Cocktail Effect at the B-Site of Hexagonal ABO3: Structural Evolution and High Entropy Low Loss Dielectrics

Abstract

High entropy oxides provide an exciting avenue to obtain superior functionalities. However, stabilizing high entropy oxides containing equimolar components is a challenge in structures that are highly sensitive to the size of constituting ions. Hexagonal ABO₃ (P6₃cm) is one such structural class that shows improper ferroelectricity due to non-centrosymmetric placement of ions. The motivation for this work was to stabilize a high entropy YInO₃-based hexagonal composition, which has not been reported earlier, and its impact on electrical properties. Tailoring synthesis conditions yielded single-phasic hexagonal polymorphs for YInO₃, Y(In_0.5Mn_0.5)O₃, Y(In_0.33Mn_0.33Fe_0.33)O₃, and Y(In_0.25Mn_0.25Fe_0.25Ga_0.25)O₃. Sharp XRD peaks with very broad Raman modes and decrease in the grain size support a single hexagonal phase with strong randomization. Entropy stabilization was established by positive enthalpy of formation determined by Calvet-calorimetry. These hexagonal polymorphs have differential thermal expansions, with a/b-axes showing double expansion compared to c-axis, which is attributed to unbuckling of BO₅ layers that provides a buffer in the c-direction. Interestingly, as the B-site randomizes, BO₅ polyhedra become more regular, accompanied by an increase in the B–O_planar–B angle, which increases local symmetry and tends to reduce inherent polarization. Y(In_0.25Mn_0.25Fe_0.25Ga_0.25)O₃ exhibits a low dielectric loss of ∼0.0085 and an ultra-low leakage current of 5.2 × 10^–10A/cm² (upto 150 °C), an order of magnitude improvement over YInO₃.