Electronic Structure of High Entropy Ceramics: Mechanisms of Property Emergence
Open Access
- Author:
- Marques Dos Santos Vieira, Francisco
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 05, 2025
- Committee Members:
- John Mauro, Program Head/Chair
Susan Sinnott, Major Field Member
Zhiqiang Mao, Major Field Member
Raymond Schaak, Outside Unit & Field Member
Jon-Paul Maria, Chair & Dissertation Advisor
Ismaila Dabo, Special Member - Keywords:
- High entropy
high-entropy
oxides
DFT
DFT+U
Band gap - Abstract:
- High-entropy ceramics (HECs) are chemically complex crystalline solid solutions of several constituents (often five or more) in various proportions (often equimolar). The sharing of a sublattice by multiple species produces significant configurational entropy (from which these materials derive their name). The entropy engineering approach leverages this configurational entropy to produce HECs in spite of the enthalpic penalty associated with mixing multiple species on a sublattice. The chemical complexity of HECs opens the door to extreme compositional flexibility which offers the opportunity to engineer material properties. Additionally, the limited short-range order (SRO) among the high-entropy sublattice produces a property dispersion differing significantly from that of the end-member compositions of a given HEC. Applications of interest include catalysis, electrocalorics, ionic conductors, radiation shielding, among others. The compositional flexibility and lack of SRO present significant challenges to first principles studies of HECs. Each composition demands a large simulation cell to accommodate the lack of SRO. This is further complicated by the fact that each composition admits an enormous number of configurations each of which may produce different predictions of material properties. This dissertation presents a framework for modeling HECs and demonstrates its use in the study of three HECs by Hubbard corrected density functional theory (DFT+U). The study of Ba-rich (Ca Sr Ba Eu Yb)MnSB2 revealed that configurational entropy-driven corrugation of the Sb sublattice affected the dimensionality of the Dirac bands near the Fermi level. The study of equimolar (Fe Co Ni Cu Zn)Al2O4 identified the variability of electronegativity and the narrowing of crystal field splitting promoted by A-site cation mixing as contributing factors to the narrowing of the band gap of the high-entropy composition vis à vis its end-member compositions. The study of equimolar (Mn Fe Co Ni Cu Zn)WO4 identified charge transfer between Fe and Cu, and the lifting of band degeneracy driven by local lattice distortions to be additional mechanisms contributing to band gap narrowing. These studies demonstrate that pragmatically sampling the composition and configuration space can identify mechanisms underpinning the emergence of electronic properties of high-entropy ceramics.