Unstable zone-boundary phonon modes drive atomic displacements linked to a rich array of properties. Yet, the electronic origin of the instability remains to be clearly explained. In this Letter, we propose that bonding interaction between Bloch states belonging to different wave vectors leads to such instability via the pseudo- or second-order Jahn–Teller effect. Our first-principles calculations and representation theory-based analyses show that rotations of anion coordinated octahedra, an archetypal example of zone-boundary phonon condensations, are induced by this bonding mechanism. The proposed mechanism is universal to any non-zone-center phonon condensations and could offer a general approach to understanding the origin of structural phase transitions in crystals.

中文翻译：

---

非区域中心声子凝聚的电子起源：以八面体旋转为例

不稳定的区域边界声子模式驱动与一系列丰富特性相关的原子位移。然而，不稳定性的电子起源仍有待清楚解释。在这封信中，我们提出属于不同波向量的布洛赫态之间的键合相互作用通过伪或二阶 Jahn-Teller 效应导致这种不稳定性。我们的第一性原理计算和基于表示理论的分析表明，阴离子配位八面体的旋转是由这种键合机制引起的，这是区域边界声子凝聚的典型例子。所提出的机制对于任何非区域中心声子凝聚都是通用的，并且可以提供一种理解晶体结构相变起源的通用方法。