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Perovskite-type transition metal oxides have great potential as storage material in resistive random-access memory (RRAM) devices. Typical non-volatile memory cells are realized in metal-insulator-metal (MIM) stacks with insulator thicknesses of few nanometers. We report on the investigation of single-crystal SrTiO3 to understand the role of volume and interface real structure for the electrical conductivity in such materials. Conductivity in SrTiO3 single crystals was established by a reducing high vacuum (HV) annealing introducing charged oxygen vacancies acting as donor centers. Titanium electrodes are evaporated on both crystal faces to obtain an MIM element.

The structure of the isomorphous tetraaquabis(saccharinate)metal(II) dihydrates was employed as structurally flexible coordination framework capable of sustaining large internal distortions to study the competitive inclusion of Jahn-Teller ( . Probing of the metal content of the solid phase showed a two-regime inclusion profile: increasing of the ratio of the distorted cation relative to the undistorted one in the solution phase of up to about 35% results in linear dependence and preferred inclusion of the former with maximum concentration of 100% in the crystal, and complete exclusion of the undistorted ion above that point. A mixed crystal with highest copper ratio of 63% was obtained from solution with 25% copper, which under the P21/c crystal symmetry corresponds to sustainable integrity of the undistorted lattice by substitution of up to 2/3 of its sites. This stability limit shows that four out of six sites around each [Mn(H2O)4(sac)2] 2+ ion can be substituted by distorted [Cu(H2O)4(sac)2]2+ guests under conditions of thermodynamically controlled, statistically averaged exchange. The undistorted ion is tolerant toward inclusion of the strongly distorted guests, whereas the distorted ion is exclusive toward the undistorted ion. Along with the expectation from the JT theory, structural refinement of seven crystals, including a mixed crystal with composition of [Cu0.126Mn0.874 (H2O)4(C7H4NO3S)2](H2O)2, showed that metal-ligand distances are significantly affected by the metal substitution.Keywords: copper and manganese saccharinates, solid solutions, Jahn-Teller distortions P08.14.143 Acta Cryst. (2008). A64, C462Evolution of phonon spectra and elastic constants from graphene to graphite Karl H Michel, Bart Verberck University of Antwerp, Department of Physics, Groenenborgerlaan 171, Antwerp, Antwerp, 2020, Belgium, E-mail karl.michel@ua.ac.beWe present a unified theory of the phonon dispersions and elastic properties of graphene, graphite and graphene multilayer systems. Starting from a fifth-nearest neighbor force constants model derived from full in-plane phonon dispersions of graphite [Mohr et al., Phys. Rev. B 76, 035439 (2007)] we use Born's long wavelength method to calculate the tension and bending coefficients of graphene. Extending the model by interplanar interactions, we study the phonon dispersions and the elastic constants of graphite and the phonon spectra of graphene multilayers. We find that the inner displacement terms due to sublattice shifts between inequivalent C atoms are quantitatively important in determining the elasto-mechanical properties of graphene but not of graphite. We investigate the evolution from graphene to graphite by studying the increase of the wB2g1(N) rigid planes optical mode as a function of the number of layers N. At N = 10 the graphite value wB2g1 127 cm-1 is practically reached near N = 15. Due to properties favorable for technical applications, ferroic nanometer film composites with perovskite-related structures have received great attention. In this report,...

The dynamical interaction in a simultaneous twenty-four beam diffraction in a two-plate silicon cavity assembly for 14.4388 keV X-rays is studied, where the diffracted X-rays are reflected back and forth between the two crystal plates via the (12 4 0) reflection. The dynamical theory of X-ray diffraction with a Cartesian coordinate system is employed to calculate the interference pattern due to Fabry-Perot type resonance. The calculated intensity distribution of the transmitted beam is in good agreement with the measured one. The dispersion surface, linear absorption coefficients, wavefield intensities, and excitations of mode are also calculated. The formation of standing waves and phase change under the cavity resonance condition will be discussed.Keywords: twenty-four beam diffraction, X-ray cavity, dynamical interaction P15.06.04 Acta Cryst. (2008). A64, C575Site-selective determination of coordination symmetries by anisotropic anomalous X-ray scattering Matthias Zschornak 1,2 , Tilmann Leisegang 1 , Hartmut Stoecker 1 , Torsten Weissbach 1 , Sibylle Gemming 2 , Dirk C. Meyer 1 1 Technische Universitaet Dresden, Institute of Structural Physics, Z e l l e s c h e r We g 1 6 , D r e s d e n , S a x o n y, 0 1 0 6 9 , G e r m a n y, 2 Forschungszentrum Dresden-Rossendorf, Bautzner Landstr. 128, Dresden, Saxony, 01328, Germany, E-mail zschornak@gmx.netBased on an experiment of Kirfel and Petcov on rutile (136) P42/ mnm, which verified anisotropic anomalous scattering (AAS) by measuring 'Forbidden Reflection near Edge Diffraction' (FRED), we aimed to extend the results with allowed reflections to extract more tensor symmetries of the Ti scattering factor tensor fij by fitting the model of AAS to the experimental data. Results from DFT calculations will also be presented. Furthermore we intended to study possible restrictions for atomic site occupation of unknown structures in an identified space group due to these local symmetry relations exemplary for this model structure. Experiments were carried out at DESY/HASYLAB BL C using a Si (111) double crystal monochromator tuned to an energy of 4985 eV. An automated optimization and Ψ-scan routine for a sample setup with rotating degree of freedom assured AAS measurements at the reflection maxima. The rutile samples investigated were 10x10x1 mm 3 wafers in (001), (110) and (111) orientation and Ψ-scans were measured for the reflections 001, 220, 110 and 111. Ti occupies Wyckoff site 2a, its tensor symmetry must follow the local symmetry m.mm leaving 3 complex elements f11, f12, f33. Simulations showed dependencies: 001 to f12, 220 to f11-f33, 110 to f11-f33, 111 to f12. For the 001 reflection intensity real and imaginary part correlate, but the 111 intensity displays asymmetric influence so the ambiguity is separable. The measured data show clear evidence of AAS and the 001 FRED and 111 intensities could be fitted: f'Ti d' 12 = -5.44 * 0.5(8) f"Ti d" 12 = 3.98 * 1.4(2) Since f13 and f23 were refined to zero, the positioning of Ti within the unit cell would by inverse ...

Resistance switching in metal -insulator -metal (MIM) structures with transition metal oxides as the insulator material is a promising concept for upcoming non-volatile memories. The electronic properties of transition metal oxides can be tailored in a wide range by doping and external fields. In this study SrTiO 3 single crystals are subjected to high temperature vacuum annealing. The vacuum annealing introduces oxygen vacancies, which act as donor centers. MIM stacks are produced by physical vapor deposition of Au and Ti contacts on the front and rear face of the SrTiO 3 crystal. The time dependent forming of the MIM stacks under an external voltage is investigated for crystals with varying bulk conductivities. For continued formation, the resistivity increases up to failure of the system where no current can be measured anymore and switching becomes impossible.