound to the metal M (SI, 180 degrees rotation of NO), can be generated if two conditions are fullfilled. First the optical excitation must lead to a change in the bond between the NO group and the central metal atom M, either by a metal-to-ligand charge transfer of type d -->pi*(NO) or by a d-->d(z(2)) transition, which changes the sigma bonding of the NO group to the metal, such that the vibrational deformation mode delta(M-N-O) can drive the system into the SII configuration. Second the excited state potential must posses a minimum close to the saddle point of the ground state surface between GS and SII, SI, or cross that surface, such that the relaxation from the excited state into the metastable minima can occur. The same is true for transfers between the two metastable states SII and SI. As a further constraint with respect to the amount of population, i.e. the number of complexes which can be transferred into SII or SI, the cross sections sigma(GS,SII,SI) of the states GS, SII, and SI must be considered. If sigma(GS) > sigma(SII) and sigma(SI) > sigma(SII) SII can be occupied while SI can be significantly occupied if sigma(GS) > sigma(SI) and sigma(SII) > sigma(SI). More simply speaking the depletion rate of the metastable state should be smaller than its population rate for a given wavelength.

The ongoing search for new photorefractive materials offering a high photosensitive flexibility is triggered by the limited tuning ability of conventional photorefractive materials, such as electro-optic crystals, photopolymers, or liquid crystals. One of the latest discovered photorefractive materials is Na 2 [Fe(CN) 5 NO] · 2H 2 O, in which metastable linkage isomers of the nitrosyl group can be generated by light irradiation resulting in huge changes of the refractive index up to Dn ∼ 10 -2 .Here we show that this type of photorefraction is a general property arising from the generation of linkage isomers and can thus be found in many compounds containing [ML 5 NO] m± complexes, where M is a transition metal, L a ligand, and m the formal charge of the anion/cation. The benefit of this generality is reflected in the tailoring properties of the photorefractive response such as the spectral sensitivity simply by selecting different representatives of the [ML 5 NO] m± compounds.Conventional photorefractive materials, [1] where the lightinduced modulation of the refractive index is caused by the linear electro-optic effect (Pockels effect), are widely applied in nonlinear optics.[2] They feature reversible photorefractivity without chemical processing allowing for dynamical holography, [3] and the kinetics of hologram formation is described by the well-known Kiev equations. [4] In recent years a second class of reversible photorefractive media for dynamical holography has been established, the unconventional photorefractive materials. Here an optical bistability is at the origin of photorefraction and the kinetics show a characteristic transient behavior.[5] The unique properties of this class were com-

Cherenkov second-harmonic generation (CSHG) is a powerful tool for three-dimensional domain wall profiling in ferroic bulk crystals. Here, we apply this noninvasive technique for tracking head-to-head charged domain walls (CDWs) across millimeter-thick ferroelectric single-crystalline lithium niobate. CSHG sensitively reveals the inclination α > 0 of any such CDW with a superb optical resolution. Moreover, we deduce fully charged head-to-head CDWs (α = 90 • ) to be much rougher and to show protrusions, domain inclusions, and novel topologies. Our findings provide insight into the mechanisms of electron transport and charge trapping in CDWs, as is mandatory for their use in prospective nanoelectronic devices.