Kavli Affiliate: Ke Wang
| First 5 Authors: Hao-Cheng Thong, Fang-Zhou Yao, Xian-Xian Cai, Ze Xu, Mao-Hua Zhang
| Summary:
The functionality of ferroelectrics is often constrained by their Curie
temperature, above which depolarization occurs. Lithium (Li) is the only
experimentally known substitute that can increase the Curie temperature in
ferroelectric niobate-based perovskites, yet the mechanism remains unresolved.
Here, the unique phenomenon in Li-substituted KNbO3 is investigated using
first-principles density functional theory. Theoretical calculations show that
Li substitution at the A-site of perovskite introduces compressive chemical
pressure, reducing Nb-O hybridization and associated ferroelectric instability.
However, the large off-center displacement of the Li cation compensates for
this reduction and further enhances the soft polar mode, thereby raising the
Curie temperature. In addition, the stability of the tetragonal phase over the
orthorhombic phase is predicted upon Li substitution, which reasonably explains
the experimental observation of a decreased orthorhombic-to-tetragonal phase
transition temperature. Finally, a metastable anti-phase polar state in which
the Li cation displaces oppositely to the Nb cation is revealed, which could
also contribute to the variation of phase transition temperatures. These
findings provide critical insights into the atomic-scale mechanisms governing
Curie temperature enhancement in ferroelectrics and pave the way for designing
advanced ferroelectric materials with improved thermal stability and functional
performance.
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