Phase coexistence in Bi1−x Pr x FeO3 ceramics

Bi_1?x Pr _x FeO₃ ceramics across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The structural phase transitions in Bi_1?x Pr _x FeO₃ driven by doping concentration and temperature are significantly different from those in BiFeO₃ compounds doped with other rare-earth elements. The features of the structural transformations have been discussed based on the specific character of the chemical bonds associated with praseodymium ions. The detailed study of the crystal structure evolution clarified the ranges of both single-phase and phase coexistence regions at different temperatures and dopant concentrations. For x = 0.125, compound extraordinary three-phase coexistence state has been observed in a narrow temperature range at about 400 °C. The results explicate driving forces of the structural transitions and elucidate the origin of the remarkable physical properties of BiFeO₃-based compounds near the morphotropic phase boundary.     

A-site ordered state in manganites with perovskite-like structure based on optimally doped compounds Ln0.70Ba0.30MnO3 (Ln = Pr,Nd)

In this paper, we report on the crystal structure and magnetic properties of the nanostructured Ba-ordered phases of rare-earth manganites obtained from the optimally doped solid solutions Ln_0.70Ba_0.30MnO₃ (Ln = Pr, Nd). The materials were studied by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and SQUID-magnetometry techniques. It is found that states with different degrees of cation ordering in the A-sublattice of the ABO₃ perovskite can be obtained by employing special conditions of chemical treatment. In particular, reduction of the parent compounds results in the formation of a nanocomposite containing ferrimagnetic anion-deficient ordered phase LnBaMn₂O₅. Oxidation of the composite does not change an average size of the nanocrystallites, but drastically alters their phase composition to stabilize ferromagnetic stoichiometric ordered phase LnBaMn₂O₆ and ferromagnetic superstoichiometric disordered phase Ln_0.90Ba_0.10MnO_3+δ. It is shown that the magnetic properties of the materials are determined by the joint action of chemical (cation ordering) and external (surface tension) pressures.     

Temperature and Composition‐Induced Structural Transitions in Bi1−xLa(Pr)xFeO3 Ceramics

Bi_1?xLa_xFeO₃ and Bi_1?xPr_xFeO₃ ceramics of the compositions near the morphotropic phase boundary have been studied by X‐ray and neutron diffraction techniques and differential thermal analysis. The structural phases characterized by the long‐range polar, antipolar, and nonpolar ordering as well as the phase coexistence regions have been identified using the diffraction data depending on the dopant concentration and temperature. As a result of these studies the three phase region has been observed for the Pr‐doped compounds and the phase diagrams have been constructed. The detailed evolution of the structural parameters permitted to itemize the factors affecting the structural phase transitions and clarify the origin of the enhanced electromechanical properties in these materials. The performed structural analysis disclosed different character of the chemical bonds in the La‐ and Pr‐doped BiFeO₃ compounds. Further, the role of the rare‐earth ions in the covalency of the chemical bonds is discussed.     

Influence of Superexchange Interaction on the Ferromagnetic Properties of Manganites and Cobaltites

The role of superexchange interaction in the formation of the ferromagnetic state of cobaltites in the La_0.5Sr_0.5Co_1–xMexO₃ (Me = Cr, Ga, Fe) systems and manganites La_0.7Sr_0.3Mn_0.85M_0.15O₃ (M–Nb, Mg) with a perovskite structure has been studied. It was found that the ferromagnetic state in cobaltites can be implemented in some compositions without the mixed-valence effect of cobalt ions. The initial compound (x = 0) is ferromagnetic (Т_С = 247 K) with the saturation magnetization close to 2μB (at T = 30 K) per formular unit. It has been shown that the chemical substitution of cobalt by chromium reduces the spontaneous magnetization to 0.3μB (at х = 0.2), while the substitution of cobalt by iron ions (х = 0.2) does not alter the magnetization. The obtained data are interpreted in the model of positive superexchange interactions between cobalt and iron ions and negative ones between cobalt and chrome. It has been shown that the La_0.7Sr_0.3Mn_0.85Nb_0.15O₃ composition is ferromagnetic with ТС=145K, with a magnetic moment of 3.1 μB/Mn at 10 K, and no evidence of a cooperative orbital ordering in the manganite compounds has been revealed. Partial chemical substitution of Nb⁵⁺ ions by Mg²⁺ ones leads to the formation of Mn⁴⁺ ions, while it does not strengthen the ferromagnetic state. The strengthening of the structural distortions reduces the ferromagnetic component. It is assumed that the ferromagnetic state is caused by a significant hybridization of eg-orbitals of the manganese and oxygen ions, which strengthens the positive component of the superexchange interactions.     

