Recent research on sliding ferroelectricity in various two-dimensional bilayers and multilayers has shown its promising application potential and emerging new physics distinct from conventional ferroelectricity. However, it stems from van der Waals interactions of asymmetrically stacked layers, so the polarizations are weak. Here, we propose that in most layered ion conductors, the asymmetric stacking of layers will give rise to a unique type of ionic sliding ferroelectricity. The major weakness is overcome since the vertical polarizations are generally enhanced by an order of magnitude, which is reversible via in-plane translation of layers and ions. Strikingly, the switching of vertical polarization is intercorrelated to various combinations of in-plane layer sliding and ion displacement modes, which may lead to quantized or fractional quantum ferroelectric switching. We also show first-principles evidence for the wide existence of ionic sliding multiferroicity with altermagnetism as well as ferroelectric control of large altermagnetic spin splitting.

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• Electric polarization
• Ferroelectricity
• First-principles calculations
• Intercalation
• Magnetoelectric effect
• Ionic solids