Stoichiometric Control over Ferroic Behavior in Ba(Ti_1–xFe_x)O₃ Nanocrystals

Abstract

Ba(Ti,Fe)O 3 is a useful system for the exploration of multiferroic properties as a function of composition and variation in structure, based upon a model of intersubstitution of the B site cation. Nanocrystals of Ba(Ti,Fe)O 3 could be used as building blocks for composite multiferroic materials, provided ferroic properties are recognizable at this length scale and Ti and Fe serve as ideal models for the case of d 0 versus d n in a ferroic perovskite. A series of iron-substituted barium titanate nanocrystals (BaTi 1−x Fe x O 3 ) were synthesized at 60 °C using a hybrid sol−gel chemical solution processing method. No further crystallization/ calcination steps were required. The as-prepared nanocrystals are fully crystalline, uniform in size (∼8 nm by TEM), and dispersible in polar organic solvents, yielding nanocrystal/alcohol formulations. Complete consumption of the reactant precursors ensures adequate control over stoichiometry of the final product, over a full range of x (0, 0.1 to 0.75, 1.0). Pair distribution function (PDF) analysis enabled in-depth structural characterization (phase, space group, unit cell parameters, etc.) and shows that, in the case of x = 0, 0.1, 0.2, 0.3, BaTiO 3 , and BaTi 1−x Fe x O 3 nanocrystals, it is concluded that they are tetragonal noncentrosymmetric P4mm with lattice parameters increasing from, e.g., c = 4.04 to 4.08 Å. XPS analysis confirms the presence of both Fe 3+ (d 5 ) and Fe 4+ (d 4 ), both candidates for multiferroicity in this system, given certain spin configurations in octahedral field splitting. The PDF cacluated lattice expansion is attributed to Fe 3+ (d 5 , HS) incorporation. The evidence of noncentrosymmetry, lattice expansion, and XPS conformation of Fe 3+ provides support for the existence of multiferroicity in these sub-10 nm uniform dispersed nanocrystals. For x > 0.5, Fe impacts the structure but still produces dispersible, relatively monodisperse nanocrystals. XPS also shows an increasing amount of Fe 4+ with increasing Fe, suggesting that Fe(IV) is evolving as charge compensation with decreasing Ti 4+ , while attempting to preserve the perovskite structure. A mixture of Fe 3+ /Fe 4+ is thought to reside at the B site: Fe 4+ helps stabilize the structure through charge balancing, while Fe 3+ may be complimented with oxygen vacancies to some extent, especially at the surface. The structure may therefore be of the form BaTi 1−x Fe x O 3-δ for increasing x. At higher concentrations (Fe > 0.5) the emergence of BaFeO 3 and/or BaFe 2 O 4 is offered as an explanation for competing phases, with BaFeO 3 as the likeliest competing phase for x = 1.0. Because of the good dispersibility of the nanocrystals in solvents, spin coating of uniform 0−3 nanocomposite BaTi 1−x Fe x O 3 /polyvinylpyrrolidone thin film capacitors (<0.5 μm) was possible. Frequency dependent dielectric measurements showed stable dielectric constants at 1 MHz of 27.0 to 22.2 for BaTi 1−x Fe x O 3 samples for x = 0−0.75, respectively. Loss tangent

When sonochemistry meets heterogeneous photocatalysis: designing a sonophotoreactor towards sustainable selective oxidation

“…It is also possible that many nanoparticles remain aggregated during the magnetic stirring via polar interactions (Fig. 6b), [61][62][63] and these aggregations move as one chunk during magnetic stirring. This leads to the creation of interstitial phases/spaces, [64][65][66] where unselective reactions, like overoxidation, can take place, due to the presence of strongly adsorbed water moieties from the environmental humidity or/ and retention of the diluted oxygen.…”

A cloud platform for atomic pair distribution function analysis: PDFitc

“…For example, the top-ranked structure entry in our Ba-Ti-O example is the Crystallography Open Database (COD) (Gražulis et al, 2009) ID 9014492 (Kwei et al, 1993) BaTiO 3 structure with space group Amm2. This structure was also found to be the best-fit structure to the PDFs from the gel-synthesized nanoparticles in the original work (Lombardi et al, 2019).…”

“…We use an experimental X-ray PDF of barium titanate nanoparticles (Lombardi et al, 2019) as an example data file here to illustrate the structureMining@PDFitc workflow in steps.…”

Mechanochemical Forces as a Synthetic Tool for Zeroand One-Dimensional Titanium Oxide-Based Nano-photocatalysts

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