Structural, ferroelectric, and optical properties of Bi3+ doped YFeO3: A first‐principles study

The orthoferrites with the general formula RFeO3 (R = Ho, Er, Lu, Sc, and Y) have recently attracted a great deal of attention because they are promising candidates for a second generation of multiferroic materials. In this computational work, the structural, ferroelectric and optical properties of the YFeO3 perovskite oxide (YFO) and a Bi‐doped YFeO3 were analyzed. Bi‐substitution in YFO leads to an increase of its lattice parameters by virtue of the larger ionic radius of Bi3+. Both compounds exhibit a G‐type antiferromagnetic ground state. The calculations disclose a significant spontaneous polarization along the [101] direction of YFO‐Bi, which originates in the asymmetric distribution of the charges around the Bi3+ ions, as a result of the Bi‐6s electrons. The electric polarizability of YFO is increased upon Bi3+‐doping and the more significant components of the real permittivity tensor of YFO‐Bi are those associated with the direction along which the maximum value of spontaneous polarization is observed. The spontaneous polarization of YFO‐Bi found in this work reveals that this compound holds the potential for the next generation of multi ferroic materials

Effect of La3+/Sr2+ ordering on the magnetic properties of La2/3Sr1/3MnO3 by first principles calculations

In this work, using DFT + U formalism, we investigate the effect of order-disorder in the A-site occupation byLa3+and Sr2+on the stability of the ferromagnetic order in La2/3Sr1/3MnO3with−Rc3symmetry. To date, adetailed theoretical discussion of such phenomenon, using a combination of different representations of theelectronic structure, is still missing in the Literature. We employed structural models consisting of 120 atomsupercells constructed according to the precise stoichiometry of the compound. Two configurations, describingrandomized and ordered occupation of the La3+/Sr2+ions, were evaluated. We demonstrate that the ferro-magnetic arrangement of La2/3Sr1/3MnO3with randomly distributed La3+and Sr2+ions is more stable. In suchconfiguration wefind that the Mn3+and Mn4+ions are not distinguished, favoring the double-exchange me-chanism, enhanced by the higher degree of covalence in the MneO bonds near the Fermi level between thespin-upMn-egorbitals and the O-porbitals

Transformative spaces in the making: key lessons from nine cases in the Global South

Creating a just and sustainable planet will require not only small changes, but also systemic transformations in how humans relate to the planet and to each other, i.e., social–ecological transformations. We suggest there is a need for collaborative environments where experimentation with new configurations of social–ecological systems can occur, and we refer to these as transformative spaces. In this paper, we seek a better understanding of how to design and enable the creation of transformative spaces in a development context. We analyse nine case studies from a previous special issue on Designing Transformative Spaces that aimed to collect examples of cutting-edge action-oriented research on transformations from the Global South. The analysis showed five design phases as being essential: Problem Definition Phase; Operationalisation Phase; Tactical Phase; Outcome Phase; and Reflection Phase. From this synthesis, we distilled five key messages that should be considered when designing research, including: (a) there are ethical dilemmas associated with creating a transformative space in a system; (b) it is important to assess the readiness of the system for change before engaging in it; (c) there is a need to balance between ‘safe’ and ‘safe-enough’ spaces for transformation; (d) convening a transformative space requires an assemblage of diverse methodological frameworks and tools; and (e) transformative spaces can act as a starting point for institutionalising transformative change. Many researchers are now engaging in transdisciplinary transformations research, and are finding themselves at the knowledge–action interface contributing to transformative space-making. We hope that by analysing experiences from across different geographies we can contribute towards better understanding of how to navigate the processes needed for the urgent global transformations that are being called for to create a more equitable and sustainable planet Earth

Phylogenetics of Chloridoideae (Gramineae): a Preliminary Study Based on Nuclear Ribosomal Internal Transcribed Spacer and Chloroplast trnL–F Sequences

The phylogeny of Chloridoideae (Gramineae) was inferred from parsimony analyses of DNA sequences from two genomes—the chloroplast trnL intron, trnL 3\u27 exon, and trnL–F intergenic spacer, and the nuclear ribosomal internal transcribed spacer region (ITS1 + 5.8S + ITS2). Eighty species representing 66 chloridoid genera were sampled, including all but four of the native New World genera. Analyses of the individual and combined data sets were performed. The phylogenies were found to be highly congruent. Of the four tribes and seven subtribes of Chloridoideae sensu Clayton and Renvoize (1986) whose phylogenetic status could be tested with our taxon sample, only Orcuttieae and Uniolinae were monophyletic. The phylogenies suggested significant homoplasy in morphological traits, including inflorescence type, number of florets per spikelet, and number of lemma nerves. We propose a new classification based on the three main clades in the phylogenies—tribes Cynodonteae, Eragrostideae, and Zoysieae. The Eragrostideae clade is well resolved and supported and is further divided into three subtribes, Cotteinae, Eragrostidinae, and Uniolinae. Cynodonteae include most of the genera in our study, but the clade is poorly resolved. However, a clade formed of Muhlenbergia and nine other genera is present in both phylogenies and is well resolved and supported. A number of interesting, well-supported relationships are evident in the phylogenies, including Pappophorum–Tridens flavus, Tragus–Willkommia, and Gouinia–Tridens muticus–Triplasis–Vaseyochloa. Except for Bouteloua, no genus represented by multiple species proved to be monophyletic in the phylogenies

