F. Lichtenberg

Synthesis of perovskite-related layered AnBnO3n+2 = ABOX type niobates and titanates and study of their structural, electric and magnetic properties

Abstract ABO X niobates and titanates belonging to the homologous series A n B n O 3 n +2 are a special group of perovskite-related layered materials. These oxides comprise the highest- T c ferroelectrics such as CaNbO 3.50 and LaTiO 3.50 , as well as thermodynamically stable bulk compounds involving well-ordered stacking sequences of layers with different thickness such as SrNbO 3.45 . An extensive overview on many AB O X compositions of the A n B n O 3 n +2 family and its properties is presented. The crystal structure type is given by n and can be tuned by adjusting the oxygen content X . The charge carrier concentration of the electrical conducting oxides can be varied by appropriate substitutions at the A or B site. To investigate the properties of these systems, more than 150 different compositions were prepared. Most of them were grown by floating zone melting, of which many were fabricated as single crystals with precise control of the oxygen content X . For these crystalline compounds, the synthesis, structural, electric and magnetic features are discussed. Attempts to prepare series members beyond the known structure types n =4, 4.33, 4.5, 5 and 6 were not successful. For some of the known structures types n , however, pronounced non-stoichiometric homogeneity ranges with respect to the oxygen content X and cation ratio A / B were found. Thus, these systems offer many possibilities to vary the compositional, structural, chemical and physical properties. Further, measurements of the resistivity as a function of temperature T are reported for crystals of the n =4 type Sr 0.8 La 0.2 NbO 3.50 , n =4.5 type Sr 0.96 Ba 0.04 NbO 3.45 and n =5 types Sr 1− Y La Y NbO 3.41 (Y=0, 0.035, 0.1), Sr 0.95 NbO 3.37 , CaNbO 3.41 and LaTiO 3.41 . These measurements, which were performed in the temperature range 4 K≤ T ≤290 K and along the a -, b - and c -axis, revealed a highly anisotropic conductivity and intricate behavior. In parts of the temperature range, these materials are quasi-1D metals which display temperature-driven metal-semiconductor transitions at lower temperatures. The niobates and titanates investigated represent a new group of quasi-1D metals which are in compositional, structural and electronical proximity to non-conducting layered (anti)ferroelectrics. Furthermore, measurements of the magnetic susceptibility as a function of temperature are reported for many compounds. As a typical property at elevated temperatures, it was observed that the magnetic susceptibility rises with increasing temperature.

Two-Dimensional Fermi Liquid Behavior of the Superconductor Sr2RuO4

We report detailed normal state measurements of the resistivity, magnetic susceptibility and specific heat of high-quality single crystals of Sr 2 RuO 4 , the only copper-free, layered perovskite superconductor ( T c ≈1 K). It is intriguing that despite its very strong anisotropy (the ratio of the interlayer to in-plane resistivity exceeds 500) arising from the essentially two-dimensional Fermi surface, the normal state properties are consistent with a universal description for a strongly correlated Fermi liquid. Therefore, Sr 2 RuO 4 serves as an ideal reference material for the investigation of the limits of applicability of Fermi liquid theory to other highly correlated layered compounds, including high- T c cuprates.

Sr2RuO4: A metallic substrate for the epitaxial growth of YBa2Cu3O7−δ

Single crystals of Sr2RuO4 were grown by the floating zone melting technique. The crystals have the K2NiF4 structure and display a metallic resistivity behavior in the a‐b‐plane between 300 and 4.2 K (ρab≂10−4 Ω cm at 300 K). The in‐plane lattice mismatch between YBa2Cu3O7−δ (001) and Sr2RuO4 (001) is smaller than 1.3%, better than that to SrTiO3 {100}. Epitaxial films of YBa2Cu3O7−δ with Tc(R=0) as high as 86 K have been grown on Sr2RuO4 crystals. The epitaxial growth of YBa2Cu3O7−δ on Sr2RuO4 was revealed by four‐circle x‐ray diffraction as well as by transmission electron microscopy.

Spin-triplet superconductivity in Sr2RuO4 probed by andreev reflection

The superconducting gap function of Sr2RuO4 was investigated by means of quasiparticle reflection and transmission at the normal conductor-superconductor interface of Sr2RuO4-Pt point contacts. We found two distinctly different types of dV/dI vs V spectra either with a double-minimum structure or with a zero-bias conductance anomaly. Both types of spectra are expected in the limit of high and low transparency, respectively, of the interface barrier between a normal metal and a spin-triplet superconductor. Together with the temperature dependence of the spectra this result strongly supports a spin-triplet superconducting order parameter for Sr2RuO4.

