While ferroelectric hafnia (HfO2) has become a technically important material for microelectronics, the physical origin of its ferroelectricity remains poorly understood. The tetragonal P42/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 Pca21 ferroelectric family can be described by a Pcca—Pca21 transition, where the Pcca mother phase will evolve into either the Pca21 ferroelectric phase or the centrosymmetric P21/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.

中文翻译：

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典型萤石型铁电体中的相变


虽然铁电铫 （HfO2） 已成为微电子学中技术上重要的材料，但对其铁电性的物理起源仍然知之甚少。四方 P42/nmc 相通常被指定为其顺电母相，但在布里渊区中心没有软模式。在这项工作中，我们提出萤石型 Pca21 铁电族中的顺电-铁电转变可以用 Pcca-Pca21 转变来描述，其中 Pcca 母相将演变成 Pca21 铁电相或中心对称 P21/c 单斜相，具体取决于应变条件。在连续相变的背景下，Pcca 相与这两个相直接相关。Hafnia 被认为是该家族的一个特例，因为它具有偶然的原子简并性，因为所有阴离子都是氧。该理论还与从化学角度解释哈夫尼亚中铁电性的七配位理论相关。此外，还讨论了促进 hafnia 中铁电相的应变条件。