  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. Here, we propose that bonding interaction between
Bloch states belonging to different wavevectors 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 understand the origin of structural phase transitions in crystals.