Spin-triplet superconductivity in Sr

The superconducting gap function of Sr2RuO4 was investigated by means of quasiparticle reflection and transmission at the normal conductor-superconductor interface of Sr2RuO4-Pt point contacts. We found two distinctly different types of dV/dI vs V spectra either with a double-minimum structure or with a zero-bias conductance anomaly. Both types of spectra are expected in the limit of high and low transparency, respectively, of the interface barrier between a normal metal and a spin-triplet superconductor. Together with the temperature dependence of the spectra this result strongly supports a spin-triplet superconducting order parameter for Sr2RuO4.

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The crystal and magnetic structures of LaTiO 3 have been studied by x-ray and neutron-diffraction techniques using nearly stoichiometric samples. We find a strong structural anomaly near the antiferromagnetic ordering T N = 146 K. In addition, the octahedra in LaTiO 3 exhibit an intrinsic distortion, which implies a splitting of the t 2 g levels. Our results indicate that LaTiO 3 should be considered as a Jahn-Teller system where the structural distortion and the resulting level splitting are enhanced by the magnetic ordering.

RuO

LaTiOx compounds are structurally related to perovskites and there are two known phases. The first,x=3.50, is a 2D layered-type ferroelectric. The second,x=3.00, is a weak ferromagnet with a 3D orthorhombic distorted perovskite structure. 20 samples with varying oxygen stoichiometry between these end members were prepared by floating zone melting, and then characterized by means of X-ray powder diffraction, electron microscopy, thermogravimetric analysis, resistivity and magnetic measurement. A phase diagram is established which displays the following physical and structural properties. A structural phase boundary atx=3.20 separates a new series of 2D layered structures from the 3D orthorhombic one. The former series represents the first conducting titanium oxides with a 2D layered structure to be reported. Atx=3.10 a phase boundary exists between a metallic and a weak ferromagnetic state where the magnetic transition temperatureTc can be sensitively tuned by the oxygen stoichiometryx. Samples withTc between 100 K and 130 K exhibit a metal-semiconductor transition whereas samples with higherTc, up to 149 K, are semiconductors between room temperature and 4.2 K.

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We report on the transport properties of LaTiO3+e and LaTiO3.5 films. The LaTiO3+e samples show metallic transport and several samples exhibit a hysteretic drop of resistance during cooldown at ∼240 K. The ferroelectric LaTiO3.5 samples, grown in capacitor structures, have nonlinear, diode-like and hysteretic V(I) characteristics. Two charge-controlled transport regimes are found, which can be utilized for switching the devices between two voltage states.

probed by Andreev reflection

Abstract Rechargeable alkaline nickel–metal hydride (Ni–MH) batteries have recently started to penetrate the consumer battery market. In contrast to the widely used Ni–Cd batteries they display up to 50% higher storage capacity and, due to the absence of Cd, an improved environmental compatibility. The electrochemically active material at the negative electrode is a hydrogen storage alloy. In most commercial applications so-called AB 5 alloys having a CaCu 5 type crystal structure have been used so far. These commercial AB 5 alloys are usually of the type Mm(Ni,Co,Al,Mn) 5 , containing typically 10 wt% Co. Mm denotes Mischmetal, a cost-effective mixture of the rare earths La, Ce, Pr and Nd. The large amount of Co is added to produce an alloy with a reasonable cycle life, but increases the alloy cost considerably. The Ni–MH system is considered as a promising energy source for pure electric vehicles and hybrid cars. However for this application it is necessary to decrease the Co content in these alloys without diminishing the cycle life endurance. Hydrogen storage alloy production by gas atomisation represents a promising way to achieve this goal.

Crystal and magnetic structure of LaTiO 3 : Evidence for nondegenerate t 2 g orbitals

Abstract Metal hydride alloys as electrode material for battery application contain up to 15 at.% cobalt. Alloys without cobalt show a much shorter cycle life compared to cobalt containing alloys. The mechanism of how cobalt influences the cycle life is still not well understood. The aim of this work is to investigate the influence of cobalt on the properties of the electrode. A series of alloys with different cobalt content and several other substituents for nickel (Fe, Cu,...) were prepared in two different ways. A set of samples was conventionally melted. A second set of samples was prepared by gas atomization. The volume expansion upon hydriding was analyzed by means of X-ray diffraction. Electrochemical measurements, e.g., discharge capacity as a function of cycle number, were performed. The volume expansion upon hydriding decreases with increasing cobalt content of the alloy. Cobalt substitution for nickel improves the cycle life of an electrode, especially at elevated temperatures (40°C). However, alloys where cobalt is partially substituted by iron show an even better cyclic stability and rate capability.