CRYSTAL GROWTH OF HIGH-ENTROPY OXIDES

Author

Matheus Pianassola, University of Tennessee, KnoxvilleFollow

Orcid ID

https://orcid.org/0000-0001-7468-2794

Date of Award

12-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Mariya Zhuravleva

Committee Members

Mariya Zhuravleva, Chuck Melcher, Veerle Keppens, Bryan Chakoumakos, Jacob McMurray

Abstract

This dissertation advances the field of crystal growth of emerging high-entropy oxides with great potential for novel structural and functional properties. The high-temperature structural stability of high-entropy rare-earth (RE) sesquioxides (RE₂O₃) upon cooling from the melt was studied for the first time to inform crystal growth experiments. While high-entropy oxides are mainly synthesized as polycrystalline ceramics, this work demonstrates their practical crystal growth for further exploration. The micro-pulling-down (mPD) crystal growth of high-entropy RE sesquioxides (RE₂O₃), aluminates (RE₃Al₅O₁₂, REAlO₃, and RE₄Al₂O₉), and perovskites (RE’RE”O₃) was demonstrated. Additionally, the scalable Czochralski (Cz) growth of aluminate crystals was explored. Compositions were formulated taking into consideration the type of RE and the average ionic radius (AIR) of the high-entropy compound. The correlation between phase formation and AIR is demonstrated as a first step towards developing a predictive capability for more complex compositions. The potential for the success of high-entropy scintillators was evaluated by growing and assessing the scintillation properties mPD Ce-doped garnet crystals.

The structural evolution of high-entropy oxides upon cooling from the melt was assessed via in-situ high-temperature neutron and X-ray diffraction on aerodynamically levitated samples. Melting point temperatures were determined via the cooling traces method. The effect of composition on phase formation was evaluated via powder and single-crystal X-ray diffraction (XRD). The microstructure and composition of crystal defects were evaluated with scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS). The elemental distribution in the single crystals was determined via electron microprobe analysis (EPMA). The luminescence properties assessed for complex garnet scintillators include radioluminescence, photoluminescence, light yield, and afterglow.

The demonstration of the crystal growth of high-entropy oxides in this dissertation opens the door to further discovery and development of highly complex crystals. This work will inspire the investigation and unlock the full potential of such complex materials since the availability of single crystals will allow the assessment of unique properties that cannot be replicated in polycrystals. The field of aluminate optical crystals may benefit the most from the results presented in this dissertation since their high-entropy analogous crystals were the most promising in terms of optical quality and phase purity.

Recommended Citation

Pianassola, Matheus, "CRYSTAL GROWTH OF HIGH-ENTROPY OXIDES. " PhD diss., University of Tennessee, 2022.
https://trace.tennessee.edu/utk_graddiss/11565