Colossal dielectric constant in high entropy oxides (1602.07842v1)

Abstract: Entropic contributions to the stability of solids are very well understood and the mixing entropy has been used for forming various solids, for instance such as inverse spinels. A particular development was related to high entropy alloys in which the configurational disorder is responsible for forming simple solid solutions and which are thoroughly studied for various applications especially due to their mechanical properties but also electrical properties, hydrogen storage, magnetic properties. Many unexplored compositions and properties still remain for this class of materials due to their large phase space. In a recent report it has been shown that the configurational disorder can be used for stabilizing simple solid solutions of oxides, which should normally not form solid solutions, these new materials were called "entropy-stabilized oxides". In this pioneering report, it was shown that mixing five equimolar binary oxides yielded, after heating at high temperature and quenching, an unexpected rock salt structure compound with statistical distribution of the cations in a face centered cubic lattice. Following this seminal study, we show here that these high entropy oxides (named HEOx hereafter) can be substituted by aliovalent elements with a charge compensation mechanism. This possibility largely increases the potential development of new materials by widening their (already complex) phase space. As a first example, we report here that at least one HEOx composition exhibits colossal dielectric constants, which could make it very promising for applications as large-k dielectric materials.

• The paper reports a colossal dielectric constant near 20,000 in HEOx achieved through precise aliovalent substitution and intrinsic charge compensation.
• The synthesis methodology, involving mechanical grinding, sintering at 1000°C, and air quenching, preserves a metastable FCC structure confirmed by XRD and XPS.
• The findings highlight promising applications in miniaturized electronics and encourage further research into defect chemistry and solid-state phenomena.

Colossal Dielectric Constant in High Entropy Oxides: Implications and Innovations

This paper presents the synthesis and characterization of high entropy oxides (HEOx), focusing on their extraordinary dielectric properties. High entropy materials typically exploit configurational entropy to stabilize novel solid solutions that are inaccessible in traditional alloy systems. The research highlights a specific composition of HEOx that shows a colossal dielectric constant (CDC), presenting potential as large-k dielectrics for diverse applications.

Key Findings

The authors report on the synthesis of oxide compositions such as (Mg, Ni, Co, Cu, Zn)O, commonly denoted as HEOx-0, and its variants doped with lithium or a combination of lithium and other aliovalent elements. The foundation of this paper stems from the ability of high entropy oxides to stabilize face-centered cubic structures at high temperatures, maintained at room temperature due to minimal diffusion rates.

Synthesis Methodology: The compounds are synthesized using mechanical grinding followed by sintering at 1000°C and air quenching, which retains a metastable state. X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) were employed to investigate the structural and chemical state of these materials, confirming the FCC structure and 2+ oxidation state of cations.

Substitution Insights: It is found that aliovalent substitution, particularly lithium incorporation, significantly influences the dielectric properties through a charge compensation mechanism. The intrinsic charge compensation mechanism involves the oxidation of Co to its 3+ state, or potentially the formation of oxygen vacancies. The lattice parameter variations adhere to Vegard's law up to a substantial lithium fraction, suggesting efficient integration into the lattice.

Colossal Dielectric Properties: The dielectric constant values recorded are immensely high, with HEOx samples exhibiting relative permittivity magnitudes reaching close to 20,000 at elevated temperatures and low frequencies. The room temperature resistivity and frequency dependency of capacitance signal potential contributions from multiple charge compensation mechanisms intrinsic to substituted HEOx.

Impact and Implications:

The implications of this research are twofold, addressing practical and theoretical perspectives:

• Practical Applications: The CDC materials discovered suggest viable alternatives for components requiring high dielectric constants, such as capacitors and other electronics where energy storage and efficiency are paramount. This could significantly influence designs in miniaturized electronic circuits and expand functionalities in telecommunications, computing, and beyond.
• Theoretical Considerations: The paper broadens the exploration within the large compositional phase space inherent to high entropy systems. The findings encourage examination into the electronic structure and defect chemistry, potentially uncovering new phenomena pertinent to solid-state physics. Furthermore, the clear tunability through aliovalent substitution emphasizes the critical role of charge compensation—an area rich for further exploration.

Speculative Future Directions

Continued paper into HEOx materials can be anticipated in the following areas:

• Mechanism Elucidation: Further work to understand the dielectric behavior and delineate the relative roles of defects, dipolar interactions, and Maxwell-Wagner polarization could contribute to a robust theoretical framework.
• Material Optimization: Optimization of synthesis variables and dopant selection to fine-tune electrical properties remains a fertile area of research. This addresses not only academic curiosity but also directly relates to industry demands for enhanced performance materials.
• Device Integration: Efforts may extend toward aligning these materials with current manufacturing practices, ensuring compatibility with existing technologies while exploring novel device architectures.

In conclusion, the discoverability of colossal dielectric phenomena in high entropy oxides not only showcases their potential as pivotal materials in electronics but also lays the groundwork for expanded understanding in high entropy systems, driving both scientific inquiry and technological advancement.