Daniel Widmer

Phase diagram of LaTiO x : from 2D layered ferroelectric insulator to 3D weak ferromagnetic semiconductor

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.

Electrical-stress-induced conductivity increase in SrTiO3 films

The charge transport of polycrystalline SrTiO3 films doped with Cr is investigated by means of temperature- and electric-field-dependent current measurements. In particular, the effect of electrical stress on the conduction mechanism is analyzed to understand the forming process of samples exhibiting resistance-switching phenomena. The temperature dependence of the conductivity can be described with the variable-range hopping model. The electrical-stress-induced conductivity increase of SrTiO3:Cr films is accompanied by a change of the temperature dependence corresponding to a significant increase of the density of localized states.

On the crystal structures of La2Ti2O7 and La5Ti5O17: High-resolution electron microscopy

Abstract The structure of La5Ti5O17 has been examined by high-resolution transmission electron microscopy combined with powder X-ray and thermogravimetric analysis. It comprises five [TiO6] octahedra-thick layers isomorphous with {1 0 0} slabs of the LaTiO3 orthorhombic perovskite structure, bounded by crystallographic shear parallel with the perovskite [0 0 1] direction. La atoms occupy the distorted perovskitic A sites. The structure is derived from the homologous La2Ti2O7 by elimination of 1 35 of the oxygens from this four-octahedra-thick layered structure. Both compounds are members of an homologous series based on variation in the anion to cation ratio.

Electric and magnetic properties of the first layered conducting titanium and niobium oxides

Three-dimensional titanium and niobium oxides are superconducting at temperatures considerably lower than layered copper oxides. Therefore it seems worthwhile to study layered titanium and niobium oxides. Layered conducting phases, however, were not known until recently, when the study of the LaTiOx system led to the first layered conducting titanium oxides. Single crystals of isostructural niobium oxides, CaNbOx, SrNbOx and Sr0.9La0.1NbO3.4, were prepared with the floating zone melting technique. In common with LaTiOx they have the following properties. For x≅3.4, its layered perovskite-related structure of a known type is derived from the layered highest-Tc (>1600 K) ferroelectrics LaTiO3.5, CaNbO3.5 and SrNbO3.5. In the range 3.42 <x<3.50 well ordered intergrowth takes place between the ferroelectricx=3.5 structure and thex=3.4 structure. This intergrowth was found to be representative of the bulk and the stacking sequence is dependent onx. Forx<3.5, magnetic and resistivity measurements were performed between room temperature and 4.2 K. The temperature dependence of the magnetic susceptibility along the layers displays a minimum. Semiconducting behavior was found along and perpendicular to the layers. At low temperatures the resistivity exhibits thermally activated behavior with unexpectedly small activation energies in the meV range. This might indicate that a further lowering of the average titanium and niobium valencies could lead to metallic behavior.

Centrosymmetric or Noncentrosymmetric? Case Study, Generalization and Structural Redetermination of Sr5Nb5O17

The possibility that the structure of the novel semiconducting perovskite-related material strontium niobium oxide, Sr5Nb5O17, refined by Schmalle et al. [Acta Cryst. (1995), C51, 1243–1246] in space group Pnn2, might instead belong to space group Pnnm has been investigated following an analysis of the atomic coordinates that indicated the latter space group to be more likely. All Iobs were carefully remeasured, first those within a hemisphere containing c*, then all that lay within the full sphere of reflection. Refinement was undertaken, with each of two different sets of weights, in each space group. Each data set was used under three limiting intensity conditions: Iobs > 4σ(Iobs), Iobs > 2σ(Iobs) and finally with all reflections, but setting magnitudes with Iobs ≤ 0 equal to 0. Fourteen separate tests based only upon the X-ray diffraction data may be used to distinguish between Pnn2 and Pnnm. Nine tests favored the latter choice, four were indeterminate and one was not used. Seven further tests may be made on the basis of physical measurement; of these, three strongly indicated Pnnm, one was indeterminate and three could not be used. The evidence clearly suggests the space group is Pnnm. The use of all reflections, including those with negative magnitude set equal to zero, is essential to avoid ambiguity in the X-ray diffraction tests and achieve the highest reliability. Refinement with weights based on variances of Type A and Type B [Schwarzenbach et al. (1995). Acta Cryst. A51, 565–569] resulted in improved reliability compared with that obtained from a popular empirical weighting scheme. The revised structure differs in several respects from that published previously.