Phase transitions in typical fluorite-type ferroelectrics

While ferroelectric hafnia (HfO2) has become a technically important material for microelectronics, the physical origin of its ferroelectricity remains poorly understood. The tetragonal P4(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 the fluorite-type Pca2(1) ferroelectric family can be described by a Pcca- Pca2(1) transition, where the Pcca mother phase will evolve into either the Pca2(1) ferroelectric phase or the centrosymmetric P2(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 with 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.