Tailoring dielectric performance in flexible HEO/PVDF composites: A comparative study of spinel vs. rock-salt high-entropy oxides

This study investigates the structural and dielectric properties of polyvinylidene fluoride (PVDF) composite films reinforced with two distinct high-entropy oxides (HEOs): rock-salt (Mg Co Cu Ni Zn)O (HEO) and spinel (Co Cr Fe Ni Mn) 3 O 4 (HEO-2). The composites were fabricated via multi-step hot pressing, achieving nearly homogeneous filler dispersion confirmed by SEM/EDS. Structural characterization (FTIR) revealed the phase content in HEO/PVDF (60.99 %) compared to HEO-2/PVDF (69.49 %). Dielectric spectroscopy, conducted at temperature and frequency range of 20–140 °C, 10 -2 –10 6 Hz, demonstrated colossal dielectric constant (CDC) behavior in both composites, with HEO-2/PVDF exhibiting superior dielectric characteristic (permittivity), which is attributed to the presence of multiple valence states of transition metal ions and more energetically favorable electron hopping pathway that enhance electron hopping and interfacial charge accumulation. Conduction mechanisms diverged, with correlated barrier hopping (CBH) dominant for both composite films at high temperatures, which transit from hopping-dominated transport. Nyquist analysis reveals a linear response for the rock-salt and spinel composite with notable bend toward the axis transiting from ordinary like Warburg diffusion behavior to bounded diffusion (finite diffusion length) or strong ideal capacitor behavior. These results demonstrate that preferential cation occupation in spinel HEOs plays a key role in enhancing dielectric performance, providing a pathway for designing high-efficiency dielectric composites.