Antiferroelectricity is a fundamental concept in physics and materials science. Conventional antiferroelectrics (AFEs) have the picture of alternating local electric dipoles defined in real space. Here, we discover a new class of AFEs, termed type-II AFEs, which possess opposite polarizations defined in momentum space across a pair of symmetry decoupled subspaces. Unlike conventional AFEs, the order parameter of type-II AFEs is rigorously formulated through Berry-phase theory and can be quantitatively extracted from the electronic band structure. Focusing on a subclass of type-II AFEs that preserve spin-rotation symmetry, we establish the relevant symmetry constraints and identify all compatible spin point groups. Remarkably, we find that type-II AFE order intrinsically coexists with antiferromagnetism, revealing a robust form of magnetoelectric coupling. We construct an altermagnetic model and identify several concrete antiferromagnetic and altermagnetic materials, such as FeS, ${\text{Cr}}_2{\mathrm{O}}_3$, ${\mathrm{MgMnO}}_3$, monolayer ${\mathrm{MoICl}}_2$ and bilayer ${\mathrm{CrI}}_3$, that exhibit this novel ordering. Furthermore, we uncover unique physical phenomena associated with type-II spin-AFEs including spin current generation upon AFE switching and localized spin polarization at boundaries and domain walls. Our findings reveal a previously hidden class of quantum materials with intertwined ferroic orders, offering exciting opportunities for both fundamental exploration and technological applications.

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• Antiferroelectricity
• First-principles calculations
• Magnetoelectric effect
• Crystal structures
• Insulators
• Multiferroics
• Symmetries in condensed matter
• Tight-binding model