Exploring Metastable Phases in Cerium-Doped Zirconia: Insights from X-ray Diffraction, Raman, X-ray Absorption, and Luminescence Spectroscopy

TL;DR

This study investigates phase changes in cerium-doped zirconia, revealing new structural insights that could improve materials for energy and catalytic applications.

Contribution

The study identifies a miscibility gap and metastable phases in ZrO2–CeO2 using advanced spectroscopic techniques.

Findings

A miscibility gap was observed between 20 and 50 mol % cerium in Zr1–xCexO2.

Tetragonal prime (t′) and tetragonal double prime (t″) phases form depending on cerium content.

Structural distortions in the t′ phase are due to oxygen displacement, not Ce3+ formation.

Abstract

The ZrO2–CeO2 system is
fundamental
to various technological applications, yet unresolved questions persist
regarding cation miscibility and the occurrence of metastable phases
in the Zr1–xCexO2 phase diagram. This work addresses these gaps
through a comprehensive investigation of Zr1–xCexO2 compositions with
varying cerium concentrations and incorporating Eu3+ as
a luminescent probe. Synchrotron powder X-ray diffraction analysis
unveiled a miscibility gap between 20 and 50 mol % cerium. Beyond
this gap, the formation of solid solutions and multiple crystalline
phases was observed, including tetragonal prime (t′) and tetragonal
double prime (t″) structures, depending on cerium content.
Raman investigations revealed a unique distortion band in all compositions
containing the t′ phase. Our high energy resolution fluorescence
detected X-ray absorption near edge structure spectroscopy (HERFD-XANES)
analysis implies that this feature results from oxygen ion displacement
in the t′ structure. Luminescence spectroscopy of the europium
environment revealed distinct excitation and emission spectra across
the various crystal phases, enabling unambiguous identification of
all metastable phases. These findings highlight the complex polymorphism
of the ZrO2–CeO2 system. The ability
to precisely control phase composition offers significant potential
for optimizing properties for diverse applications, including oxygen
sensors, three-way catalysts, and solid oxide fuel cells for clean,
sustainable energy generation.

This study
explores phase transformations in cerium-doped
zirconia (ZrO2–CeO2) solid solutions.
Synchrotron X-ray diffraction, Raman, and luminescence spectroscopy
reveal tetragonal metastable phases and a miscibility gap between
20–50 mol % Ce. Raman and HERFD-XANES confirm that Ce remains
tetravalent, with structural distortions arising from oxygen displacement
rather than Ce3+ formation. These findings enhance our
understanding of phase behavior, aiding the development of materials
with improved oxygen storage and catalytic performance.

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