Molecular dynamics simulations of oxygen diffusion in GdBaCo2O5.5

International audienceThe mechanisms of oxygen diffusion in GdBaCo2O5.5 compound are investigated by molecular dynamics simulations. The results confirm that diffusion is mainly bidimensional with oxygen moving in the a,b plane while diffusion along the c axis is much more difficult. Between 1000 and 1600 K, the activation energy for diffusion is about 0.6 eV, close to experimental values. Going deeper inside the oxygen diffusion mechanism, we see that this diffusion occurs mainly in the cobalt planes while most of the oxygen vacancies are kept in the Gd planes. Analysis of oxygen motions show that Gd planes can be seen as source-sink for the oxygen vacancies rather than as fast pathways

Thermodynamics of hydration and oxidation in the proton conductor Gd-doped barium cerate from density functional theory calculations

International audienceHydration and oxidation of gadolinium-doped barium cerate, a system with highly promising properties when used as electrolyte for protonic ceramic fuel cells, are investigated by means of density functional calculations. The energy landscape of oxygen vacancies and interstitial protons in this strongly distorted orthorhombic perovskite is computed. Although the most stable sites for protons are found in the close vicinity of the dopant, the picture of a very complex energy landscape emerges, in which some sites far away from Gd are found more stable than other ones in its close vicinity, due to the highly distorted geometry of the hostmaterials. The fully hydrated phase can be approximated by a structure with 16 local minima. Both hydration (water incorporation) and oxidation (oxygen incorporation) are found to be exothermic processeswith reaction enthalpies of−1.34 eV/H2Omolecule and −0.70 eV/O atom, respectively. The hole polaron resulting from the exothermic incorporation of oxygen is found localized on oxygens around the dopant (small polaron) and carries a spin magnetic moment. Finally, the competition between hydration and oxidation is studied and discussed as a function of oxygen and water partial pressures

Influence of synthesis route and composition on electrical properties of La9.33 + xSi6O26 + 3x/2 oxy-apatite compounds

Oxy-apatite materials La9.33 + xSi6O26 + 3x/2 are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well-known difficulties related to their densification prevent them from being used as such. This paper presents strategies to obtain oxyapatite dense materials. First, freeze-drying has been optimized to obtain ultrafine and very homogeneous La9.33 + xSi6O26 + 3x/2 (0≤x≤0.67) nanopowders. From these powders, conventional and Spark Plasma Sintering (SPS) have been used leading to very dense samples obtained at temperatures rather lower than those previously reported. For instance, SPS has allowed to prepare fully dense and transparent ceramics from 1200 °C under 100 MPa. The microstructure and transport properties of such samples have been then evaluated as a function of sintering conditions and lanthanum content. It will be show that for lanthanum content higher than 9.60 per unit formula, the parasitic phase La2SiO5 appears leading to a egradation of conduction properties.We also show that grain boundaries and porosity (for conventionally-sintered materials) seem to have blocking effects on oxygen transport. The highest overall conductivity values at 700 °C, i.e. σ700 °C=7.33.10−3 S cm−1, were measured for La9.33Si6O26 material conventionally-sintered at 1500 °C which contains bigger grains' size by comparison with σ700 °C=4.77.10−3 S cm−1 for SPS-sintered materials at the same temperature but for few minutes. These values are associated with activation energies close to 0.83–0.91 eV, regardless of sintering condition, which are commonly encountered for anionic conductivity into such materials

Interface control of emergent ferroic order in Ruddlesden-Popper Srn+1_{n+1}Tin_nO3n+1_{3n+1}

We have discovered from first-principles an unusual polar state in the low n Sr$_{n+1}$Ti$_n$O$_{3n+1}$ Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low dimensional RP materials as is well known to occur in SrTiO$_3$. These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface

Multiferroic phase transition near room temperature in BiFeO3 films

In multiferroic BiFeO3 thin films grown on highly mismatched LaAlO3 substrates, we reveal the coexistence of two differently distorted polymorphs that leads to striking features in the temperature dependence of the structural and multiferroic properties. Notably, the highly distorted phase quasi-concomitantly presents an abrupt structural change, transforms from a hard to a soft ferroelectric and transitions from antiferromagnetic to paramagnetic at 360+/-20 K. These coupled ferroic transitions just above room temperature hold promises of giant piezoelectric, magnetoelectric and piezomagnetic responses, with potential in many applications fields

Surface phase transitions in BiFeO3 below room temperature

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour

Lpd depletion reveals that SRF specifies radial versus tangential migration of pyramidal neurons

During corticogenesis, pyramidal neurons (~80% of cortical neurons) arise from the ventricular zone, pass through a multipolar stage to become bipolar and attach to radial glia[superscript 1, 2], and then migrate to their proper position within the cortex[superscript 1, 3]. As pyramidal neurons migrate radially, they remain attached to their glial substrate as they pass through the subventricular and intermediate zones, regions rich in tangentially migrating interneurons and axon fibre tracts. We examined the role of lamellipodin (Lpd), a homologue of a key regulator of neuronal migration and polarization in Caenorhabditis elegans, in corticogenesis. Lpd depletion caused bipolar pyramidal neurons to adopt a tangential, rather than radial-glial, migration mode without affecting cell fate. Mechanistically, Lpd depletion reduced the activity of SRF, a transcription factor regulated by changes in the ratio of polymerized to unpolymerized actin. Therefore, Lpd depletion exposes a role for SRF in directing pyramidal neurons to select a radial migration pathway along glia rather than a tangential migration mode.Ruth L. Kirschstein National Research Service Award (grant F32- GM074507)National Institutes of Health (U.S.) (grant # GM068678

‘A sword of Damocles’ : patient and caregiver beliefs, attitudes and perspectives on presymptomatic testing for autosomal dominant polycystic kidney disease: a focus group study

Background and objectives: Presymptomatic testing is available for early diagnosis of hereditary autosomal dominant polycystic kidney disease (ADPKD). However, the complex ethical and psychosocial implications can make decision-making challenging and require an understanding of patients’ values, goals and priorities. This study aims to describe patient and caregiver beliefs and expectations regarding presymptomatic testing for ADPKD. Design, setting and participants: 154 participants (120 patients and 34 caregivers) aged 18 years and over from eight centres in Australia, France and Korea participated in 17 focus groups. Transcripts were analysed thematically. Results: We identified five themes: avoiding financial disadvantage (insecurity in the inability to obtain life insurance, limited work opportunities, financial burden); futility in uncertainty (erratic and diverse manifestations of disease limiting utility, taking preventive actions in vain, daunted by perplexity of results, unaware of risk of inheriting ADPKD); lacking autonomy and support in decisions (overwhelmed by ambiguous information, medicalising family planning, family pressures); seizing control of well-being (gaining confidence in early detection, allowing preparation for the future, reassurance in family resilience); and anticipating impact on quality of life (reassured by lack of symptoms, judging value of life with ADPKD). Conclusions: For patients with ADPKD, presymptomatic testing provides an opportunity to take ownership of their health through family planning and preventive measures. However, these decisions can be wrought with tensions and uncertainty about prognostic implications, and the psychosocial and financial burden of testing. Healthcare professionals should focus on genetic counselling, mental health and providing education to patients’ families to support informed decision-making. Policymakers should consider the cost burden and risk of discrimination when informing government policies. Finally, patients are recommended to focus on self-care from an early age