Intermediate structural state in Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Pr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeO<Subscript>3</Subscript> ceramics at the rhombohedral–orthorhombic phase boundary

Crystal structure of the Bi_1?x Pr _x FeO₃ ceramics of the compositions corresponding to the threshold concentrations separating the polar rhombohedral (R3c) and anti-polar orthorhombic (Pbam) phases has been investigated with X-ray diffraction, transmission electron microscopy and differential scanning calorimetry measurements performed in a broad temperature range. The structural study specifies the peculiarities of the temperature-driven transition into the non-polar orthorhombic (Pnma) phase depending on the structural state of the compounds at room temperature. The crystal structure analysis reveals the revival of the anti-polar orthorhombic phase upon the temperature-induced transition, thus assuming that it can be considered as the bridge phase between the polar rhombohedral and the non-polar orthorhombic phases.     

Effect of iron doping on magnetic properties of Sr0.78Y0.22CoO2.625+δ-layered perovskite

The Sr_0.78Y_0.22Co_1−x Fe _x O_2.625+δ (x = 0 and 0.12) system has been studied using neutron diffraction technique, magnetization, and elastic properties measurements. Undoped sample exhibits superstructure with 2√2a _p  * 2√2a _p  * 4a _p metrics and structural phase transition at T = 360 K. Magnetic ordering starts to develop below 360 K. Spontaneous magnetization shows anomalous behavior and reaches maximal value nearly room temperature. A magnetic structure of the both compounds has been described assuming G-type antiferromagnetic ordering. There are two different magnetic moment values for Co ions in CoO₆ and CoO_4.5 layers. Magnetic moments in CoO_4.5 layers are larger than those for octahedrons. The refined values of magnetic moments indicate a mixed low–high spin state of Co³⁺ for the both layers. There is no evidence for any change of the magnetic structure type with temperature lowering despite the anomalous magnetization behavior. Iron doping (x = 0.12) leads to a suppression of the small ferromagnetic component, disappearance of the 2√2a _p  * 2√2a _p  * 4a _p metrics, and strong increase of the average magnetic moment for Co_4.5 layer. It is suggested that the ferromagnetic component in the undoped samples is a result of non-collinearity of the magnetic moments within CoO_4.5 layers.     

Tin Dioxide Sensing Layer Grown on Tubular Nanostructures by a Non‐Aqueous Atomic Layer Deposition Process

A new atomic layer deposition (ALD) process for nanocrystalline tin dioxide films is developed and applied for the coating of nanostructured materials. This approach, which is adapted from non‐hydrolytic sol‐gel chemistry, permits the deposition of SnO₂ at temperatures as low as 75 °C. It allows the coating of the inner and outer surface of multiwalled carbon nanotubes with a highly conformal film of controllable thickness. The ALD‐coated tubes are investigated as active components in gas‐sensor devices. Due to the formation of a p‐n heterojunction between the highly conductive support and the SnO₂ thin film an enhancement of the gas sensing response is observed.     

High-resolution, real-time 3D imaging with fringe analysis

Real-time 3D imaging is becoming increasingly important in areas such as medical science, entertainment, homeland security, and manufacturing. Numerous 3D imaging techniques have been developed, but only a few of them have the potential to achieve realtime. Of these few, fringe analysis based techniques stand out, having many advantages over the rest. This paper will explain the principles behind fringe analysis based techniques, and will provide experimental results from systems using these techniques.