Hydrostatic pressure-induced reversible phase transformation in iron oxide nanoparticles.

We report hydrostatic pressure-induced reversible phase transformation in maghemite γ-Fe2O3 nanoparticles (cubic → tetragonal → cubic) using an in situ diamond anvil cell (DAC) technique. Thermal arc plasma-synthesized nanoparticles, particularly in a He gas medium, exhibit the reversible phase transformation under pressure ranging from 0 to 2.58 GPa. Rietveld refinement reflects that cubic to tetragonal maghemite phase transformation coexists with a cubic metallic Fe phase at 0.55 GPa pressure. The generation of two new superlattice reflections at 6.93° and 8.11°, respectively, reflects the phase transformation. The presence of a core-shell-type nanostructure observed from transmission electron microscopy micrographs is found to exhibit a spin glass shell-type behavior. This triggers pressure-induced fluctuating magnetization and interparticle interaction-induced surface anisotropy and spin disorder with broken bonds, translational symmetry, and incomplete coordination. This leads to overcoming the nucleation barrier at the surface, subsequently denser nucleation sites and increased nucleation probability. This further leads to an atomic rearrangement and tetragonality in the maghemite phase. Furthermore, with increasing pressure, the reversible structural change, i.e. from the tetragonal to cubic maghemite phase, has been explained in the light of the "internal stress model". The grains are again forced back to the cubic phase via generation of uniform compression along the c-axis and tension along the a and b axes. The spin glass behavior of the core-shell nanostructure along with the "internal stress model" explain the whole reversible phase transformation phenomenon in the γ-Fe2O3 phase.

High-Temperature Dielectric Relaxation and Electric Conduction Mechanisms in a LaCoO3-Modified Na0.5Bi0.5TiO3 System.

This research study examines the high-temperature dielectric relaxation and electric conduction mechanisms in (x)LaCoO3-(1 - x)Na0.5Bi0.5TiO3 samples, where x is 0.05, 0.10, and 0.15. The findings demonstrate that all the samples exhibit two dielectric transitions: first, a frequency-dispersive shoulder at a lower temperature (Ts) around 425-450 K, which is associated with polar nanoregions (PNRs), and second, from ferroelectric to paraelectric transition at the Curie temperature (Tc) approximately between 580 and 650 K. The impedance analysis reveals the negative temperature coefficient of resistance behavior of the specimens. The broad and asymmetric relaxation peaks obtained from modulus spectroscopy demonstrate a wide range of relaxations, suggesting non-Debye-type behavior. Furthermore, the conductivity studies provide insights into understanding the transport phenomena in the samples. The oxygen vacancies resulting from the addition of LaCoO3 into the Na0.5Bi0.5TiO3 ceramics are responsible for the relaxation and conduction processes, and the charge carrier is doubly ionized oxygen ion vacancies. All samples except for LCNBT10 at 1 kHz exhibit a negative magnetodielectric response.

Spin and valence variation in cobalt doped barium strontium titanate ceramics.

In the present decade, owing to half-metallic ferromagnetism, controlled 3d transition metal-doping based defect engineering in oxide perovskites attracts considerable attention in the pursuit of spintronics. We aim to investigate the electronic structure of Co-doped barium strontium titanate (Ba0.8Sr0.2CoxTi1-xO3 where x = 0, 0.1, 0.2) solid solution. Structural, vibrational and microscopic properties indicate the cationic substitution of Co at the octahedral Ti position along with a displacive kind of tetragonal-to-cubic phase transformation. X-ray photoelectron spectroscopy evidences the reduction in the valence state from Co3+ to Co2+ and Ti K edge X-ray absorption spectroscopy endorses the higher lattice symmetry with increasing Co doping. Orbital hybridization triggered electron hopping between O 2p and Co eg orbitals results in a spin fluctuation from the occupation t62ge0g for x = 0.1 to the occupation t62ge1gL for x = 0.20 (L designates a hole in the O 2p shell) aligned state observed from density functional theory calculations. The dominating crystal field energy as compared to intra-atomic exchange (Hund) energy decides the spin-orbital degeneracy for the Co 3d orbital to induce spin fluctuations.

Orbital hybridization-induced band offset phenomena in NixCd1-xO thin films.

