Impact of Cr Doping on the Structural, Optical, and Magnetic Properties of Sol-Gel-Synthesized Bi0.80Ba0.10Pr0.10FeO3 Nanopowders.

The sol-gel route was used to synthesize a series of compounds of the system Bi0.8Ba0.10Pr0.10Fe1-x Cr x O3 within the 0 ≤ x ≤ 0.15 compositional range. To explore the impact of Cr3+ ion substitution on the structural, dielectric, optical, and magnetic properties, we introduced varying concentrations of Cr3+ while maintaining a fixed 10% atomic concentration of each Ba2+ and Pr2+ in BiFeO3. X-ray diffraction analysis revealed a structural phase transition from rhombohedral (R3c) for an undoped (i.e., without Cr) sample to two coexisting phases, i.e., a mix of rhombohedral and orthorhombic (Pbnm) phases for the Cr-doped samples. Cr3+ doping significantly changes the band gap energy from 1.84 eV (x = 0.0) to 1.93 eV (x = 0.15), which makes this material suitable for photovoltaic applications. Furthermore, each sample exhibited ferromagnetic behavior due to the disruption of the spiral spin structures and adjustments in superexchange interactions, attributed to modifications in the Fe-O and Fe-O-Fe bond lengths. A reduction in magnetization is observed at higher Cr concentrations that can be ascribed to the dilution of magnetic moments due to the increase of the orthorhombic phase percentage and the introduction of nonmagnetic Cr3+ ions. Our results show that Cr doping in the Bi0.8Ba0.10Pr0.10FeO3 system induces enhanced multiferroic properties at room temperature.

Design of new hole transport materials based on triphenylamine derivatives using different π-linkers for the application in perovskite solar cells. A theoretical study.

New organic molecules containing five different compounds, commonly called p-linkers, located between the triphenylamine units, were theoretically designed and analyzed in order to be proposed as new hole transport materials (HTMs) in perovskite solar cells, in total ten new molecules were analyzed. The electronic, optical and hole transport properties were determined, similarly, the relationship of these properties with their molecular structure was also investigated by Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB) calculations. Eight of the ten analyzed compounds exhibited the main absorption band out of the visible region; therefore these compounds did not present an overlap with the absorption spectra of the typical methylammonium lead iodide (MAPI) hybrid-perovskite. The results showed that the Highest occupied molecular orbital (HOMO) levels of the compounds are higher than the perovskite HOMO level, and in some cases these are even higher than the Spiro-OMeTAD HOMO. The calculated electronic couplings and the reorganization energy values provided useful information in order to determine if the systems were hole or electron transport materials.

Reverse micelle synthesis of oxide nanopowders: mechanisms of precipitate formation and agglomeration effects.

We present an analysis of reverse micelle stability in four model systems. The first two systems, composed of unstable microemulsions of isooctane, water, and Na-AOT with additions of either iron sulfate or yttrium nitrate, were used for the synthesis of iron oxide or yttrium oxide powders. These oxide powders were of nanocrystalline character, but with some level of agglomeration that was dependent on calcination temperature and cleaning procedures. Results show that even though the reverse micellar solutions were unstable, nanocrystalline powders with very low levels of agglomeration could be obtained. This effect can be attributed to the protective action of the surfactant on the surfaces of the powders that prevents neck formation until after all the surfactant has volatilized. A striking feature of the IR spectra collected on the iron oxide powders is the absence of peaks in the ~1715 cm(-1) to 1750 cm(-1) region, where absorption due to the symmetric C=O (carbonyl) stretching occurs. The lack of such peaks strongly suggests the carbonyl group is no longer free, but is actively participating in the surfactant-precipitate interaction. The final two microemulsion systems, containing CTAB as the surfactant, showed that loss of control of the reverse micelle synthesis process can easily occur when the amount of salt in the water domains exceeds a critical concentration. Both model systems eventually resulted in agglomerated powders of broad size distributions or particles that were large compared to the sizes of the reverse micelles, consistent with the notion that the microemulsions were not stable and the powders were precipitated in an uncontrolled fashion. This has implications for the synthesis of nanopowders by reverse micelle synthesis and provides a benchmark for process control if powders of the highest quality are desired.

Mechanisms of combustion synthesis and magnetic response of high-surface-area hexaboride compounds.

We present an analysis of the combustion synthesis mechanisms for the preparation of hexaboride materials using three compounds as model systems: EuB(6), YbB(6), and YB(6). These three hexaborides were chosen because of the differences in ionic radii between Eu(3+), Yb(3+), and Y(3+), which is a factor in their stability. The powders were prepared using metal nitrates, carbohydrazide, and two different boron precursor powders. The resulting materials were analyzed by X-ray diffraction, which showed that combustion synthesis is effective for the synthesis of EuB(6), since the Eu(3+) ion has an ionic radius greater than ∼1 Å. The synthesis of YbB(6) and YB(6) is not as effective because of the small size of the Yb(3+) and Y(3+) ions, making the hexaborides of these metals less stable and resulting in the synthesis of borates due to the presence of oxygen during the combustion process. Scanning electron microscopy and dynamic light scattering of the EuB(6) powders shows that the particle size of the hexaboride product is dependent on the particle size of the boron precursor. The magnetic susceptibility of our EuB(6) powders manifests irreversible behavior at low applied fields, which disappears at higher fields. This behavior can be attributed to the increase in size and number of magnetic polarons with increasing magnetic field.