Phase transitions in typical fluorite-type ferroelectrics

Abstract

While ferroelectric hafnia ($\mathrm{Hf}{\mathrm{O}}_2$) has become a technically important material for microelectronics, the physical origin of its ferroelectricity remains poorly understood. The tetragonal $P{4}_2/nmc$ phase is commonly assigned as its paraelectric mother phase but has no soft mode at the Brillouin zone center. In this work, we propose that the paraelectric-ferroelectric transition in hafnia-like $Pca{2}_1$ ferroelectric family can be described by a $Pcca$-$Pca{2}_1$ transition, where the $Pcca$ mother phase will evolve into either the $Pca{2}_1$ ferroelectric phase or the centrosymmetric $P{2}_1/c$ monoclinic phase, depending on the strain conditions. The $Pcca$ phase is directly linked to both phases in the context of continuous phase transition. Hafnia is regarded as a special case of this family, in that it has accidental atomic degeneracy because all anions are oxygen. The theory is also correlated to the seven-coordination theory that explains the ferroelectricity in hafnia from a chemical perspective. In addition, the strain conditions to promote the ferroelectric phase in hafnia are discussed.

12 pages, 4 figures