Herein, we present the cationic impurity-assisted band offset phenomena in NixCd1-xO (x = 0, 0.02, 0.05, 0.1, 0.2, 0.4, 0.8, and 1) thin films and further discuss them based on orbital hybridization modification. The compositional and structural studies revealed that the cationic substitution of Cd2+ by Ni2+ ions leads to a monotonic shift in the (220) diffraction peak, indicating the suppression of lattice distortion, while the evolution of local strain with an increase in Ni concentration is mainly associated with the mismatch in the electronegativity of the Cd2+ and Ni2+ ions. In fact, Fermi level pinning towards the conduction band minimum takes place with an increase in the Ni concentration at the cost of electronically compensated oxygen vacancies, resulting in the modification of the distribution of carrier concentration, which eventually affects the band edge effective mass of the conduction band electrons and further endorses band gap renormalization. Besides, the appearance of a longitudinal optical (LO) mode at 477 cm-1, as manifested by Raman spectroscopy, also indicates the active involvement of electron-phonon scattering, whereas modification in the local coordination environment, particularly anti-crossing interaction in conjunction with the presence of satellite features and shake-up states with Ni doping, was confirmed by X-ray absorption near-edge and X-ray photoelectron spectroscopy studies. These results manifest the gradual reduction of orbital hybridization upon the incorporation of Ni, leading to a decrement in the band edge effective electron mass. Finally, the molecular dynamics simulation reflected a 13% reduction in the lattice parameter for the NiO thin film compared to the undoped film, while the projected density of states calculation further supports the experimental observation of reduced orbital hybridization with an increase in Ni concentration.

High temperature-mediated rocksalt to wurtzite phase transformation in cadmium oxide nanosheets and its theoretical evidence.

Herein, a high temperature-induced phase transformation (PT) in chemically grown CdO thin films is demonstrated, and its corresponding electronic origin further investigated by density functional theory. In particular, the cubic rocksalt to hexagonal wurtzite PT in the CdO thin film annealed at 900 °C was confirmed by X-ray diffraction (XRD), which was consistent with the high-resolution transmission electron microscopy (TEM) results. Moreover, atomic force microscopy and scanning electron microscopy clearly evidenced the morphological evolution via the formation of a nanosheet network in the wurtzite-phase CdO film. The high temperature treatment also led to a significant enhancement in the optical band gap from 2.2 to 3.2 eV, as manifested by UV-visible spectroscopy. The enhanced surface roughness of the nanosheet caused a deviation in the net dipole moment, which may break the polarizable bonds and help in reducing the average dielectric constant, resulting in a band gap opening for the transformed phase. Furthermore, X-ray absorption spectroscopy at the oxygen k-edge revealed a notable shift in the inflection point of the absorption edge, while the X-ray photoelectron spectroscopy (XPS) Cd 3d and O 1s spectra suggested a gradual reduction in the CdO2 phase with an increase in annealing temperature. In addition, different complementary techniques including Rutherford backscattering and Raman spectroscopy were exploited to understand the aforementioned PT and its structural correlation. Finally, molecular dynamics simulation together with density functional theory calculation suggested that the symmetry modification at the Brillouin zone boundary provides a succinct signature for the PT in the CdO thin film.

5,6-Benzoflavones as cholesterol esterase inhibitors: synthesis, biological evaluation and docking studies.

In a continued effort to develop potent cholesterol esterase (CEase) inhibitors, a series of 5,6-benzoflavone derivatives was rationally designed and synthesized by changing the position of the benzene ring attached to the flavone skeleton in previously reported 7,8-benzoflavones. All the synthesized compounds were checked for their inhibitory potential against cholesterol esterase (CEase) using a spectrophotometric assay. Among the series of forty compounds, seven derivatives (B-10 to B-16) exhibited above 90 percent inhibition against CEase in an in vitro enzymatic assay. Compound B-16 showed the most promising activity with an IC50 value of 0.73 nM against cholesterol esterase. To determine the type of inhibition, enzyme kinetic studies were carried out for B-16, which revealed its mixed-type inhibition approach. Moreover, to figure out the key binding interactions of B-16 with the amino acid residues of the enzyme's active site, molecular protein-ligand docking studies were also performed. B-16 completely blocks the catalytic assembly of CEase and prevents it from participating in the ester hydrolysis mechanism. The favorable binding conformation of B-16 suggests its prevailing role as a CEase inhibitor. Overall, the study showed that the cis-orientation of ring A with respect to the carbonyl group of ring C is responsible for the potent CEase inhibitory activity of the newly synthesized compounds.