First report of the ferroelectric behavior of a metal, as shown for solder

Ferroelectric behavior is well known for nonconductive dielectrics. This work provides the first report of this behavior in a metal, specifically a solder (Sn–4Ag), which is highly conductive. As required for ferroelectric behavior, the polarization changes sign when the applied electric field changes sign. The curve of polarization vs. electric field over positive and negative values of the electric field is asymmetric, with the forward polarization being greater than the subsequent reverse polarization, regardless of the direction of the initial current (electric field), due to the hysteresis. A minimum electric field magnitude of 1 × 10⁻³ V/m (corresponding to an energy of 0.5 meV) is needed for the polarization magnitude to be nonzero. Above this value, the polarization magnitude increases with increasing field magnitude, with the slope of the curve increasing with the field magnitude, reaching a constant value when the field magnitude exceeds 4 × 10⁻³ V/m. The polarization does not level off, indicating the absence of saturation in the polarization, as expected from the polarization stemming from the free electrons interacting with the atoms. This is in contrast to nonconductive ferroelectric materials, which exhibit polarization saturation. The maximum slope of the curve gives the DC relative permittivity of 6.10 × 10³, which is smaller than the AC permittivity based on capacitance measurement by three orders of magnitude. The ferroelectric behavior is potentially useful for data storage, voltage-based tuning of the capacitance and polarization polarity switching through the reversal of the polarity of the electric field.