Several candidate reference phases have been proposed to discuss phase transitions and ferroelectricity in hafnia in recent years. Although these proposals comply with crystallographic requirements, a physically compelling rationale connecting parent and daughter phases is often lacking. This problem is aggravated by the absence of clearly dominant polarization switching pathways, making it difficult to formulate a robust physical criterion for identifying the relevant high-symmetry states. Here, we use first-principles calculations to show that pressure reveals clear parent-daughter relationships among many polymorphs of hafnia and thereby provides a physically motivated hierarchy of structures, including the monoclinic ground state and the technologically relevant ferroelectric phase, and their respective parent structures. Our simulations also reveal several additional phases, including higher-energy structures that act as common ancestors of the widely discussed cubic and orthorhombic reference phases.

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• Phase diagrams
• Pressure effects
• Structural properties
• Ferroelectrics
• Density functional calculations