Highly textured Sr, Nb, co-doped BiFeO3 thin films grown on SrRuO3/Si substrates by rf-sputtering

In this study, (011)-highly oriented Sr, Nb co-doped BiFeO3 (BFO) thin films were successfully grown on SrRuO3/Si substrates by rf-magnetron sputtering. The presence of parasite magnetic phases was ruled out based on the high resolution x-ray diffraction data. BFO films exhibited a columnar-like grain growth with rms surface roughness values of 5.3 nm and average grain sizes of 65-70 nm for samples with different thicknesses. Remanent polarization values (2Pr) of 54 lC cm 2 at room temperature were found for the BFO films with a ferroelectric behavior characteristic of an asymmetric device structure. Analysis of the leakage mechanisms for this structure in negative bias suggests Schottky injection and a dominant Poole-Frenkel trap-limited conduction at room temperature. Oxygen vacancies and Fe3þ/Fe2þ trap centers are consistent with the surface chemical bonding states analysis from x-ray photoelectron spectroscopy data. The (011)-BFO/ SrRuO3/Si film structure exhibits a strong magnetic interaction at the interface between the multiferroic film and the substrate layer where an enhanced ferromagnetic response at 5 K was observed. Zero-field cooled (ZFC) and field cooled (FC) magnetization curves of this film system revealed a possible spin glass behavior at spin freezing temperatures below 30 K depending on the BFO film thickness

Study by hysteresis measurements of the influence of grain size on the dielectric properties of ceramics of the Sr0.4Ba0.60TiO3 type prepared under different sintering conditions

The ferroelectric properties of the Ba0.6Sr0.4TiO3 system prepared at 1250, 1350 and 1450 degrees C sintering temperatures for up to 4 h, are studied through the measurements of the hysteresis loops at different temperatures and at a constant field of 40 kV/m. Grain size analysis was performed with a scanning electron microscope (SEM) with a statistical method, The dielectric properties of the samples are reported as a function of temperature for the different sintering conditions. The dielectric permittivity and the remanent polarization are correlated to the grain size distributions obtained by SEM. A derivative process of the remanent polarization is used to analyze the phase transitions as the temperature is varied. (C) 1999 Published by Elsevier Science Ltd. All rights reserved

Heat utilisation technologies: A critical review of heat pipes

In electrical or thermal appliances, heat (thermal energy) must either be added into or removed from a system to maintain operational stability. Heat pipes can enhance the heat transfer capabilities without needing a significant temperature gradient between heat sources and heat sinks. The effectiveness of heat pipes is due to the latent heat of phase change of the working fluid within (i) condensation and (ii) evaporation stages. The latent heat of phase change greatly exceeds the sensible heat capacity. Heat pipes may rely on gravity, wicks, centrifugal force or in some cases even a magnetic field to help return condensate flow from the condenser to the evaporator. Wicks in heat pipes are classified into three groups: sintered, groove and mesh types. This review attempts to cover various types of heat pipes such as thermal diodes, variable conductance, pulsating, etc. The application of nanotechnology in heat pipes can be separated into two groups: nanoparticles and nanobubbles, with the latter receiving considerably less attention than the former. The hybridisation of heat pipe technology is also possible and has been discussed along with its future research potential

First-principles study of the coexisting ferroelectric and ferromagnetic properties of the La0.75Bi0.25CrO3 compound

Despite the interesting properties of La1-xBixCrO3, the origin of its multiferroic properties has not been yet established by first-principles calculations. Toward this effect, in this work we present a comparative study between the structural, electronic, magnetic and ferroelectric properties of pristine LaCrO3 and La0.75Bi0.25CrO3 using density functional theory (DFT) plus the Hubbard potential (U) formalism and the Berry phase approach for the polarization. Additionally, we present a comparative analysis between reported experimental results and the theoretical calculations of this work, from which we have determined that LaCrO3 is stable under a G-type antiferromagnetic configuration, while the La0.75Bi0.25CrO3 is stable in a ferromagnetic one. This is due to the decrease of the Cr-O-Cr angle which is strongly related to the high degree of covalence of the spin-up Cr-t2g and O-2p orbitals in the Cr-O bonds. On the other hand, La0.75Bi0.25CrO3 turns out to be ferroelectric due to the structural distortion induced by the lone pair of Bi in site A, which predicts a spontaneous polarization of 30.41 μC/cm2 in the [011] direction. That is, first-principles calculations satisfactory explain the origin of ferromagnetism and ferroelectricity of the La1−xBixCrO3 compound