Spin-driven ferroelectricity and possible antiferroelectricity in   triangular lattice antiferromagnets ACrO2 (A = Cu, Ag, Li, or Na)
  Correlation between dielectric and magnetic properties was investigated on the triangular lattice antiferromagnets ACrO2 (A= Cu, Ag, Li, or Na) showing 120-degree spiral spin structure with easy-axis anisotropy. For the A= Cu and Ag compounds with delafossite structure, ferroelectric polarization emerges upon the spiral spin order, implying the strong coupling between the ferroelectricity and spiral spin structure. On the other hand, for the A = Li and Na compounds with ordered rock salt structure, no polarization but only clear anomalies in dielectric constant can be observed upon the spiral spin order. The absence of polarization can be interpreted as the antiferroelectric state induced by the alternate stacking of Cr3+ layer with opposite spin vector chirality. These results imply that a vast range of trigonally stacked triangular-lattice systems with 120-degree spin structure can be multiferroic, irrespective of their magnetic anisotropy. 
S. Seki, Y. Onose, and Y. Tokura