Structural Microstrain Research Articles

Hydrogenation Influences on the Structural, Optical, and Insulating Properties of (Bi+Cr) Codoped Anatase TiO2 Nanoparticles

AbstractThis paper presents the investigation results for pure and (Bi/Cr) codoped TiO2 nanoparticles (NPs) synthesized by using a hydrothermal precipitation technique. The synthesized NPs are characterized by using X‐ray diffraction (XRD) and optical spectroscopy. The XRD patterns show a single anatase phase of an average nano‐crystallite size ≈6–9 nm. In addition, it was noticed that the hydrogenation of the NPs of the ceramic samples reduced the crystallite sizes due to the creation of O‐vacancies and microstrain. The optical absorption spectra show redshift of the bandgap. Moreover, an absorption hump in the high‐wavelength range of 630–800 nm is observed and attributed to the d‐d transitions in Cr3+ ions. Then, it is studied hydrogenation's significant effect on the absorption spectrum for λ > 370 nm, which attributes to the creation of a high density of point defects and structural microstrain that enhances the redshift of bandgap. The permittivity at 1 kHz of the prepared (Bi/Cr):TiO2 ceramic is found to be of large value (≈670), which is enhanced up to ≈103 by the hydrogenation. It is concluded that the hydrogenation in the present investigation supports the effects of the Bi3+ ions on the dielectric properties rather than that of Cr3+ ions, which mostly substitute the Ti ions (TiCr) due to the similarity of Ti3+ and Cr3+ ions. Thus, the measured dielectric constant of the hydrogenated sample [(Bi/Cr):TiO2‐H] is almost equal to that value of the host TiO2 doped with only Bi ions (Bi:TiO2). Moreover, it is concluded that the hydrogenation has a negligible effect on the effective polaron hopping energy, but increases the sample maximum barrier height (WM) by ≈55% causing an increase in the measured values of the permittivity.

Mechanochemical Synthesis and Magnetic Characterization of Nanosized Cubic Spinel FeCr2S4 Particles.

Nanosized samples of the cubic thiospinel FeCr2S4 were synthesized by ball milling of FeS and Cr2S3 precursors followed by a distinct temperature treatment between 500 and 800 °C. Depending on the applied temperature, volume weighted mean (Lvol) particle sizes of 56 nm (500 °C), 86 nm (600 °C), and 123 nm (800 °C) were obtained. All samples show a transition into the ferrimagnetic state at a Curie temperature TC of ∼ 167 K only slightly depending on the annealing temperature. Above TC, ferromagnetic spin clusters survive and Curie–Weiss behavior is observed only at T ≫ TC, with T depending on the heat treatments and the external magnetic field applied. Zero-field-cooled and field-cooled magnetic susceptibilities diverge significantly below TC in contrast to what is observed for conventionally solid-state-prepared polycrystalline samples. In the low-temperature region, all samples show a transition into the orbital ordered state at about 9 K, which is more pronounced for the samples heated to higher temperatures. This observation is a clear indication that the cation disorder is very low because a pronounced disorder would suppress this magnetic transition. The unusual magnetic properties of the samples at low temperatures and different external magnetic fields can be clearly related to different factors like structural microstrain and magnetocrystalline anisotropy.

Hydrogen spillover and structural defects in a PdO/zirconia nanophase synthesized through a surfactant-templated route

Nanosized zirconia with structural defects was synthesized by using surfactant-templated method. The average crystallite size, phase composition, cationic defect concentration and structural microstrain in the monoclinic phase of zirconia annealed at 800 °C were quantitatively determined by the Rietveld refinement. An unusual behaviour of hydrogen reversibly storing into/releasing from the PdO/ZrO 2 nanophases was observed, which related to formation/decomposition of a hydride phase β-PdH 0.76 and to hydrogen storage in the cationic defects of zirconia nanophases by means of hydrogen spillover from Pd metals to zirconia support.