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

Ba­(Ti,Fe)­O₃ 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)­O₃ 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 d⁰ versus dⁿ in a ferroic perovskite. A series of iron-substituted barium titanate nanocrystals (BaTi_1–xFe_xO₃) 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, BaTiO₃, and BaTi_1–xFe_xO₃ 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 Fe³⁺(d⁵) and Fe⁴⁺(d⁴), both candidates for multiferroicity in this system, given certain spin configurations in octahedral field splitting. The PDF cacluated lattice expansion is attributed to Fe³⁺(d⁵, HS) incorporation. The evidence of noncentrosymmetry, lattice expansion, and XPS conformation of Fe³⁺ 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 Fe⁴⁺ with increasing Fe, suggesting that Fe­(IV) is evolving as charge compensation with decreasing Ti⁴⁺, while attempting to preserve the perovskite structure. A mixture of Fe³⁺/Fe⁴⁺ is thought to reside at the B site: Fe⁴⁺ helps stabilize the structure through charge balancing, while Fe³⁺ may be complimented with oxygen vacancies to some extent, especially at the surface. The structure may therefore be of the form BaTi_1–xFe_xO_3‑δ for increasing x. At higher concentrations (Fe > 0.5) the emergence of BaFeO₃ and/or BaFe₂O₄ is offered as an explanation for competing phases, with BaFeO₃ 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–xFe_xO₃/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–xFe_xO₃ samples for x = 0–0.75, respectively. Loss tangent values at 1 MHz were ∼0.04, demonstrating the ability to prepare capacitors of magnetic Ba­(Ti_1–xFe_x)­O₃ 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).