Stoichiometric Control over Ferroic Behavior in Ba(Ti<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>)O<sub>3</sub> Nanocrystals

Ba­(Ti,Fe)­O<sub>3</sub> 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<sub>3</sub> 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 <i>d</i><sup>0</sup> versus <i>d</i><sup><i>n</i></sup> in a ferroic perovskite. A series of iron-substituted barium titanate nanocrystals (BaTi<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3</sub>) 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 <i>x</i> (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 <i>x</i> = 0, 0.1, 0.2, 0.3, BaTiO<sub>3</sub>, and BaTi<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3</sub> nanocrystals, it is concluded that they are tetragonal noncentrosymmetric <i>P</i>4<i>mm</i> with lattice parameters increasing from, e.g., <i>c</i> = 4.04 to 4.08 Å. XPS analysis confirms the presence of both Fe<sup>3+</sup>(<i>d</i><sup>5</sup>) and Fe<sup>4+</sup>(<i>d</i><sup>4</sup>), both candidates for multiferroicity in this system, given certain spin configurations in octahedral field splitting. The PDF cacluated lattice expansion is attributed to Fe<sup>3+</sup>(<i>d</i><sup>5</sup>, HS) incorporation. The evidence of noncentrosymmetry, lattice expansion, and XPS conformation of Fe<sup>3+</sup> provides support for the existence of multiferroicity in these sub-10 nm uniform dispersed nanocrystals. For <i>x</i> > 0.5, Fe impacts the structure but still produces dispersible, relatively monodisperse nanocrystals. XPS also shows an increasing amount of Fe<sup>4+</sup> with increasing Fe, suggesting that Fe­(IV) is evolving as charge compensation with decreasing Ti<sup>4+</sup>, while attempting to preserve the perovskite structure. A mixture of Fe<sup>3+</sup>/Fe<sup>4+</sup> is thought to reside at the B site: Fe<sup>4+</sup> helps stabilize the structure through charge balancing, while Fe<sup>3+</sup> may be complimented with oxygen vacancies to some extent, especially at the surface. The structure may therefore be of the form BaTi<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3‑δ</sub> for increasing <i>x</i>. At higher concentrations (Fe > 0.5) the emergence of BaFeO<sub>3</sub> and/or BaFe<sub>2</sub>O<sub>4</sub> is offered as an explanation for competing phases, with BaFeO<sub>3</sub> as the likeliest competing phase for <i>x</i> = 1.0. Because of the good dispersibility of the nanocrystals in solvents, spin coating of uniform 0–3 nanocomposite BaTi<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3</sub>/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<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3</sub> samples for <i>x</i> = 0–0.75, respectively. Loss tangent values at 1 MHz were ∼0.04, demonstrating the ability to prepare capacitors of magnetic Ba­(Ti<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>)­O<sub>3</sub> with relatively high permittivity. Magnetic characterization by MPMS (both magnetic hysteresis loops and zero field and field cooling measurements) showed increased magnetization with increasing Fe ion concentration. Weak magnetic coercivity and a small remanence magnetization is observed (<5 K), implying a weak ferromagnetic state at low temperatures (<5 K).