13 research outputs found
Rietveld refinements of the paraelectric and ferroelectric structures of PbFe0.5Ta0.5O3
No full textThe high symmetry parent phase and the two derived low symmetry phases of the complex perovskite PbFe0.5Ta0.5O3 have been refined by the Rietveld method from neutron powder diffraction data. The analysed powders were obtained by grinding single crystals. Owing to the Very small distortions from the cubic structure, the lattice symmetry of the derived phases was determined by means of synchrotron radiation powder diffraction. At 350 K the cubic phase (which is known to be stable for T > 270 K) is characterized by positional disorder or anharmonic thermal motion of lead atoms, as happens in most Pb-based complex perovskites. It was refined in space group Pm (3) over bar m, with strongly anisotropic thermal motion of oxygen atoms. The synchrotron powder diffraction pattern of the intermediate phase (stable for 220 K < T < 270 K), collected at 230 K, agrees with a small tetragonal distortion. Neutron data at 230 K were refined in symmetry P4mm. Only oxygen atoms are significantly displaced from the cubic positions. The analysis of line broadening and splitting in the synchrotron radiation patterns collected at 130 K and 15 K indicate the low temperature symmetry to be monoclinic. Neutron data at 15 K were refined in space group Cm
Low-temperature magnetic behavior of perovskite compounds PbFe1/2Ta1/2O3 and PbFe1/2Nb1/2O3
No full textThe isostructural perovskite compounds PbFe(1/2)Ta(1/2)O3 and PbFe(1/2)Nb(1/2)O3 have been known for long time, and they are part of the important class of materials called multiferroic, where ferroelasticity, ferroelectricity, and ferromagnetism coexist. In the literature regarding PbFe(1/2)Ta(1/2)O3 and PbFe(1/2)Nb(1/2)O3, an "anomaly" of their low-temperature magnetic behavior has not always been reported. Moreover, both the origin of this behavior, and the cause for which it was not always observed, were never completely explained. In this paper, the magnetic behavior of the two compounds at low temperature has been extensively studied and explained as the occurring of a spin-glasslike transition
Electronic structure of PbFe1/2Ta1/2O3: Crystallographic ordering and magnetic properties
No full textWe report electronic structure calculations for the multiferroic ferromagnetoelectric perovskite PbFe1/2Ta1/2O3, within density functional theory in the local spin density approximation (LSDA) and within the LSD+U approach. Our results, corresponding to several possible crystallographic and magnetic orderings, show the link between short range cation coordination and magnetic properties. In particular, the existence of two different Néel ordering temperatures, experimentally evidenced, is explained in terms of different superexchange paths introduced by crystallographic orderings certainly present, locally, in the real samples. The introduction of ferroelectric displacements, which are found to be in good agreement with experiment, do not bring about large changes in the electronic structure of this system
Radio Frequency Sputter Deposition of Epitaxial Nanocrystalline Nd1-xSrxCoO3 Thin Films
No full textIn this paper we report the deposition of epitaxial thin films of Nd1-xSrxCoO3 with x = 0, 0.2, and 0.5 on single-crystalline substrates (SrTiO3 and LaAlO3) carried out by means of rf-magnetron sputtering. The deposited films are all completely oriented and epitaxial and characterized by a nanocrystalline morphology. As-deposited films have an average roughness around 1 nm while after the thermal treatment this increases up to 20 nm while preserving the nanocrystalline morphology. All the films deposited on SrTiO3 have shown to be under a certain degree of tensile strain while those on the LaAlO3 experience a compressive strain, thus suggesting that at about 50 nm the films are not fully relaxed, even after the thermal treatment. For the x = 0.2 composition three different thicknesses have been investigated, revealing an increased strain for the thinner films
In situ investigation of the early stage of TiO2 epitaxy on (001) SrTiO3
No full textWe report on a systematic study of the growth of epitaxial TiO2 films deposited by pulsed laser deposition on Ti-terminated SrTiO3 (001) single crystals. By using in situ reflection high energy electron diffraction, low energy electron diffraction, x-ray photoemission spectroscopy, and scanning probe microscopy, we show that the stabilization of the anatase (001) phase is preceded by the growth of a few nanometers thick pseudomorphic SrxTiO2+y (x, y < 1) intermediate layer. The data demonstrate that the formation of this intermediate phase is related to the activation of a long range Sr migration from the SrTiO3 substrate into the film. Our results enrich the phase diagram of the Sr-Ti-O system under epitaxial strain opening a route for the study of the electronic and dielectric properties of the reported Sr-deficient SrTiO3 phase. (C) 2011 American Institute of Physics
